Pool-type experimental reactors have had safe and stable operation for the past 50 years.
“It can be constructed either inner land or on the coast, making it an especially good fit for northern inland areas, and it has an expected lifespan of around 60 years. In terms of costs, the thermal price is far superior to gas, and is comparably economical with coal and combined heat and power (CHP).”
A feasibility study on constructing China’s first nuclear plant for district heating is being carried out by China General Nuclear and Tsinghua University. The plant would use the domestically-developed NHR200-II low-temperature heating reactor technology.
Pool reactors at universities since the 1960s
A common university research reactor design (67 units) is the pool-type reactor, where the core is a cluster of fuel elements sitting in a large pool of water. Among the fuel elements are control rods and empty channels for experimental materials. Each element comprises several (e.g. 18) curved aluminium-clad fuel plates in a vertical box. The water both moderates and cools the reactor, and graphite or beryllium is generally used for the reflector, although other materials may also be used. Apertures to access the neutron beams are set in the wall of the pool. Tank type research reactors (32 units) are similar, except that cooling is more active.The TRIGA (Training, Research, Isotopes, General Atomics) reactor is a common pool-type design (38 units in 2017, with 31 decommissioned) with three generations of design commissioned since 1960.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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Wow! “Superior to natural gas!” Those clever Chinese stunt scientists! This blog is even better than Sputnik News! But it appears China has never heard of geothermal heat pumping. Water in PVC piping is heated by little more than solar energy on an asphalt pavement, then superheated by a reverse HVAC compressor. The same system acts as a heat sink for cooling in the summer. So a preliminary design for a pool reactor has been completed by unspecified Chinese acting with spontaneous patriotism and wisdom. Never a mention of whether it ever will be built or marketed. Son of Zhang Zhimin anounced 3 years ago that China would take “the first bite of the apple” by building the first MSR of the 21st Century. Still no word what became of that.
Wow! Superior to natural gas!”” Those clever Chinese stunt scientists! This blog is even better than Sputnik News!But it appears China has never heard of geothermal heat pumping. Water in PVC piping is heated by little more than solar energy on an asphalt pavement”””” then superheated by a reverse HVAC compressor. The same system acts as a heat sink for cooling in the summer.So a preliminary design for a pool reactor has been completed by unspecified Chinese acting with spontaneous patriotism and wisdom. Never a mention of whether it ever will be built or marketed.Son of Zhang Zhimin anounced 3 years ago that China would take “”””the first bite of the apple”””” by building the first MSR of the 21st Century. Still no word what became of that.”””
Correction: [fast] neutrons typically interact with material in an ELASTIC (not inelastic) manner. Words mean things. Sorry
Correction: [fast] neutrons typically interact with material in an ELASTIC (not inelastic) manner. Words mean things. Sorry
.. “Light water reactors NEVER use reflectors of graphite or beryllium regardless of what crap is written on the General Atomics website.” “What NEVER? No NEVER! What! NEVER?? Well, .. hardly ever.” (Apologies to Gilbert and Sullivan. HMS Pinafore, I think.) I don’t pretend to be an expert on reactor design, though I understand basic physics well enough. I’ll admit that I see little point for wrapping a commercial PWR in a neutron reflector, beryllium or otherwise. That’s because the core is large enough, filled with enough moderating water around the fuel elements, that the neutrons emitted in fission or decay events are pretty well thermalized before they ever reach the core wall. PWRs running on enriched uranium have rich neutron economies. They can afford to let a small fraction of their neutrons escape without triggering fissions. Small reactors are another matter. All neutrons in a reactor are born fast; they have to be slowed down to reach the low energies at which the cross section for capture by a fissile nucleus becomes large. In a particularly small reactor, a lot of neutrons will reach the reactor wall before being fully moderated. They need to be reflected back into the core, and if they can be partially moderated at the same time, so much the better! I know nothing about the design of naval reactors beyond what’s general knowledge: that they’re small PWRs that run on highly enriched uranium. But I’d be willing to bet that their cores are indeed enclosed by neutron reflectors, and that those reflectors are beryllium or grahite. BTW, didn’t GA play a major role in the design of naval reactors?
.. Light water reactors NEVER use reflectors of graphite or beryllium regardless of what crap is written on the General Atomics website.””””””What NEVER? No NEVER! What! NEVER?? Well”””” .. hardly ever.”””” (Apologies to Gilbert and Sullivan. HMS Pinafore”” I think.)I don’t pretend to be an expert on reactor design though I understand basic physics well enough. I’ll admit that I see little point for wrapping a commercial PWR in a neutron reflector beryllium or otherwise. That’s because the core is large enough filled with enough moderating water around the fuel elements that the neutrons emitted in fission or decay events are pretty well thermalized before they ever reach the core wall. PWRs running on enriched uranium have rich neutron economies. They can afford to let a small fraction of their neutrons escape without triggering fissions.Small reactors are another matter. All neutrons in a reactor are born fast; they have to be slowed down to reach the low energies at which the cross section for capture by a fissile nucleus becomes large. In a particularly small reactor a lot of neutrons will reach the reactor wall before being fully moderated. They need to be reflected back into the core and if they can be partially moderated at the same time so much the better! I know nothing about the design of naval reactors beyond what’s general knowledge: that they’re small PWRs that run on highly enriched uranium. But I’d be willing to bet that their cores are indeed enclosed by neutron reflectors and that those reflectors are beryllium or grahite.BTW”” didn’t GA play a major role in the design of naval reactors?”””
.. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on, which is a small fraction of all scattering events.” Now THERE I can say with confidence that you’re wrong. Neutrons don’t “bounce off” nuclei; the force between neutrons and nuclei is short range but strongly attractive, not repulsive. The correct analogy isn’t billiard balls, but rather interstellar comets whose trajectories pass close to the sun. It’s rare for them to be aimed so directly at the sun that they collide with it, but their trajectories are nearly always parabolic. A parabolic trajectory, translated into the quantum world of particle physics, corresponds to a 180-degree backscatter. Rutherford, in his early experiments with particle scattering, was astonished to observe direct backscatter. It defied the conventional model of atoms and electrons that had prevailed up to then. That was the “plum pudding” model of electrons embedded in a nebulous blob of positively charged atomic mass. He described his astonishment at his experimental results as akin to firing a bullet at a soft tissue stretched over a target frame, and seeing the bullet bounce straight. It’s what led him to develop the nuclear model of atomic elements.
.. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on” which is a small fraction of all scattering events.””Now THERE I can say with confidence that you’re wrong. Neutrons don’t “”””bounce off”””” nuclei; the force between neutrons and nuclei is short range but strongly attractive”” not repulsive. The correct analogy isn’t billiard balls but rather interstellar comets whose trajectories pass close to the sun. It’s rare for them to be aimed so directly at the sun that they collide with it but their trajectories are nearly always parabolic. A parabolic trajectory translated into the quantum world of particle physics corresponds to a 180-degree backscatter. Rutherford in his early experiments with particle scattering”” was astonished to observe direct backscatter. It defied the conventional model of atoms and electrons that had prevailed up to then. That was the “”””plum pudding”””” model of electrons embedded in a nebulous blob of positively charged atomic mass. He described his astonishment at his experimental results as akin to firing a bullet at a soft tissue stretched over a target frame”””” and seeing the bullet bounce straight. It’s what led him to develop the nuclear model of atomic elements.”””
No, ~40% loss is the right number. I took the trouble of calculating it. It follows from the old reliable dynamic duo, conservation of energy and conservation of momentum. More precisely, the loss is 32/81 — i.e., the backscattered neutron has 7/9 the velocity, 49/81 the energy that it had before encountering the (assumed stationary) Be nucleus. And just to be picky, the energy lost in a proton-neutron collision isn’t 50%, it’s 100% (for the “head on” case). The proton flies away from the collision with all the velocity that the neutron had going into the collision, while the neutron comes to rest. Of course, if you’re not talking about the direct head-on case but a statistical average over all scattering collision, it would be quite a different matter. But in that case it wouldn’t be 50% either; it would be much less. Oh, and just to be nasty, the scattering relationships don’t depend only on the atomic number; the atomic mass plays a large role. Sorry, I just had a semi-fight with my wife, who doesn’t fight fair. I don’t have a dog to kick, whereas you look like fair game.
No ~40{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} loss is the right number. I took the trouble of calculating it. It follows from the old reliable dynamic duo conservation of energy and conservation of momentum. More precisely the loss is 32/81 — i.e. the backscattered neutron has 7/9 the velocity 49/81 the energy that it had before encountering the (assumed stationary) Be nucleus. And just to be picky the energy lost in a proton-neutron collision isn’t 50{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} it’s 100{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} (for the head on”” case). The proton flies away from the collision with all the velocity that the neutron had going into the collision”” while the neutron comes to rest.Of course if you’re not talking about the direct head-on case but a statistical average over all scattering collision it would be quite a different matter. But in that case it wouldn’t be 50{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} either; it would be much less. Oh and just to be nasty the scattering relationships don’t depend only on the atomic number; the atomic mass plays a large role.Sorry I just had a semi-fight with my wife who doesn’t fight fair. I don’t have a dog to kick”” whereas you look like fair game.”””
*nothing is fast absorber Loose talk obviously
*nothing is fast absorberLoose talk obviously
And the average energy lost for Neutron versus proton is 50% because the neutron weighs the same as a proton; average energy lost Neutron versus beryllium is probably less than an eighth of that – the relationship is simple it’s in a book – it involves only the atomic number.
And the average energy lost for Neutron versus proton is 50{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} because the neutron weighs the same as a proton; average energy lost Neutron versus beryllium is probably less than an eighth of that – the relationship is simple it’s in a book – it involves only the atomic number.
If you read down that page you gave from General Atomics, you will see that their EM2 reactor improved when the ELIMINATED the Be2C reflector and went with something of a higher atomic weight – Zr3Si2. Look at the illustration on the page. hahahaha Here is the caption from the image: “Computer simulations suggest a reflector composed of zirconium silicide can resolve power peaking issues in fast reactors and significantly improve core performance”. They are so backwards at GA, so out of practice, such a farce that they thought the beryllium was buying them something, but it was hurting them. Clowns! So, the link “high neutron reflector materials” on the General Atomics site actually confirms what I said – and we thought EM2 reactor was fast, but evidently it doesn’t have an average neutron energy above 1.8MeV, ’cause their getting no benefit out of all that Be, it is actually hurting them relative to zirconium and silicon (heavier elements). Ha.
If you read down that page you gave from General Atomics you will see that their EM2 reactor improved when the ELIMINATED the Be2C reflector and went with something of a higher atomic weight – Zr3Si2. Look at the illustration on the page. hahahaha Here is the caption from the image: Computer simulations suggest a reflector composed of zirconium silicide can resolve power peaking issues in fast reactors and significantly improve core performance””.They are so backwards at GA”” so out of practice such a farce that they thought the beryllium was buying them something but it was hurting them. Clowns! So”” the link “”””high neutron reflector materials”””” on the General Atomics site actually confirms what I said – and we thought EM2 reactor was fast”” but evidently it doesn’t have an average neutron energy above 1.8MeV ’cause their getting no benefit out of all that Be”” it is actually hurting them relative to zirconium and silicon (heavier elements). Ha.”””
Light water reactors NEVER use reflectors of graphite or beryllium regardless of what chrap is written on the General Atomics website. I gave several real sources where thick, thick steel is used as the reflector in PWRs – GA hasn’t built anything since Ft. Saint Vrain and that was a failure. Russian RBMK is a graphite moderated light water cooled reactor – it is so big it is almost infinite and whatever the “reflector” is outside the core – it is probably graphite too – doesn’t matter. Beryllium doesn’t have near 100% 180-degree backscatter; that doesn’t make sense. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on, which is a small fraction of all scattering events. Inelastic scattering emits the neutron after some delay isotropically favoring no direction over another. In short, fast neutrons elastic scatter more often then they inelastic scatter (absorb). Fast fission is kind of like spallation. At some point past carbon (sodium?) the scattering events don’t slow the neutrons much. “Reflector” in this case would keep the neutrons fast and be a high Z material. I’m holding my ground bro. Be is not “ideal for enabling small reactors to achieve criticality”; the Be is acting through a completely different mechanism – it is adding fast neutrons to a fast reactor – it becomes a SOURCE of neutrons if the reactor is fast. It does nothing better than inert steel if the neutrons are thermal. Scattering events in Be take a lot of energy from the neutron; Be moderates. Sure, some fraction of the neutrons will bounce 180 degrees, but slower – steel bounces them back with no slowing. It works better in LWR; it is used in LWR; Be is not used in LWR or anything else beside FLiBe or LPPFusion cathodes.
Light water reactors NEVER use reflectors of graphite or beryllium regardless of what chrap is written on the General Atomics website. I gave several real sources where thick thick steel is used as the reflector in PWRs – GA hasn’t built anything since Ft. Saint Vrain and that was a failure. Russian RBMK is a graphite moderated light water cooled reactor – it is so big it is almost infinite and whatever the reflector”” is outside the core – it is probably graphite too – doesn’t matter.Beryllium doesn’t have near 100{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} 180-degree backscatter; that doesn’t make sense. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on”” which is a small fraction of all scattering events. Inelastic scattering emits the neutron after some delay isotropically favoring no direction over another.In short”” fast neutrons elastic scatter more often then they inelastic scatter (absorb). Fast fission is kind of like spallation. At some point past carbon (sodium?) the scattering events don’t slow the neutrons much. “”””Reflector”””” in this case would keep the neutrons fast and be a high Z material. I’m holding my ground bro. Be is not “”””ideal for enabling small reactors to achieve criticality””””; the Be is acting through a completely different mechanism – it is adding fast neutrons to a fast reactor – it becomes a SOURCE of neutrons if the reactor is fast. It does nothing better than inert steel if the neutrons are thermal. Scattering events in Be take a lot of energy from the neutron; Be moderates. Sure”” some fraction of the neutrons will bounce 180 degrees”” but slower – steel bounces them back with no slowing. It works better in LWR; it is used in LWR; Be is not used in LWR or anything else beside FLiBe or LPPFusion cathodes.”””
..Reflectors smooth the neutron flux by scattering back neutrons that would otherwise escape the core, promoting a more consistent fuel burn and more efficient operation. Light-water reactors typically use reflectors made of graphite or beryllium.” – from General Atomics web page on “high neutron reflector materials”. A fast neutron loses about 40% of its energy in backscattering off a beryllium nucleus, so you’re right that it isn’t what you’d want as a reflector in a fast neutron reactor. But the pool type reactors that the article is about aren’t fast neutron reactors. Beryllium has near-zero neutron absorption, and near-100% 180-degree backscatter (as opposed to deflection) for neutrons with which it interacts. So it’s ideal for enabling small reactors to achieve criticality. Or so I understand.
..Reflectors smooth the neutron flux by scattering back neutrons that would otherwise escape the core” promoting a more consistent fuel burn and more efficient operation. Light-water reactors typically use reflectors made of graphite or beryllium.”” – from General Atomics web page on “”””high neutron reflector materials””””.A fast neutron loses about 40{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} of its energy in backscattering off a beryllium nucleus”” so you’re right that it isn’t what you’d want as a reflector in a fast neutron reactor. But the pool type reactors that the article is about aren’t fast neutron reactors. Beryllium has near-zero neutron absorption”” and near-100{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} 180-degree backscatter (as opposed to deflection) for neutrons with which it interacts. So it’s ideal for enabling small reactors to achieve criticality. Or so I understand.”””
I mostly agree.
I mostly agree.
Be is used in weapons because it increases the multiplication factor due to the n -> 2n reaction which occurs above 1.86MeV which means that only super-fast (i.e. unmoderated) neutrons can cause the reaction. Those would be the neutrons that didn’t hit any hydrogen in a LWR – effectively born in the last row of fuel pins because MFP in LWR is on the order of 2cm. So, the Be plays two functions in a weapon or fast reactor: external moderator (vanishingly useful for a weapon) and fast neutron source from n ->2n, which is the dominant factor because iron or uranium would be a better fast neutron reflector. In LWR a typical cut-off for “fast” as a bin or bucket term would be 1MeV – that cut-off would be for steel component embrittlement – as far as the fuel is concerned the cut off is lower (10s or 100s of eV) or anything not sub-eV. Per your definition (scattering vs. capture) I recall that the frontal area of the nucleus is the dominant parameter for fast neutron cross sections – unlike the strange resonance behavior seen at thermal energies, which is like magic by comparison. Unfortunately, many people look at the widely available thermal neutron cross section data (an endpoint reference for LWR) and think they can infer fast neutron characteristics from it. In reality, for fast neutrons, which are marbles, everything looks like a bowling ball – some bowling balls might be 6 pounds (sodium) and some bowling balls might be 15 pounds (Iron). If a marble is shot at a bowling ball the marble retains nearly all of its momentum upon scattering and the scattering angles are not isotropic or equal probability (the forward angles are naturally more probable in a high speed elastic collision) – if a golf ball hits a golf/lacross/baseball (Be/carbon/oxygen) this is less true. Now, without going through the effort of pulling a lot of references, fast neutron total cross sections (all scattering+absorption) are generally less than 5b compared to the hundreds or thousand
Be is used in weapons because it increases the multiplication factor due to the n -> 2n reaction which occurs above 1.86MeV which means that only super-fast (i.e. unmoderated) neutrons can cause the reaction. Those would be the neutrons that didn’t hit any hydrogen in a LWR – effectively born in the last row of fuel pins because MFP in LWR is on the order of 2cm. So the Be plays two functions in a weapon or fast reactor: external moderator (vanishingly useful for a weapon) and fast neutron source from n ->2n which is the dominant factor because iron or uranium would be a better fast neutron reflector.In LWR a typical cut-off for fast”” as a bin or bucket term would be 1MeV – that cut-off would be for steel component embrittlement – as far as the fuel is concerned the cut off is lower (10s or 100s of eV) or anything not sub-eV. Per your definition (scattering vs. capture) I recall that the frontal area of the nucleus is the dominant parameter for fast neutron cross sections – unlike the strange resonance behavior seen at thermal energies”” which is like magic by comparison. Unfortunately many people look at the widely available thermal neutron cross section data (an endpoint reference for LWR) and think they can infer fast neutron characteristics from it. In reality for fast neutrons which are marbles everything looks like a bowling ball – some bowling balls might be 6 pounds (sodium) and some bowling balls might be 15 pounds (Iron). If a marble is shot at a bowling ball the marble retains nearly all of its momentum upon scattering and the scattering angles are not isotropic or equal probability (the forward angles are naturally more probable in a high speed elastic collision) – if a golf ball hits a golf/lacross/baseball (Be/carbon/oxygen) this is less true. Now without going through the effort of pulling a lot of references”” fast neutron total cross sections (all scattering+absorption) are generally less than 5b compared to the hundreds or thousan”
From the US-EPR (big Framatome PWR) – for your information and education. An important design feature of the U.S. EPR is the heavy reflector, a large steel structure that replaces the thin baffle plates used in existing reactors (see Figure 3.9.5-3—Reactor Pressure Vessel Heavy Reflector). This reflector reduces fast neutron leakage and flattens the core power distribution. The reflector resides between the fuel and the core barrel and above the lower core support plate. To avoid any welded or bolted connections close to the core, the reflector consists of stacked forged slabs (rings) positioned one above the other (see slabs I-XII in Figure 3.9.5-3). Keys are used to align the slabs, and they are axially restrained by tie rods bolted to the lower core support plate. The heavy reflector is cooled by water flowing through cooling channels running axially through each slab. The heavy reflector reduces the fast flux on the pressure vessel and improves the neutron economy in the active core. With a volume ratio of approximately 95 percent metal to 5 percent water, the heavy reflector efficiently reflects fast neutrons back to the fuel. In addition, the thermal neutron flux drops off immediately outside the core because there is only a small amount of water present (in the reflector cooling holes) and 4-8 in of steel separating the core from the water outside the reflector.
From the US-EPR (big Framatome PWR) – for your information and education.An important design feature of the U.S. EPR is the heavy reflector a large steelstructure that replaces the thin baffle plates used in existing reactors (seeFigure 3.9.5-3—Reactor Pressure Vessel Heavy Reflector). This reflector reduces fastneutron leakage and flattens the core power distribution. The reflector residesbetween the fuel and the core barrel and above the lower core support plate. To avoidany welded or bolted connections close to the core the reflector consists of stackedforged slabs (rings) positioned one above the other (see slabs I-XII in Figure 3.9.5-3).Keys are used to align the slabs and they are axially restrained by tie rods bolted to the lower core support plate. The heavy reflector is cooled by water flowing throughcooling channels running axially through each slab.The heavy reflector reduces the fast flux on the pressure vessel and improves theneutron economy in the active core. With a volume ratio of approximately 95 percentmetal to 5 percent water the heavy reflector efficiently reflects fast neutrons back tothe fuel. In addition the thermal neutron flux drops off immediately outside the corebecause there is only a small amount of water present (in the reflector cooling holes)and 4-8 in of steel separating the core from the water outside the reflector.”
One final thing – in a LWR – the best choice for a reflector would be drumroll more fuel – even natural fuel – even plates of depleted uranium alloy. That is why big cores are more fuel efficient than small cores and don’t need as much excess reactivity as SMRs do. Can’t make a “reflector” out of uranium tho, because then it would be fuel – special nuclear material. There is not one LWR in the world that would not have its neutron economy enhanced by an extra ring of depleted uranium fuel assemblies (you could even use Thorium) – both are guess what – drumroll – thermal neutron absorbers. See, it’s not a straightforward as you expect. That is why I am paid for what I know.
One final thing – in a LWR – the best choice for a reflector would bedrumrollmore fuel – even natural fuel – even plates of depleted uranium alloy.That is why big cores are more fuel efficient than small cores and don’t need as much excess reactivity as SMRs do. Can’t make a reflector”” out of uranium tho”” because then it would be fuel – special nuclear material. There is not one LWR in the world that would not have its neutron economy enhanced by an extra ring of depleted uranium fuel assemblies (you could even use Thorium) – both are guess what – drumroll – thermal neutron absorbers.See”” it’s not a straightforward as you expect. That is why I am paid for what I know.”””
There was also a Manhattan Project criticality accident involving tungsten carbide reflector bricks – look it up.
There was also a Manhattan Project criticality accident involving tungsten carbide reflector bricks – look it up.
I found a figure showing you just how thick of a stainless steel reflector is these days: Figure 4.3-25 of the NuScale FSAR on page 130 nrc(dot)gov(slash)docs(slash)ML1808(slash)ML18086A172(dot)pdf Nice you gave a CYA “at least for thermal neutrons”… So, to further clarify, heavy reflectors of high Z material are used to bounce a fraction of the leaking fast neutrons back into the active region of the core. My own set of calculations performed for the B&W mPower reactor back in 2010 showed that 9 inches of steel was better than 6 inches of steel was better than 3 inches of steel. We went with about 8 inches. The worth of the reflector was several dollars positive vs. a boundary condition of H2O. Reflectors in thermal reactors should be high Z and not moderating.
I found a figure showing you just how thick of a stainless steel reflector is these days: Figure 4.3-25 of the NuScale FSAR on page 130nrc(dot)gov(slash)docs(slash)ML1808(slash)ML18086A172(dot)pdfNice you gave a CYA at least for thermal neutrons””…So”” to further clarify”” heavy reflectors of high Z material are used to bounce a fraction of the leaking fast neutrons back into the active region of the core. My own set of calculations performed for the B&W mPower reactor back in 2010 showed that 9 inches of steel was better than 6 inches of steel was better than 3 inches of steel. We went with about 8 inches. The worth of the reflector was several dollars positive vs. a boundary condition of H2O. Reflectors in thermal reactors should be high Z and not moderating.”””
Nothing is a fast neutron absorber, and that is what we are trying to “reflect”. Thermal neutrons in the reflector region are effectively lost to the system. Go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. Stainless steel is used. Sure, W and SS are thermal neutron absorbers.
Nothing is a fast neutron absorber and that is what we are trying to reflect””. Thermal neutrons in the reflector region are effectively lost to the system. Go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. Stainless steel is used. Sure”””” W and SS are thermal neutron absorbers.”””
Stainless steel is used for neutron reflectors and anything with a very high Z is a good neutron reflector. Beryllium is a moderator and it also emits neutrons when bombarded increasing the multiplication of the system. Thermal neutrons in the reflector are lost to the system; they don’t come back into the fuel zone. A reflector with a high Z, like W or SS (as I mentioned) is what is used in modern cores. Some fraction of the fast neutrons that leak the periphery are bounced back at an angle greater than 180-degree and they re-enter the fuel/moderator region where they have a chance to react. AFAIK Beryllium is used in bombs and variable reflector (rotational) elements in fast space reactors – it is not used in LWR and would not reflect fast neutrons as I mentioned. It is a rare material because the only ore is aquamarine/emerald and there is not a lot of it. Beryllium metal has some niche uses due to transparency to xrays. The machine turnings are toxic to people and cause respiratory illness. The material is not used as a reflector for LWR especially cheap LWR used for district heating. High Z materials are what is used (SS) – go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. BTW, challenges are fine; if you’re rude, I’m just going to put egg on your face.
Stainless steel is used for neutron reflectors and anything with a very high Z is a good neutron reflector. Beryllium is a moderator and it also emits neutrons when bombarded increasing the multiplication of the system. Thermal neutrons in the reflector are lost to the system; they don’t come back into the fuel zone. A reflector with a high Z like W or SS (as I mentioned) is what is used in modern cores. Some fraction of the fast neutrons that leak the periphery are bounced back at an angle greater than 180-degree and they re-enter the fuel/moderator region where they have a chance to react. AFAIK Beryllium is used in bombs and variable reflector (rotational) elements in fast space reactors – it is not used in LWR and would not reflect fast neutrons as I mentioned. It is a rare material because the only ore is aquamarine/emerald and there is not a lot of it. Beryllium metal has some niche uses due to transparency to xrays. The machine turnings are toxic to people and cause respiratory illness. The material is not used as a reflector for LWR especially cheap LWR used for district heating. High Z materials are what is used (SS) – go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. BTW challenges are fine; if you’re rude I’m just going to put egg on your face.
Wow! “Superior to natural gas!” Those clever Chinese stunt scientists! This blog is even better than Sputnik News! But it appears China has never heard of geothermal heat pumping. Water in PVC piping is heated by little more than solar energy on an asphalt pavement, then superheated by a reverse HVAC compressor. The same system acts as a heat sink for cooling in the summer. So a preliminary design for a pool reactor has been completed by unspecified Chinese acting with spontaneous patriotism and wisdom. Never a mention of whether it ever will be built or marketed. Son of Zhang Zhimin anounced 3 years ago that China would take “the first bite of the apple” by building the first MSR of the 21st Century. Still no word what became of that.
Wow! Superior to natural gas!”” Those clever Chinese stunt scientists! This blog is even better than Sputnik News!But it appears China has never heard of geothermal heat pumping. Water in PVC piping is heated by little more than solar energy on an asphalt pavement”””” then superheated by a reverse HVAC compressor. The same system acts as a heat sink for cooling in the summer.So a preliminary design for a pool reactor has been completed by unspecified Chinese acting with spontaneous patriotism and wisdom. Never a mention of whether it ever will be built or marketed.Son of Zhang Zhimin anounced 3 years ago that China would take “”””the first bite of the apple”””” by building the first MSR of the 21st Century. Still no word what became of that.”””
Wow! “Superior to natural gas!” Those clever Chinese stunt scientists! This blog is even better than Sputnik News!
But it appears China has never heard of geothermal heat pumping. Water in PVC piping is heated by little more than solar energy on an asphalt pavement, then superheated by a reverse HVAC compressor. The same system acts as a heat sink for cooling in the summer.
So a preliminary design for a pool reactor has been completed by unspecified Chinese acting with spontaneous patriotism and wisdom. Never a mention of whether it ever will be built or marketed.
Son of Zhang Zhimin anounced 3 years ago that China would take “the first bite of the apple” by building the first MSR of the 21st Century. Still no word what became of that.
Correction: [fast] neutrons typically interact with material in an ELASTIC (not inelastic) manner. Words mean things. Sorry
Correction: [fast] neutrons typically interact with material in an ELASTIC (not inelastic) manner. Words mean things. Sorry
.. “Light water reactors NEVER use reflectors of graphite or beryllium regardless of what crap is written on the General Atomics website.” “What NEVER? No NEVER! What! NEVER?? Well, .. hardly ever.” (Apologies to Gilbert and Sullivan. HMS Pinafore, I think.) I don’t pretend to be an expert on reactor design, though I understand basic physics well enough. I’ll admit that I see little point for wrapping a commercial PWR in a neutron reflector, beryllium or otherwise. That’s because the core is large enough, filled with enough moderating water around the fuel elements, that the neutrons emitted in fission or decay events are pretty well thermalized before they ever reach the core wall. PWRs running on enriched uranium have rich neutron economies. They can afford to let a small fraction of their neutrons escape without triggering fissions. Small reactors are another matter. All neutrons in a reactor are born fast; they have to be slowed down to reach the low energies at which the cross section for capture by a fissile nucleus becomes large. In a particularly small reactor, a lot of neutrons will reach the reactor wall before being fully moderated. They need to be reflected back into the core, and if they can be partially moderated at the same time, so much the better! I know nothing about the design of naval reactors beyond what’s general knowledge: that they’re small PWRs that run on highly enriched uranium. But I’d be willing to bet that their cores are indeed enclosed by neutron reflectors, and that those reflectors are beryllium or grahite. BTW, didn’t GA play a major role in the design of naval reactors?
.. Light water reactors NEVER use reflectors of graphite or beryllium regardless of what crap is written on the General Atomics website.””””””What NEVER? No NEVER! What! NEVER?? Well”””” .. hardly ever.”””” (Apologies to Gilbert and Sullivan. HMS Pinafore”” I think.)I don’t pretend to be an expert on reactor design though I understand basic physics well enough. I’ll admit that I see little point for wrapping a commercial PWR in a neutron reflector beryllium or otherwise. That’s because the core is large enough filled with enough moderating water around the fuel elements that the neutrons emitted in fission or decay events are pretty well thermalized before they ever reach the core wall. PWRs running on enriched uranium have rich neutron economies. They can afford to let a small fraction of their neutrons escape without triggering fissions.Small reactors are another matter. All neutrons in a reactor are born fast; they have to be slowed down to reach the low energies at which the cross section for capture by a fissile nucleus becomes large. In a particularly small reactor a lot of neutrons will reach the reactor wall before being fully moderated. They need to be reflected back into the core and if they can be partially moderated at the same time so much the better! I know nothing about the design of naval reactors beyond what’s general knowledge: that they’re small PWRs that run on highly enriched uranium. But I’d be willing to bet that their cores are indeed enclosed by neutron reflectors and that those reflectors are beryllium or grahite.BTW”” didn’t GA play a major role in the design of naval reactors?”””
.. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on, which is a small fraction of all scattering events.” Now THERE I can say with confidence that you’re wrong. Neutrons don’t “bounce off” nuclei; the force between neutrons and nuclei is short range but strongly attractive, not repulsive. The correct analogy isn’t billiard balls, but rather interstellar comets whose trajectories pass close to the sun. It’s rare for them to be aimed so directly at the sun that they collide with it, but their trajectories are nearly always parabolic. A parabolic trajectory, translated into the quantum world of particle physics, corresponds to a 180-degree backscatter. Rutherford, in his early experiments with particle scattering, was astonished to observe direct backscatter. It defied the conventional model of atoms and electrons that had prevailed up to then. That was the “plum pudding” model of electrons embedded in a nebulous blob of positively charged atomic mass. He described his astonishment at his experimental results as akin to firing a bullet at a soft tissue stretched over a target frame, and seeing the bullet bounce straight. It’s what led him to develop the nuclear model of atomic elements.
.. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on” which is a small fraction of all scattering events.””Now THERE I can say with confidence that you’re wrong. Neutrons don’t “”””bounce off”””” nuclei; the force between neutrons and nuclei is short range but strongly attractive”” not repulsive. The correct analogy isn’t billiard balls but rather interstellar comets whose trajectories pass close to the sun. It’s rare for them to be aimed so directly at the sun that they collide with it but their trajectories are nearly always parabolic. A parabolic trajectory translated into the quantum world of particle physics corresponds to a 180-degree backscatter. Rutherford in his early experiments with particle scattering”” was astonished to observe direct backscatter. It defied the conventional model of atoms and electrons that had prevailed up to then. That was the “”””plum pudding”””” model of electrons embedded in a nebulous blob of positively charged atomic mass. He described his astonishment at his experimental results as akin to firing a bullet at a soft tissue stretched over a target frame”””” and seeing the bullet bounce straight. It’s what led him to develop the nuclear model of atomic elements.”””
No, ~40% loss is the right number. I took the trouble of calculating it. It follows from the old reliable dynamic duo, conservation of energy and conservation of momentum. More precisely, the loss is 32/81 — i.e., the backscattered neutron has 7/9 the velocity, 49/81 the energy that it had before encountering the (assumed stationary) Be nucleus. And just to be picky, the energy lost in a proton-neutron collision isn’t 50%, it’s 100% (for the “head on” case). The proton flies away from the collision with all the velocity that the neutron had going into the collision, while the neutron comes to rest. Of course, if you’re not talking about the direct head-on case but a statistical average over all scattering collision, it would be quite a different matter. But in that case it wouldn’t be 50% either; it would be much less. Oh, and just to be nasty, the scattering relationships don’t depend only on the atomic number; the atomic mass plays a large role. Sorry, I just had a semi-fight with my wife, who doesn’t fight fair. I don’t have a dog to kick, whereas you look like fair game.
No ~40{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} loss is the right number. I took the trouble of calculating it. It follows from the old reliable dynamic duo conservation of energy and conservation of momentum. More precisely the loss is 32/81 — i.e. the backscattered neutron has 7/9 the velocity 49/81 the energy that it had before encountering the (assumed stationary) Be nucleus. And just to be picky the energy lost in a proton-neutron collision isn’t 50{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} it’s 100{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} (for the head on”” case). The proton flies away from the collision with all the velocity that the neutron had going into the collision”” while the neutron comes to rest.Of course if you’re not talking about the direct head-on case but a statistical average over all scattering collision it would be quite a different matter. But in that case it wouldn’t be 50{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} either; it would be much less. Oh and just to be nasty the scattering relationships don’t depend only on the atomic number; the atomic mass plays a large role.Sorry I just had a semi-fight with my wife who doesn’t fight fair. I don’t have a dog to kick”” whereas you look like fair game.”””
Correction: [fast] neutrons typically interact with material in an ELASTIC (not inelastic) manner. Words mean things. Sorry
*nothing is fast absorber Loose talk obviously
*nothing is fast absorberLoose talk obviously
And the average energy lost for Neutron versus proton is 50% because the neutron weighs the same as a proton; average energy lost Neutron versus beryllium is probably less than an eighth of that – the relationship is simple it’s in a book – it involves only the atomic number.
And the average energy lost for Neutron versus proton is 50{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} because the neutron weighs the same as a proton; average energy lost Neutron versus beryllium is probably less than an eighth of that – the relationship is simple it’s in a book – it involves only the atomic number.
If you read down that page you gave from General Atomics, you will see that their EM2 reactor improved when the ELIMINATED the Be2C reflector and went with something of a higher atomic weight – Zr3Si2. Look at the illustration on the page. hahahaha Here is the caption from the image: “Computer simulations suggest a reflector composed of zirconium silicide can resolve power peaking issues in fast reactors and significantly improve core performance”. They are so backwards at GA, so out of practice, such a farce that they thought the beryllium was buying them something, but it was hurting them. Clowns! So, the link “high neutron reflector materials” on the General Atomics site actually confirms what I said – and we thought EM2 reactor was fast, but evidently it doesn’t have an average neutron energy above 1.8MeV, ’cause their getting no benefit out of all that Be, it is actually hurting them relative to zirconium and silicon (heavier elements). Ha.
If you read down that page you gave from General Atomics you will see that their EM2 reactor improved when the ELIMINATED the Be2C reflector and went with something of a higher atomic weight – Zr3Si2. Look at the illustration on the page. hahahaha Here is the caption from the image: Computer simulations suggest a reflector composed of zirconium silicide can resolve power peaking issues in fast reactors and significantly improve core performance””.They are so backwards at GA”” so out of practice such a farce that they thought the beryllium was buying them something but it was hurting them. Clowns! So”” the link “”””high neutron reflector materials”””” on the General Atomics site actually confirms what I said – and we thought EM2 reactor was fast”” but evidently it doesn’t have an average neutron energy above 1.8MeV ’cause their getting no benefit out of all that Be”” it is actually hurting them relative to zirconium and silicon (heavier elements). Ha.”””
Light water reactors NEVER use reflectors of graphite or beryllium regardless of what chrap is written on the General Atomics website. I gave several real sources where thick, thick steel is used as the reflector in PWRs – GA hasn’t built anything since Ft. Saint Vrain and that was a failure. Russian RBMK is a graphite moderated light water cooled reactor – it is so big it is almost infinite and whatever the “reflector” is outside the core – it is probably graphite too – doesn’t matter. Beryllium doesn’t have near 100% 180-degree backscatter; that doesn’t make sense. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on, which is a small fraction of all scattering events. Inelastic scattering emits the neutron after some delay isotropically favoring no direction over another. In short, fast neutrons elastic scatter more often then they inelastic scatter (absorb). Fast fission is kind of like spallation. At some point past carbon (sodium?) the scattering events don’t slow the neutrons much. “Reflector” in this case would keep the neutrons fast and be a high Z material. I’m holding my ground bro. Be is not “ideal for enabling small reactors to achieve criticality”; the Be is acting through a completely different mechanism – it is adding fast neutrons to a fast reactor – it becomes a SOURCE of neutrons if the reactor is fast. It does nothing better than inert steel if the neutrons are thermal. Scattering events in Be take a lot of energy from the neutron; Be moderates. Sure, some fraction of the neutrons will bounce 180 degrees, but slower – steel bounces them back with no slowing. It works better in LWR; it is used in LWR; Be is not used in LWR or anything else beside FLiBe or LPPFusion cathodes.
Light water reactors NEVER use reflectors of graphite or beryllium regardless of what chrap is written on the General Atomics website. I gave several real sources where thick thick steel is used as the reflector in PWRs – GA hasn’t built anything since Ft. Saint Vrain and that was a failure. Russian RBMK is a graphite moderated light water cooled reactor – it is so big it is almost infinite and whatever the reflector”” is outside the core – it is probably graphite too – doesn’t matter.Beryllium doesn’t have near 100{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} 180-degree backscatter; that doesn’t make sense. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on”” which is a small fraction of all scattering events. Inelastic scattering emits the neutron after some delay isotropically favoring no direction over another.In short”” fast neutrons elastic scatter more often then they inelastic scatter (absorb). Fast fission is kind of like spallation. At some point past carbon (sodium?) the scattering events don’t slow the neutrons much. “”””Reflector”””” in this case would keep the neutrons fast and be a high Z material. I’m holding my ground bro. Be is not “”””ideal for enabling small reactors to achieve criticality””””; the Be is acting through a completely different mechanism – it is adding fast neutrons to a fast reactor – it becomes a SOURCE of neutrons if the reactor is fast. It does nothing better than inert steel if the neutrons are thermal. Scattering events in Be take a lot of energy from the neutron; Be moderates. Sure”” some fraction of the neutrons will bounce 180 degrees”” but slower – steel bounces them back with no slowing. It works better in LWR; it is used in LWR; Be is not used in LWR or anything else beside FLiBe or LPPFusion cathodes.”””
.. “Light water reactors NEVER use reflectors of graphite or beryllium regardless of what crap is written on the General Atomics website.”
“What NEVER? No NEVER! What! NEVER?? Well, .. hardly ever.” (Apologies to Gilbert and Sullivan. HMS Pinafore, I think.)
I don’t pretend to be an expert on reactor design, though I understand basic physics well enough. I’ll admit that I see little point for wrapping a commercial PWR in a neutron reflector, beryllium or otherwise. That’s because the core is large enough, filled with enough moderating water around the fuel elements, that the neutrons emitted in fission or decay events are pretty well thermalized before they ever reach the core wall. PWRs running on enriched uranium have rich neutron economies. They can afford to let a small fraction of their neutrons escape without triggering fissions.
Small reactors are another matter. All neutrons in a reactor are born fast; they have to be slowed down to reach the low energies at which the cross section for capture by a fissile nucleus becomes large. In a particularly small reactor, a lot of neutrons will reach the reactor wall before being fully moderated. They need to be reflected back into the core, and if they can be partially moderated at the same time, so much the better! I know nothing about the design of naval reactors beyond what’s general knowledge: that they’re small PWRs that run on highly enriched uranium. But I’d be willing to bet that their cores are indeed enclosed by neutron reflectors, and that those reflectors are beryllium or grahite.
BTW, didn’t GA play a major role in the design of naval reactors?
..Reflectors smooth the neutron flux by scattering back neutrons that would otherwise escape the core, promoting a more consistent fuel burn and more efficient operation. Light-water reactors typically use reflectors made of graphite or beryllium.” – from General Atomics web page on “high neutron reflector materials”. A fast neutron loses about 40% of its energy in backscattering off a beryllium nucleus, so you’re right that it isn’t what you’d want as a reflector in a fast neutron reactor. But the pool type reactors that the article is about aren’t fast neutron reactors. Beryllium has near-zero neutron absorption, and near-100% 180-degree backscatter (as opposed to deflection) for neutrons with which it interacts. So it’s ideal for enabling small reactors to achieve criticality. Or so I understand.
..Reflectors smooth the neutron flux by scattering back neutrons that would otherwise escape the core” promoting a more consistent fuel burn and more efficient operation. Light-water reactors typically use reflectors made of graphite or beryllium.”” – from General Atomics web page on “”””high neutron reflector materials””””.A fast neutron loses about 40{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} of its energy in backscattering off a beryllium nucleus”” so you’re right that it isn’t what you’d want as a reflector in a fast neutron reactor. But the pool type reactors that the article is about aren’t fast neutron reactors. Beryllium has near-zero neutron absorption”” and near-100{22800fc54956079738b58e74e4dcd846757aa319aad70fcf90c97a58f3119a12} 180-degree backscatter (as opposed to deflection) for neutrons with which it interacts. So it’s ideal for enabling small reactors to achieve criticality. Or so I understand.”””
“.. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on, which is a small fraction of all scattering events.”
Now THERE I can say with confidence that you’re wrong. Neutrons don’t “bounce off” nuclei; the force between neutrons and nuclei is short range but strongly attractive, not repulsive. The correct analogy isn’t billiard balls, but rather interstellar comets whose trajectories pass close to the sun. It’s rare for them to be aimed so directly at the sun that they collide with it, but their trajectories are nearly always parabolic. A parabolic trajectory, translated into the quantum world of particle physics, corresponds to a 180-degree backscatter.
Rutherford, in his early experiments with particle scattering, was astonished to observe direct backscatter. It defied the conventional model of atoms and electrons that had prevailed up to then. That was the “plum pudding” model of electrons embedded in a nebulous blob of positively charged atomic mass. He described his astonishment at his experimental results as akin to firing a bullet at a soft tissue stretched over a target frame, and seeing the bullet bounce straight. It’s what led him to develop the nuclear model of atomic elements.
No, ~40% loss is the right number. I took the trouble of calculating it. It follows from the old reliable dynamic duo, conservation of energy and conservation of momentum. More precisely, the loss is 32/81 — i.e., the backscattered neutron has 7/9 the velocity, 49/81 the energy that it had before encountering the (assumed stationary) Be nucleus. And just to be picky, the energy lost in a proton-neutron collision isn’t 50%, it’s 100% (for the “head on” case). The proton flies away from the collision with all the velocity that the neutron had going into the collision, while the neutron comes to rest.
Of course, if you’re not talking about the direct head-on case but a statistical average over all scattering collision, it would be quite a different matter. But in that case it wouldn’t be 50% either; it would be much less.
Oh, and just to be nasty, the scattering relationships don’t depend only on the atomic number; the atomic mass plays a large role.
Sorry, I just had a semi-fight with my wife, who doesn’t fight fair. I don’t have a dog to kick, whereas you look like fair game.
I mostly agree.
I mostly agree.
Be is used in weapons because it increases the multiplication factor due to the n -> 2n reaction which occurs above 1.86MeV which means that only super-fast (i.e. unmoderated) neutrons can cause the reaction. Those would be the neutrons that didn’t hit any hydrogen in a LWR – effectively born in the last row of fuel pins because MFP in LWR is on the order of 2cm. So, the Be plays two functions in a weapon or fast reactor: external moderator (vanishingly useful for a weapon) and fast neutron source from n ->2n, which is the dominant factor because iron or uranium would be a better fast neutron reflector. In LWR a typical cut-off for “fast” as a bin or bucket term would be 1MeV – that cut-off would be for steel component embrittlement – as far as the fuel is concerned the cut off is lower (10s or 100s of eV) or anything not sub-eV. Per your definition (scattering vs. capture) I recall that the frontal area of the nucleus is the dominant parameter for fast neutron cross sections – unlike the strange resonance behavior seen at thermal energies, which is like magic by comparison. Unfortunately, many people look at the widely available thermal neutron cross section data (an endpoint reference for LWR) and think they can infer fast neutron characteristics from it. In reality, for fast neutrons, which are marbles, everything looks like a bowling ball – some bowling balls might be 6 pounds (sodium) and some bowling balls might be 15 pounds (Iron). If a marble is shot at a bowling ball the marble retains nearly all of its momentum upon scattering and the scattering angles are not isotropic or equal probability (the forward angles are naturally more probable in a high speed elastic collision) – if a golf ball hits a golf/lacross/baseball (Be/carbon/oxygen) this is less true. Now, without going through the effort of pulling a lot of references, fast neutron total cross sections (all scattering+absorption) are generally less than 5b compared to the hundreds or thousand
Be is used in weapons because it increases the multiplication factor due to the n -> 2n reaction which occurs above 1.86MeV which means that only super-fast (i.e. unmoderated) neutrons can cause the reaction. Those would be the neutrons that didn’t hit any hydrogen in a LWR – effectively born in the last row of fuel pins because MFP in LWR is on the order of 2cm. So the Be plays two functions in a weapon or fast reactor: external moderator (vanishingly useful for a weapon) and fast neutron source from n ->2n which is the dominant factor because iron or uranium would be a better fast neutron reflector.In LWR a typical cut-off for fast”” as a bin or bucket term would be 1MeV – that cut-off would be for steel component embrittlement – as far as the fuel is concerned the cut off is lower (10s or 100s of eV) or anything not sub-eV. Per your definition (scattering vs. capture) I recall that the frontal area of the nucleus is the dominant parameter for fast neutron cross sections – unlike the strange resonance behavior seen at thermal energies”” which is like magic by comparison. Unfortunately many people look at the widely available thermal neutron cross section data (an endpoint reference for LWR) and think they can infer fast neutron characteristics from it. In reality for fast neutrons which are marbles everything looks like a bowling ball – some bowling balls might be 6 pounds (sodium) and some bowling balls might be 15 pounds (Iron). If a marble is shot at a bowling ball the marble retains nearly all of its momentum upon scattering and the scattering angles are not isotropic or equal probability (the forward angles are naturally more probable in a high speed elastic collision) – if a golf ball hits a golf/lacross/baseball (Be/carbon/oxygen) this is less true. Now without going through the effort of pulling a lot of references”” fast neutron total cross sections (all scattering+absorption) are generally less than 5b compared to the hundreds or thousan”
From the US-EPR (big Framatome PWR) – for your information and education. An important design feature of the U.S. EPR is the heavy reflector, a large steel structure that replaces the thin baffle plates used in existing reactors (see Figure 3.9.5-3—Reactor Pressure Vessel Heavy Reflector). This reflector reduces fast neutron leakage and flattens the core power distribution. The reflector resides between the fuel and the core barrel and above the lower core support plate. To avoid any welded or bolted connections close to the core, the reflector consists of stacked forged slabs (rings) positioned one above the other (see slabs I-XII in Figure 3.9.5-3). Keys are used to align the slabs, and they are axially restrained by tie rods bolted to the lower core support plate. The heavy reflector is cooled by water flowing through cooling channels running axially through each slab. The heavy reflector reduces the fast flux on the pressure vessel and improves the neutron economy in the active core. With a volume ratio of approximately 95 percent metal to 5 percent water, the heavy reflector efficiently reflects fast neutrons back to the fuel. In addition, the thermal neutron flux drops off immediately outside the core because there is only a small amount of water present (in the reflector cooling holes) and 4-8 in of steel separating the core from the water outside the reflector.
From the US-EPR (big Framatome PWR) – for your information and education.An important design feature of the U.S. EPR is the heavy reflector a large steelstructure that replaces the thin baffle plates used in existing reactors (seeFigure 3.9.5-3—Reactor Pressure Vessel Heavy Reflector). This reflector reduces fastneutron leakage and flattens the core power distribution. The reflector residesbetween the fuel and the core barrel and above the lower core support plate. To avoidany welded or bolted connections close to the core the reflector consists of stackedforged slabs (rings) positioned one above the other (see slabs I-XII in Figure 3.9.5-3).Keys are used to align the slabs and they are axially restrained by tie rods bolted to the lower core support plate. The heavy reflector is cooled by water flowing throughcooling channels running axially through each slab.The heavy reflector reduces the fast flux on the pressure vessel and improves theneutron economy in the active core. With a volume ratio of approximately 95 percentmetal to 5 percent water the heavy reflector efficiently reflects fast neutrons back tothe fuel. In addition the thermal neutron flux drops off immediately outside the corebecause there is only a small amount of water present (in the reflector cooling holes)and 4-8 in of steel separating the core from the water outside the reflector.”
One final thing – in a LWR – the best choice for a reflector would be drumroll more fuel – even natural fuel – even plates of depleted uranium alloy. That is why big cores are more fuel efficient than small cores and don’t need as much excess reactivity as SMRs do. Can’t make a “reflector” out of uranium tho, because then it would be fuel – special nuclear material. There is not one LWR in the world that would not have its neutron economy enhanced by an extra ring of depleted uranium fuel assemblies (you could even use Thorium) – both are guess what – drumroll – thermal neutron absorbers. See, it’s not a straightforward as you expect. That is why I am paid for what I know.
One final thing – in a LWR – the best choice for a reflector would bedrumrollmore fuel – even natural fuel – even plates of depleted uranium alloy.That is why big cores are more fuel efficient than small cores and don’t need as much excess reactivity as SMRs do. Can’t make a reflector”” out of uranium tho”” because then it would be fuel – special nuclear material. There is not one LWR in the world that would not have its neutron economy enhanced by an extra ring of depleted uranium fuel assemblies (you could even use Thorium) – both are guess what – drumroll – thermal neutron absorbers.See”” it’s not a straightforward as you expect. That is why I am paid for what I know.”””
There was also a Manhattan Project criticality accident involving tungsten carbide reflector bricks – look it up.
There was also a Manhattan Project criticality accident involving tungsten carbide reflector bricks – look it up.
I found a figure showing you just how thick of a stainless steel reflector is these days: Figure 4.3-25 of the NuScale FSAR on page 130 nrc(dot)gov(slash)docs(slash)ML1808(slash)ML18086A172(dot)pdf Nice you gave a CYA “at least for thermal neutrons”… So, to further clarify, heavy reflectors of high Z material are used to bounce a fraction of the leaking fast neutrons back into the active region of the core. My own set of calculations performed for the B&W mPower reactor back in 2010 showed that 9 inches of steel was better than 6 inches of steel was better than 3 inches of steel. We went with about 8 inches. The worth of the reflector was several dollars positive vs. a boundary condition of H2O. Reflectors in thermal reactors should be high Z and not moderating.
I found a figure showing you just how thick of a stainless steel reflector is these days: Figure 4.3-25 of the NuScale FSAR on page 130nrc(dot)gov(slash)docs(slash)ML1808(slash)ML18086A172(dot)pdfNice you gave a CYA at least for thermal neutrons””…So”” to further clarify”” heavy reflectors of high Z material are used to bounce a fraction of the leaking fast neutrons back into the active region of the core. My own set of calculations performed for the B&W mPower reactor back in 2010 showed that 9 inches of steel was better than 6 inches of steel was better than 3 inches of steel. We went with about 8 inches. The worth of the reflector was several dollars positive vs. a boundary condition of H2O. Reflectors in thermal reactors should be high Z and not moderating.”””
Nothing is a fast neutron absorber, and that is what we are trying to “reflect”. Thermal neutrons in the reflector region are effectively lost to the system. Go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. Stainless steel is used. Sure, W and SS are thermal neutron absorbers.
Nothing is a fast neutron absorber and that is what we are trying to reflect””. Thermal neutrons in the reflector region are effectively lost to the system. Go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. Stainless steel is used. Sure”””” W and SS are thermal neutron absorbers.”””
Stainless steel is used for neutron reflectors and anything with a very high Z is a good neutron reflector. Beryllium is a moderator and it also emits neutrons when bombarded increasing the multiplication of the system. Thermal neutrons in the reflector are lost to the system; they don’t come back into the fuel zone. A reflector with a high Z, like W or SS (as I mentioned) is what is used in modern cores. Some fraction of the fast neutrons that leak the periphery are bounced back at an angle greater than 180-degree and they re-enter the fuel/moderator region where they have a chance to react. AFAIK Beryllium is used in bombs and variable reflector (rotational) elements in fast space reactors – it is not used in LWR and would not reflect fast neutrons as I mentioned. It is a rare material because the only ore is aquamarine/emerald and there is not a lot of it. Beryllium metal has some niche uses due to transparency to xrays. The machine turnings are toxic to people and cause respiratory illness. The material is not used as a reflector for LWR especially cheap LWR used for district heating. High Z materials are what is used (SS) – go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. BTW, challenges are fine; if you’re rude, I’m just going to put egg on your face.
Stainless steel is used for neutron reflectors and anything with a very high Z is a good neutron reflector. Beryllium is a moderator and it also emits neutrons when bombarded increasing the multiplication of the system. Thermal neutrons in the reflector are lost to the system; they don’t come back into the fuel zone. A reflector with a high Z like W or SS (as I mentioned) is what is used in modern cores. Some fraction of the fast neutrons that leak the periphery are bounced back at an angle greater than 180-degree and they re-enter the fuel/moderator region where they have a chance to react. AFAIK Beryllium is used in bombs and variable reflector (rotational) elements in fast space reactors – it is not used in LWR and would not reflect fast neutrons as I mentioned. It is a rare material because the only ore is aquamarine/emerald and there is not a lot of it. Beryllium metal has some niche uses due to transparency to xrays. The machine turnings are toxic to people and cause respiratory illness. The material is not used as a reflector for LWR especially cheap LWR used for district heating. High Z materials are what is used (SS) – go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. BTW challenges are fine; if you’re rude I’m just going to put egg on your face.
*nothing is fast absorber
Loose talk obviously
And the average energy lost for Neutron versus proton is 50% because the neutron weighs the same as a proton; average energy lost Neutron versus beryllium is probably less than an eighth of that – the relationship is simple it’s in a book – it involves only the atomic number.
If you read down that page you gave from General Atomics, you will see that their EM2 reactor improved when the ELIMINATED the Be2C reflector and went with something of a higher atomic weight – Zr3Si2. Look at the illustration on the page. hahahaha Here is the caption from the image: “Computer simulations suggest a reflector composed of zirconium silicide can resolve power peaking issues in fast reactors and significantly improve core performance”.
They are so backwards at GA, so out of practice, such a farce that they thought the beryllium was buying them something, but it was hurting them. Clowns!
So, the link “high neutron reflector materials” on the General Atomics site actually confirms what I said – and we thought EM2 reactor was fast, but evidently it doesn’t have an average neutron energy above 1.8MeV, ’cause their getting no benefit out of all that Be, it is actually hurting them relative to zirconium and silicon (heavier elements). Ha.
Light water reactors NEVER use reflectors of graphite or beryllium regardless of what chrap is written on the General Atomics website. I gave several real sources where thick, thick steel is used as the reflector in PWRs – GA hasn’t built anything since Ft. Saint Vrain and that was a failure. Russian RBMK is a graphite moderated light water cooled reactor – it is so big it is almost infinite and whatever the “reflector” is outside the core – it is probably graphite too – doesn’t matter.
Beryllium doesn’t have near 100% 180-degree backscatter; that doesn’t make sense. Think of billiard balls – the scattering angle has to do with how square the individual impact is (how centered). The only way you get 180-degree backscatter is dead-nuts straight on, which is a small fraction of all scattering events. Inelastic scattering emits the neutron after some delay isotropically favoring no direction over another.
In short, fast neutrons elastic scatter more often then they inelastic scatter (absorb). Fast fission is kind of like spallation. At some point past carbon (sodium?) the scattering events don’t slow the neutrons much. “Reflector” in this case would keep the neutrons fast and be a high Z material. I’m holding my ground bro. Be is not “ideal for enabling small reactors to achieve criticality”; the Be is acting through a completely different mechanism – it is adding fast neutrons to a fast reactor – it becomes a SOURCE of neutrons if the reactor is fast. It does nothing better than inert steel if the neutrons are thermal. Scattering events in Be take a lot of energy from the neutron; Be moderates. Sure, some fraction of the neutrons will bounce 180 degrees, but slower – steel bounces them back with no slowing. It works better in LWR; it is used in LWR; Be is not used in LWR or anything else beside FLiBe or LPPFusion cathodes.
“..Reflectors smooth the neutron flux by scattering back neutrons that would otherwise escape the core, promoting a more consistent fuel burn and more efficient operation. Light-water reactors typically use reflectors made of graphite or beryllium.” – from General Atomics web page on “high neutron reflector materials”.
A fast neutron loses about 40% of its energy in backscattering off a beryllium nucleus, so you’re right that it isn’t what you’d want as a reflector in a fast neutron reactor. But the pool type reactors that the article is about aren’t fast neutron reactors.
Beryllium has near-zero neutron absorption, and near-100% 180-degree backscatter (as opposed to deflection) for neutrons with which it interacts. So it’s ideal for enabling small reactors to achieve criticality. Or so I understand.
I mostly agree.
Be is used in weapons because it increases the multiplication factor due to the n -> 2n reaction which occurs above 1.86MeV which means that only super-fast (i.e. unmoderated) neutrons can cause the reaction. Those would be the neutrons that didn’t hit any hydrogen in a LWR – effectively born in the last row of fuel pins because MFP in LWR is on the order of 2cm. So, the Be plays two functions in a weapon or fast reactor: external moderator (vanishingly useful for a weapon) and fast neutron source from n ->2n, which is the dominant factor because iron or uranium would be a better fast neutron reflector.
In LWR a typical cut-off for “fast” as a bin or bucket term would be 1MeV – that cut-off would be for steel component embrittlement – as far as the fuel is concerned the cut off is lower (10s or 100s of eV) or anything not sub-eV.
Per your definition (scattering vs. capture) I recall that the frontal area of the nucleus is the dominant parameter for fast neutron cross sections – unlike the strange resonance behavior seen at thermal energies, which is like magic by comparison. Unfortunately, many people look at the widely available thermal neutron cross section data (an endpoint reference for LWR) and think they can infer fast neutron characteristics from it. In reality, for fast neutrons, which are marbles, everything looks like a bowling ball – some bowling balls might be 6 pounds (sodium) and some bowling balls might be 15 pounds (Iron). If a marble is shot at a bowling ball the marble retains nearly all of its momentum upon scattering and the scattering angles are not isotropic or equal probability (the forward angles are naturally more probable in a high speed elastic collision) – if a golf ball hits a golf/lacross/baseball (Be/carbon/oxygen) this is less true. Now, without going through the effort of pulling a lot of references, fast neutron total cross sections (all scattering+absorption) are generally less than 5b compared to the hundreds or thousands of barns for important isotopes at thermal energies.
Everything looks the same to fast neutrons – unless the material contains a neutron source (i.e. fissions or gives neutron reactions). Everything looks the same with a cross section of 1-2b and very little energy lost per collision except for elements carbon and below – the energy lost per collision is a relatively straightforward function of ‘A’.
From the US-EPR (big Framatome PWR) – for your information and education.
An important design feature of the U.S. EPR is the heavy reflector, a large steel
structure that replaces the thin baffle plates used in existing reactors (see
Figure 3.9.5-3—Reactor Pressure Vessel Heavy Reflector). This reflector reduces fast
neutron leakage and flattens the core power distribution. The reflector resides
between the fuel and the core barrel and above the lower core support plate. To avoid
any welded or bolted connections close to the core, the reflector consists of stacked
forged slabs (rings) positioned one above the other (see slabs I-XII in Figure 3.9.5-3).
Keys are used to align the slabs, and they are axially restrained by tie rods bolted to the lower core support plate. The heavy reflector is cooled by water flowing through
cooling channels running axially through each slab.
The heavy reflector reduces the fast flux on the pressure vessel and improves the
neutron economy in the active core. With a volume ratio of approximately 95 percent
metal to 5 percent water, the heavy reflector efficiently reflects fast neutrons back to
the fuel. In addition, the thermal neutron flux drops off immediately outside the core
because there is only a small amount of water present (in the reflector cooling holes)
and 4-8 in of steel separating the core from the water outside the reflector.
One final thing – in a LWR – the best choice for a reflector would be
drumroll
more fuel – even natural fuel – even plates of depleted uranium alloy.
That is why big cores are more fuel efficient than small cores and don’t need as much excess reactivity as SMRs do. Can’t make a “reflector” out of uranium tho, because then it would be fuel – special nuclear material. There is not one LWR in the world that would not have its neutron economy enhanced by an extra ring of depleted uranium fuel assemblies (you could even use Thorium) – both are guess what – drumroll – thermal neutron absorbers.
See, it’s not a straightforward as you expect. That is why I am paid for what I know.
There was also a Manhattan Project criticality accident involving tungsten carbide reflector bricks – look it up.
I found a figure showing you just how thick of a stainless steel reflector is these days:
Figure 4.3-25 of the NuScale FSAR on page 130
nrc(dot)gov(slash)docs(slash)ML1808(slash)ML18086A172(dot)pdf
Nice you gave a CYA “at least for thermal neutrons”…
So, to further clarify, heavy reflectors of high Z material are used to bounce a fraction of the leaking fast neutrons back into the active region of the core. My own set of calculations performed for the B&W mPower reactor back in 2010 showed that 9 inches of steel was better than 6 inches of steel was better than 3 inches of steel. We went with about 8 inches. The worth of the reflector was several dollars positive vs. a boundary condition of H2O.
Reflectors in thermal reactors should be high Z and not moderating.
Nothing is a fast neutron absorber, and that is what we are trying to “reflect”. Thermal neutrons in the reflector region are effectively lost to the system. Go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. Stainless steel is used. Sure, W and SS are thermal neutron absorbers.
Stainless steel is used for neutron reflectors and anything with a very high Z is a good neutron reflector. Beryllium is a moderator and it also emits neutrons when bombarded increasing the multiplication of the system. Thermal neutrons in the reflector are lost to the system; they don’t come back into the fuel zone. A reflector with a high Z, like W or SS (as I mentioned) is what is used in modern cores. Some fraction of the fast neutrons that leak the periphery are bounced back at an angle greater than 180-degree and they re-enter the fuel/moderator region where they have a chance to react. AFAIK Beryllium is used in bombs and variable reflector (rotational) elements in fast space reactors – it is not used in LWR and would not reflect fast neutrons as I mentioned. It is a rare material because the only ore is aquamarine/emerald and there is not a lot of it. Beryllium metal has some niche uses due to transparency to xrays. The machine turnings are toxic to people and cause respiratory illness. The material is not used as a reflector for LWR especially cheap LWR used for district heating. High Z materials are what is used (SS) – go look at chapter four of the AP1000 or the NuScale Design Certification documents on the NRC website. BTW, challenges are fine; if you’re rude, I’m just going to put egg on your face.
Maybe throw in a thermal battery.
Maybe throw in a thermal battery.
Maybe throw in a thermal battery.
Maybe throw in a thermal battery.
Do you actually know what a neutron reflector is? It’s not intended for shielding, and tungsten (W) would be a terrible reflector. The property that determines whether an element is a good neutron reflector is a high ratio between the cross sections for scattering vs. capture. No other element, AFAIK, matches beryllium — at least for thermal neutrons. It’s always been the standard material for that use, and isn’t particularly rare.
Do you actually know what a neutron reflector is? It’s not intended for shielding and tungsten (W) would be a terrible reflector. The property that determines whether an element is a good neutron reflector is a high ratio between the cross sections for scattering vs. capture. No other element AFAIK matches beryllium — at least for thermal neutrons. It’s always been the standard material for that use and isn’t particularly rare.
Do you actually know what a neutron reflector is? It’s not intended for shielding, and tungsten (W) would be a terrible reflector. The property that determines whether an element is a good neutron reflector is a high ratio between the cross sections for scattering vs. capture. No other element, AFAIK, matches beryllium — at least for thermal neutrons. It’s always been the standard material for that use, and isn’t particularly rare.
Do you actually know what a neutron reflector is? It’s not intended for shielding and tungsten (W) would be a terrible reflector. The property that determines whether an element is a good neutron reflector is a high ratio between the cross sections for scattering vs. capture. No other element AFAIK matches beryllium — at least for thermal neutrons. It’s always been the standard material for that use and isn’t particularly rare.
Maybe throw in a thermal battery.
Would they be better off just building a Gen III+ or Gen IV reactor and getting electricity and heat at the same time for not much more cost? I suppose the big advantage is that swimming pool reactors could be built in the short term with no need for years of prototype testing?
Would they be better off just building a Gen III+ or Gen IV reactor and getting electricity and heat at the same time for not much more cost?I suppose the big advantage is that swimming pool reactors could be built in the short term with no need for years of prototype testing?
Would they be better off just building a Gen III+ or Gen IV reactor and getting electricity and heat at the same time for not much more cost? I suppose the big advantage is that swimming pool reactors could be built in the short term with no need for years of prototype testing?
Would they be better off just building a Gen III+ or Gen IV reactor and getting electricity and heat at the same time for not much more cost?I suppose the big advantage is that swimming pool reactors could be built in the short term with no need for years of prototype testing?
Solar panel in the northern China winter? What do you do on an overcast day? Freeze?
Solar panel in the northern China winter? What do you do on an overcast day? Freeze?
Solar panel in the northern China winter? What do you do on an overcast day? Freeze?
Solar panel in the northern China winter? What do you do on an overcast day? Freeze?
Do you actually know what a neutron reflector is? It’s not intended for shielding, and tungsten (W) would be a terrible reflector.
The property that determines whether an element is a good neutron reflector is a high ratio between the cross sections for scattering vs. capture. No other element, AFAIK, matches beryllium — at least for thermal neutrons. It’s always been the standard material for that use, and isn’t particularly rare.
Pool-type nuclear reactor design for heating Chinese cities with lower cost than Chinese gas and comparable price to Chinese coal” Fixed.
Pool-type nuclear reactor design for heating Chinese cities with lower cost than Chinese gas and comparable price to Chinese coal””Fixed.”””
Would they be better off just building a Gen III+ or Gen IV reactor and getting electricity and heat at the same time for not much more cost?
I suppose the big advantage is that swimming pool reactors could be built in the short term with no need for years of prototype testing?
Solar panel in the northern China winter? What do you do on an overcast day? Freeze?
A well insulated house with a solar panel and a heat pump heater/cooler can achieve the same result with a lot less effort.
A well insulated house with a solar panel and a heat pump heater/cooler can achieve the same result with a lot less effort.
It is a lot of trouble for heat. No doubt
It is a lot of trouble for heat. No doubt
“Pool-type nuclear reactor design for heating Chinese cities with lower cost than Chinese gas and comparable price to Chinese coal”
Fixed.
A well insulated house with a solar panel and a heat pump heater/cooler can achieve the same result with a lot less effort.
The water both moderates and cools the reactor, and graphite or beryllium is generally used for the reflector, although other materials may also be used.” Statements like that (mentioning beryllium) make me wonder if it was mistranslated. It is in a pool of water for gawdsakes. Nobody uses Be for anything – it’s toxic, rare, etc.. Be actually becomes a neutron source when used. Best ‘reflector’ option for a small leaky core would be something like W in theory or Fe/Ni/Cr in practice (i.e. SS INOX). At 6″ thickness, this will bounce quite a few fast leak neutrons back into the fuel. That is even used on AP1000 nowadays.
The water both moderates and cools the reactor and graphite or beryllium is generally used for the reflector” although other materials may also be used.””Statements like that (mentioning beryllium) make me wonder if it was mistranslated. It is in a pool of water for gawdsakes. Nobody uses Be for anything – it’s toxic”” rare”” etc.. Be actually becomes a neutron source when used. Best ‘reflector’ option for a small leaky core would be something like W in theory or Fe/Ni/Cr in practice (i.e. SS INOX). At 6″””” thickness”””” this will bounce quite a few fast leak neutrons back into the fuel. That is even used on AP1000 nowadays.”””
It is a lot of trouble for heat. No doubt
“The water both moderates and cools the reactor, and graphite or beryllium is generally used for the reflector, although other materials may also be used.”
Statements like that (mentioning beryllium) make me wonder if it was mistranslated. It is in a pool of water for gawdsakes. Nobody uses Be for anything – it’s toxic, rare, etc.. Be actually becomes a neutron source when used. Best ‘reflector’ option for a small leaky core would be something like W in theory or Fe/Ni/Cr in practice (i.e. SS INOX). At 6″ thickness, this will bounce quite a few fast leak neutrons back into the fuel. That is even used on AP1000 nowadays.