Thorium-fuelled Molten Salt Reactors (MSRs) offer a potentially safer, more efficient and sustainable form of nuclear power. Pioneered in the US at Oak Ridge National Laboratory (ORNL) in the 1960s and 1970s, MSRs benefit from novel safety and operational features such as passive temperature regulation, low operating pressure and high thermal to electrical conversion efficiency. Some MSR designs, such as the Liquid Fluoride Thorium Reactor (LFTR), provide continuous online fuel reprocessing, enabling very high levels of fuel burn-up. Although MSRs can be fuelled by any fissile material, the use of abundant thorium as fuel enables breeding in the thermal spectrum, and produces only tiny quantities of plutonium and other long-lived actinides.
Current international research and development efforts are led by China, where a $350 million MSR programme has recently been launched, with a 2MW test MSR scheduled for completion by around 2020. Smaller MSR research programmes are ongoing in France, Russia and the Czech Republic. The MSR programme at ORNL concluded that there were no insurmountable technical barriers to the development of MSRs. Current research and development priorities include integrated demonstration of online fuel reprocessing, verification of structural materials and development of closed cycle gas turbines for power conversion.
Thorium is a mildly radioactive element, three times as abundant as uranium. There is enough energy in 5000 tons of thorium to meet world energy demand for a year. Reserves of thorium are widely dispersed around the world with large deposits in Australia, the USA, Turkey and India. China also has substantial thorium reserves,
especially within its rare earth mineral deposits in Inner Mongolia. In Europe, Norway has 132,000 tons of proven reserves, 5% of the world’s total.
As fuel in an MSR is already in liquid form, it cannot melt down, as solid fuel rods in a LWR can. The liquid fuel can be quickly drained from the reactor and left to cool in dump tanks. As molten fluoride salt is heavy, it cannot be dispersed by wind. In the very unlikely event of a terrorist attack or missile strike on the reactor, spilled fuel would only create a small contamination zone in the immediate surroundings of the reactor.
Types of MSR
* Two Fluid MSR (which includes LFTR)
* Single Fluid MSR (which includes denatured MSR)
* Thorium fuelled MSR
* Fast neutron MSR
Two Fluid MSRs
Interest in a two-fluid iteration of the MSBR, the LFTR, has grown over recent years underpinned by the campaign work of Kirk Sorensen of Flibe Energy, and the publication of popular science title ‘SuperFuel: Thorium, The Green Energy Source for the Future’ by US journalist Richard Martin.
In 2008, a solution was proposed to the plumbing problem which led ORNL to abandon its two-fluid designs. Nuclear physicist David Leblanc has a patent pending on an alternative plumbing design which simplifies the complex series of graphite tubes separating the thorium blanket from the core present in ORNL designs.
Liquid Fluoride Thorium Reactor (LFTR)
The reactor core contains fissile fuel (U-233, U-235 or Pu-239 in liquid fluoride form) held in a graphite container which acts as a neutron moderator. The core is surrounded by an outer vessel, the blanket, which contains thorium dissolved in a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2) known as FLiBe. When the fuel in the core fissions, neutrons are released which penetrate the walls of the core and bombard the surrounding blanket of thorium causing it to transmute into U-233. The U-233 is transferred to the core and new thorium is added to the blanket.
The LFTR contains two separate piping loops. The first loop carries the irradiated liquid thorium into a decay tank where the U-233 can be moved to the inner core. The second loop transfers the heated U-233 molten salt from the inner core to the heat exchanger which drives a turbine which generates electricity. The U-233 salt is then transferred back to the reactor core to continue fissioning.
The amount of fuel the reactor breeds is equal to the amount that it consumes. To keep the fission reaction going, thorium must be added to the reactor at the same rate that it generates and fissions uranium
Single Fluid MSRs
Single fluid MSRs combine both fertile thorium and fissile material in one salt. The two MSRs constructed at ORNL were both single fluid systems. ORNL focussed on single fluid systems to avoid the plumbing complexities associated with two fluid reactors.
Denatured Molten Salt Reactor (DMSR) ‘30 Year Once Through’
The DMSR was designed in the late 1970s at ORNL as part of a program to develop proliferation-resistant reactors. Denatured fuel contains fissile fuel diluted with at least 80% U-238 which renders the fissile component unsuitable for weapons applications.
As the DMSR was designed for simplicity without continuous fission product removal, it does not make enough of its own fuel to self-sustain, so a fresh mixture of around 80% U-238/20% U-235 must be continuously added to the fuel salt. The DMSR is thus fuelled, at least partly, by uranium, but a majority of its energy would come from the thorium to U-233 chain.
The DMSR’s graphite core is larger and thus less power dense than the earlier MSBR. This enables the graphite moderator to last for the lifetime of the reactor.
Gaseous fission products and noble metals are continuously removed, but the remaining fission products are left in the molten salt for the full 30 year lifetime of the reactor. As there is no online processing of the salt, the DMSR cannot fission all of the uranium in the core, as the LFTR can. Despite this, the DMSR is a much more efficient burner of uranium than an LWR.
Thermal and Fast Spectrum Reactors
The terms ‘thermal’ and ‘fast’ refer to the speed of neutrons in a reactor. Thermal neutrons move at around 2000 meter/second, while fast neutrons move at around 9000 kilometers/second (4500 times faster than thermal and 3% of lightspeed). In the fast spectrum the fission cross-section (the probability that neutrons will fission fissile atoms) is small. When neutrons are released from fissioning atoms they tend to be travelling at high speed. In the same way that it would be hard to take a motorway exit if you were travelling at 300mph, so neutrons generally do not hit the target and split atoms when moving fast.
In the thermal spectrum, the speed of neutrons is reduced by the presence of a neutron moderator, thus increasing the fission cross-section. Common moderators include water, helium and graphite. Neutrons lose thermal energy when they bounce off the nuclei of the atoms in the moderator which slows them down. This process is known as thermalising.
Uranium consists of fissile isotope U-235 (0.7%) and fertile isotope U-238 (99.3%). With a thermal spectrum, even this natural uranium can support a chain reaction in reactors such as the CANDU or Magnox, while LWRs require only modest enrichment of 3% to 5% U-235 to operate. While operating they also produce and partially burn off Pu-239 produced when U-238 absorbs a neutron. In the thermal spectrum however, Pu-239 is an inefficient fuel, emitting on average less than two neutrons for every one absorbed. Conversely in a fast spectrum, while a far greater fraction of fissile material must be present, the Pu239 produced from U238 is a far more efficient fuel and can allow operation as a breeder where more fissile fuel is made than consumed. This is not possible for U-238 to Pu-239 in the thermal spectrum.
Thorium-Fuelled Molten Salt Fast Reactors
Unlike U-238, thorium can readily be consumed in its entirety in a thermal reactor. ORNL’s MSR work focussed only on thermal spectrum designs, and the current Chinese MSR programme is also dedicated to thermal spectrum reactors.
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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