Sorensen Rebuttal of IEER/PSR Thorium Paper

There is a “fact sheet” about thorium issued by the Institute for Energy and Environmental Research (IEER) and Physicians for Social Responsibility (PSR) called “Thorium Fuel: No Panacea for Nuclear Power.” The authors of this sheet were Arjun Makhijani and Michele Boyd. A letter was sent to them in 2010 to correct errors, but no corrections were made

Thorium “fuel” has been proposed as an alternative to uranium fuel in nuclear reactors. There are not “thorium reactors,” but rather proposals to use thorium as a “fuel” in different types of reactors, including existing light-water reactors and various fast breeder reactor designs.

It would seem that Mr. Makhijani and Ms. Boyd are unaware of the work done at Oak Ridge National Laboratory under Dr. Alvin Weinberg from 1955 to 1974 on the subject of fluid-fueled reactors, particularly those that used liquid-fluoride salts as a medium in which to sustain nuclear reactions. The liquid-fluoride reactor was the most promising of these fluid-fueled designs, and indeed it did have the capability to use thorium as fuel. It was not a light-water reactor, nor was it a fast-breeder reactor. It has a thermal (slowed-down) neutron spectrum which made it easier to control and vastly improved the amount of fissile fuel it needed to start. It operated at atmospheric pressure rather than the high pressure of water-cooled reactors. It was also singularly suited to the use of thorium due to the nature of its chemistry and the chemistry of thorium and uranium.

Thorium, which refers to thorium-232, is a radioactive metal that is about three times more abundant than uranium in the natural environment. Large known deposits are in Australia, India, and Norway. Some of the largest reserves are found in Idaho in the U.S. The primary U.S. company advocating for thorium fuel is Thorium Power ( Contrary to the claims made or implied by thorium proponents, however, thorium doesn’t solve the proliferation, waste, safety, or cost problems of nuclear power, and it still faces major technical hurdles for commercialization.

Mr. Makhijani and Ms. Boyd may wish to update their document since “Thorium Power” is now called “Lightbridge” and no longer advocates for the use of thorium, whereas the community of supporters of liquid-fluoride thorium reactors (LFTR) still maintains strong support for the use of thorium because it is indeed a solution to the issues of proliferation, waste, safety, and cost that accompany the present use of solid-fueled, water-cooled reactors.

Thorium is not actually a “fuel” because it is not fissile and therefore cannot be used to start or sustain a nuclear chain reaction. A fissile material, such as uranium-235 (U-235) or plutonium-239 (which is made in reactors from uranium-238), is required to kick-start the reaction. The enriched uranium fuel or plutonium fuel also maintains the chain reaction until enough of the thorium target material has been converted into fissile uranium-233 (U-233) to take over much or most of the job. An advantage of thorium is that it absorbs slow neutrons relatively efficiently (compared to uranium-238) to produce fissile uranium-233.

On the contrary, thorium is very much a fuel because in the steady-state operation of a LFTR, it is the only thing that is consumed to make energy. Makhijani and Boyd are correct that any nuclear reactor needs fissile material to start the chain reaction, and the LFTR is no different, but the important point is that once started on fissile material, LFTR can run indefinitely on only thorium as a feed—it will not continue to consume fissile material. That is very much the characteristic of a true fuel. “Burning thorium” in this manner is possible because the LFTR uses the neutrons from the fissioning of uranium-233 to convert thorium into uranium-233 at the same rate at which it is consumed. The “inventory” of uranium-233 remains stable over the life of the reactor when production and consumption are balanced. Today’s reactors use solid-uranium oxide fuel that is covalently-bonded and sustains radiation damage during its time in the reactor. The fluoride fuel used in LFTR is ionically-bonded and impervious to radiation damage no matter what the exposure duration. LFTR can be used to consume uranium-235 or plutonium-239 recovered from nuclear weapons and “convert” it, for all intents and purposes, to uranium-233 that will enable the production of energy from thorium indefinitely. Truly this is a reactor design that can “beat swords into plowshares” in a safe and economically attractive way.

The use of enriched uranium or plutonium in thorium fuel has proliferation implications. Although U-235 is found in nature, it is only 0.7 percent of natural uranium, so the proportion of U-235 must be industrially increased to make “enriched uranium” for use in reactors. Highly enriched uranium and separated plutonium are nuclear weapons materials.

Since so many nuclear weapons have already been built and are being decommissioned, one might assume that Makhijani and Boyd would welcome a technology like LFTR that could safely consume these sensitive materials in an economically-advantageous way, beating swords into plowshares and using material that was once fashioned as a weapon as a material that can provide light and energy to billions. Enriched uranium or plutonium can’t simply be “thrown away”. LFTR puts these materials to productive use as they are destroyed in the reactor and uranium-233 is generated.

In addition, U-233 is as effective as plutonium-239 for making nuclear bombs. In most proposed thorium fuel cycles, reprocessing is required to separate out the U-233 for use in fresh fuel. This means that, like uranium fuel with reprocessing, bomb-making material is separated out, making it vulnerable to theft or diversion. Some proposed thorium fuel cycles even require 20% enriched uranium in order to get the chain reaction started in existing reactors using thorium fuel. It takes 90% enrichment to make weapons-usable uranium, but very little additional work is needed to move from 20% enrichment to 90% enrichment. Most of the separative work is needed to go from natural uranium, which has 0.7% uranium-235 to 20% U-235.

In a fluoride reactor, all of the fuel processing equipment will be located in a containment region containing the reactor and its primary heat exchangers, under very high radiation fields, and under the high heat needed to keep the fuel liquid. Once the system is properly designed to direct uranium-233 created in the outer regions of the reactor (the “blanket”) to the central regions of the reactor (the “core”) there will be no possibility of redirection of the material flow. Such a redirection would necessitate a rebuild of the entire reactor and would be vastly beyond the capabilities of the operators. Furthermore, the nature of U-233 removal and transfer from blanket to core involves the operation of an electrolytic cell that will allow very precise control and accountability of the material in question. Unlike solid-fueled reactors the uranium-233 never needs to leave the secure area of the containment building or come in contact with humans in order to continue the operation of the reactor. This is another important point that the authors have failed to distinguish as they have ignored the existence or implications of fluid-fueled thorium reactors.

To claim that uranium-233 is just as effective as plutonium-239 for nuclear weapons is gross simplification bordering on outright deception. They have similar values for critical mass, but this leaves out a very important point. The nuclear reactions that consume uranium-233 also produce small amounts of uranium-232, a contaminant that will later be mentioned by the authors but ignored at this stage of the criticism. U-232 has a decay sequence that includes the hard gamma-ray-emitting radioisotopes bismuth-212 and thallium-208. Indeed, the half-life of U-232 is short enough that this decay chain begins to set up within days of the purification of the uranium, and within a few months that gamma-ray flux from the material is intense. These gamma rays destroy the electronics of a nuclear weapon, compromise the chemical explosives, and clearly signal to detection systems where the fissile material is located. This is one of the key reasons why no operational nuclear weapons have ever been built using uranium-233 as the fissile material.

It has been claimed that thorium fuel cycles with reprocessing would be much less of a proliferation risk because the thorium can be mixed with uranium-238. In this case, fissile uranium-233 is also mixed with non-fissile uranium-238. The claim is that if the uranium-238 content is high enough, the mixture cannot be used to make bombs without a complex uranium enrichment plant. This is misleading. More uranium-238 does dilute the uranium-233, but it also results in the production of more plutonium-239 as the reactor operates. So the proliferation problem remains either bomb-usable uranium-233 or bomb-usable plutonium is created and can be separated out by reprocessing.

In my opinion, mixing uranium-238 with uranium-233 during the normal operation of a LFTR is a bad idea because it compromises the capability of the reactor to “burn” thorium to a degree that it then becomes necessary to add fissile material to keep the reactor running. This is because uranium-238 will absorb many of the neutrons that would otherwise convert thorium into uranium-233, instead converting uranium-238 into plutonium-239. Plutonium-239 is a poor fuel in a LFTR due to the limited solubility of plutonium trifluoride (PuF3) and the poor performance of plutonium in a thermal-neutron spectrum (only 2/3 of the plutonium-239 will fission when struck by a neutron).

But something is possible in the fluid fuel of a LFTR that is impossible in the solid fuel of a conventional reactor with regards to the “downblending” of uranium. Under extreme scenarios, it may be desireable to have a separate supply of uranium-238 inside the reactor containment that could be irreversibly mixed with the uranium-233 in the core. This would have the effect of making the reactor unable to restart, and despite the contention of Makhajani and Boyd, there is no feasible way to isotopically separate uranium-233 (contaminated with uranium-232) from uranium-238 because of the severe gamma radiation that would be emitted during any attempt to separate the isotopes. This approach to “just-in-time” downblending is only possible with fluid fuel, and its absence of consideration in the document again shows that the authors are unaware of the fluid fuel option and its implications.

Further, while an enrichment plant is needed to separate U-233 from U-238, it would take less separative work to do so than enriching natural uranium. This is because U-233 is five atomic weight units lighter than U-238, compared to only three for U-235. It is true that such enrichment would not be a straightforward matter because the U-233 is contaminated with U-232, which is highly radioactive and has very radioactive radionuclides in its decay chain. The radiation-dose-related problems associated with separating U-233 from U-238 and then handling the U-233 would be considerable and more complex than enriching natural uranium for the purpose of bomb making. But in principle, the separation can be done, especially if worker safety is not a primary concern; the resulting U-233 can be used to make bombs. There is just no way to avoid proliferation problems associated with thorium fuel cycles that involve reprocessing. Thorium fuel cycles without reprocessing would offer the same temptation to reprocess as today’s once-through uranium fuel cycles.

Makhijani and Boyd really betray a fundamental lack of understanding of the nature of uranium isotope separation facilities with their simplistic and cursory description of U-233 separation from U-238. Such a process would be so difficult due to the presence of U-232 that it simply would not be considered, even by the hypothetical “suicide” operators that they postulate. Anyone who had invested the large sums of money into a uranium isotope separation system would never risk permanently crippling its ability to operate by introducing U-232-contaminated feed into the system.

Proponents claim that thorium fuel significantly reduces the volume, weight and long-term radiotoxicity of spent fuel. Using thorium in a nuclear reactor creates radioactive waste that proponents claim would only have to be isolated from the environment for 500 years, as opposed to the irradiated uranium-only fuel that remains dangerous for hundreds of thousands of years. This claim is wrong. The fission of thorium creates long-lived fission products like technetium-99 (half-life over 200,000 years). While the mix of fission products is somewhat different than with uranium fuel, the same range of fission products is created. With or without reprocessing, these fission products have to be disposed of in a geologic repository.

Again, the authors make blanket statements about “thorium” but then confine their examination to some variant of solid thorium fuel in a conventional reactor. In a LFTR, thorium can be consumed with exceptionally high efficiency, approaching completeness. Unburned thorium and valuable uranium-233 is simply recycled to the next generation of fluoride reactor when a reactor is decommissioned. The fuel is not damaged by radiation. Thus thorium and uranium-233 would not enter a waste stream during the use of a LFTR.

All fission produces a similar set of fission products, each with roughly half the mass of the original fissile material. Most have very short half-lives, and are highly radioactive and highly dangerous. A very few have very long half-lives, very little radioactivity, and little concern. A simple but underappreciated truth is that the longer the half-life of a material, the less radioactive and the less dangerous it is. Technetium-99 (Tc-99) has a half-life of 100,000 years and indeed is a product of the fission of uranium-233, just as it is a product of the fission of uranium-235 or plutonium-239. Its immediate precursor, technetium-99m (Tc-99m), has a half-life of six hours and so is approximately 150 million times more radioactive than Tc-99.

Nevertheless, it might come as a surprise to the casual reader that hundreds of thousands of people intentionally ingest Tc-99m every year as part of medical imaging procedures because it produces gamma rays that allow radiography technicians to image internal regions of the body and diagnose concerns. The use of Tc-99m thus allows physicians to forego thousands of exploratory and invasive surgeries that would otherwise risk patient health. The Tc-99m decays over the period of a few days to Tc-99, with its 100,000 half-life, extremely low levels of radiation, and low risk.

What is the ultimate fate of the Tc-99? It is excreted from the body through urination and ends up in the municipal water supply. If the medical community and radiological professionals intentionally cause patients to ingest a form of technetium that is 150 million times more radioactive than Tc-99, with the intent that its gamma rays be emitted within the body, and then sees no risk from the excretion of Tc-99 into our water supply, where is the concern? It is yet another example of fear, uncertainty, and doubt that Makhijani and Boyd would raise this issue as if it represented some sort of condemnation of the use of thorium for nuclear power.

If the spent fuel is not reprocessed, thorium-232 is very-long lived (half-life:14 billion years) and its decay products will build up over time in the spent fuel. This will make the spent fuel quite radiotoxic, in addition to all the fission products in it. It should also be noted that inhalation of a unit of radioactivity of thorium-232 or thorium-228 (which is also present as a decay product of thorium-232) produces a far higher dose, especially to certain organs, than the inhalation of uranium containing the same amount of radioactivity. For instance, the bone surface dose from breathing an amount (mass) of insoluble thorium is about 200 times that of breathing the same mass of uranium.

Statements like this really cause me to wonder if Makhijani and Boyd understand the nature of radioactivity. Yes, thorium-232 has a 14-billion-year half-life, which means that the radioactivity of thorium is exceptionally low. It will rise as the decay chain of Th-232 begins to form, but it is still at a very low level. To be concerned with the radioactivity of thorium in spent fuel, while neglecting to mention the five billion kilograms of thorium contained in each meter of the Earth’s continental crust again appears to be another example of fear, uncertainty, and doubt levied unfairly against the use of thorium. The buildup of thorium-228 as part of the decay of thorium will happen on a scale within the Earth’s crust so titanically in excess of any activity on the part of man so as to render that point utterly immaterial to any discussion of thorium as a nuclear fuel.

Since both thorium and uranium are natural and common constituents of the Earth’s crust, discussing a bone surface dose obtained by breathing insoluble thorium—a very strange exposure pathway—and contrasting it with uranium is again utterly immaterial to the use of thorium as a nuclear fuel. Do Makhijani and Boyd mean to say that it would be preferable to be breathing uranium instead? The criticism seems to have no structure.

Furthermore, LFTR will not reject thorium to a waste stream nor generate “spent fuel” in the conventional sense. Thorium remains in the reactor until consumed for energy. At shutdown, unconsumed thorium is transferred to the next generation of reactor.

Finally, the use of thorium also creates waste at the front end of the fuel cycle. The radioactivity associated with these is expected to be considerably less than that associated with a comparable amount of uranium milling. However, mine wastes will pose long-term hazards, as in the case of uranium mining. There are also often hazardous non-radioactive metals in both thorium and uranium mill tailings.

Thorium is found with rare-earth mineral deposits, and global demand for rare-earth mining will inevitably bring up thorium deposits. At the present time, we in the US have the strange policy of considering this natural material as a “radioactive waste” that must be disposed at considerable cost. Other countries like China have taken a longer view on the issue and simply stockpile the thorium that they recover during rare-earth mining for future use in thorium reactors. In addition, the United States has an already-mined supply of 3200 metric tonnes of thorium in Nevada that will meet energy needs for many decades. The issues surrounding thorium mining are immaterial to its discussion as a nuclear energy source because thorium will be mined under any circumstance, but if we use it as a nuclear fuel we can save time and effort by avoiding the expense of trying to throw it away.

Research and development of thorium fuel has been undertaken in Germany, India, Japan, Russia, the UK and the U.S. for more than half a century. Besides remote fuel fabrication and issues at the front end of the fuel cycle, thorium-U-233 breeder reactors produce fuel (“breed”) much more slowly than uranium-plutonium-239 breeders. This leads to technical complications. India is sometimes cited as the country that has successfully developed thorium fuel. In fact, India has been trying to develop a thorium breeder fuel cycle for decades but has not yet done so commercially.

Thorium/U233 reactors like LFTR produce sufficient U-233 to make up for U-233 consumed in the fission process. This may be what the authors meant by “breeding more slowly”, but since they consider plutonium a dangerous substance and eschew the use of nuclear power, it is a wonder why they would consider a reactor that does not produce plutonium as having some sort of deficiency. They neglect to elaborate on what sort of “technical complications” this very attractive feature would entail.

The thorium effort in India has been centered around the use of thorium in solid-oxide form, and has suffered from the deficiencies of using this approach, which are transcended through the use of thorium in liquid fluoride form. This is further evidence that the authors are unaware of the implications of the liquid-fluoride thorium reactor.

One reason reprocessing thorium fuel cycles haven’t been successful is that uranium-232 (U 232) is created along with uranium-233. U-232, which has a half-life of about 70 years, is extremely radioactive and is therefore very dangerous in small quantities: a single small particle in a lung would exceed legal radiation standards for the general public. U-232 also has highly radioactive decay products. Therefore, fabricating fuel with U-233 is very expensive and difficult.

Previously I mentioned the implications of the presence of uranium-232 contamination within uranium-233 and its anti-proliferative nature with regards to nuclear weapons. U-232 contamination also makes fabrication of solid thorium-oxide fuel containing uranium-233-oxide very difficult. In the liquid-fluoride reactor, fuel fabrication is unnecessary and this difficulty is completely averted.

Thorium may be abundant and possess certain technical advantages, but it does not mean that it is economical. Compared to uranium, thorium fuel cycle is likely to be even more costly. In a once-through mode, it will need both uranium enrichment (or plutonium separation) and thorium target rod production. In a breeder configuration, it will need reprocessing, which is costly. In addition, as noted, inhalation of thorium-232 produces a higher dose than the same amount of uranium-238 (either by radioactivity or by weight). Reprocessed thorium creates even more risks due to the highly radioactive U-232 created in the reactor. This makes worker protection more difficult and expensive for a given level of annual dose.

The liquid-fluoride thorium reactor has an exceptionally simple and self-contained fuel cycle that has every promise of being less-expensive than today’s wasteful and complicated “once-through” approach to uranium fuel utilization. Makhijani and Boyd try to assign thorium to the wasteful “once-through” fuel cycle, point out deficiencies, and then condemn thorium as having no promise. This might analogous to putting diesel fuel in a gasoline-powered car and then pointing out how deficient diesel fuel is when the car will no longer operate. It is disingenuous and deceptive, and the kindest thing that can be said is that Makhijani and Boyd are ignorant of the implications of the liquid-fluoride thorium reactor and its fuel cycle, which they should not be if they presume to issue a “position paper” such as this.

Finally, the use of thorium also creates waste at the front end of the fuel cycle. The radioactivity associated with these is expected to be considerably less than that associated with a comparable amount of uranium milling. However, mine wastes will pose long-term hazards, as in the case of uranium mining. There are also often hazardous non-radioactive metals in both thorium and uranium mill tailings.

This is a repeat of the issue previously considered, as is immaterial as a factor for or against the use of thorium in nuclear powered reactors since thorium will be mined anyway during the mining of rare-earth minerals. The only question will be whether the mined thorium will be wasted or not.

In conclusion, Makhijani and Boyd fail to consider the implications of the liquid-fluoride thorium reactor on all aspects relating to the benefits of thorium as a nuclear fuel. They fail to consider its strong benefits with regards to nuclear proliferation, since no operational nuclear weapon has ever been fabricated from thorium or uranium-233. They fail to consider how LFTR can be used to productively consume nuclear weapons material made excess by the end of the Cold War. They fail to consider the reduction in nuclear waste that would accompany the use of LFTR. They fail entirely to account for the safety features inherent in a LFTR—how low-pressure operation and a chemically-stable fuel form allow the reactor to have a passive safety response to severe accidents. They fail to account for the improvement in cost that would be realized if LFTRs were to efficiently use thorium, reduce the need for mining fossil fuels, and increase the availability of energy.

Mr. Makhijani and Ms. Boyd should retract this statement in its entirety as flawed and deceptive to a public that needs clear and accurate information about our energy future.

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