Understanding Molten Salt Reactors:
Using liquid fuel avoids the risk, unavoidable with solid fuels, of generating radioactive gases as by-products of the fission process. These gases, principally of iodine and cesium create a serious hazard if dispersed in the atmosphere, which can happen if there is catastrophic failure with a solid-fuelled reactor. In a molten fuel they form stable salts, not gases. These radioactive salts are not dispersed far beyond the reactor site in any accident scenario.
Using liquid fuel avoids the risk, unavoidable with solid fuels, of generating radioactive gases as by-products of the fission process. These gases, principally of iodine and cesium create a serious hazard if dispersed in the atmosphere, which can happen if there is catastrophic failure with a solid-fuelled reactor. In a molten fuel they form stable salts, not gases. These radioactive salts are not dispersed far beyond the reactor site in any accident scenario.
First, Why nuclear?
Fission of the uranium atom releases about 100 million times as much energy as oxidation of an atom of carbon. Nuclear fission is the only available technology with sufficient energy density to address global energy poverty.
Second, Why Molten Salt Reactors?
In a molten salt reactor hot liquid fluoride or chloride salt is used as coolant and, with dissolved fissile and fertile isotopes, as fuel in the reactor core. The experimental reactors at Oak Ridge Laboratory used thermal spectrum neutrons in the reactor core slowed down or ‘moderated’ by graphite. Today both thermal (moderated) and fast (without moderation) neutron spectrum reactors are under consideration.
Liquid or molten salt is used at from about 560°C to above 700°C. It boils at about 1400ºC so these reactors do not need to be pressurised. For the 1950-70 Oak Ridge Laboratory experimental reactors, heat from fission energy was removed with forced circulation of molten salt by pumping. A secondary coolant loop transfers heat to where it is needed as process heat.
Unconsumed fission products including actinides are soluble in the salt. They can be separated as needed from the salt either near the reactor or, alternatively, at separate salt processing facilities. These operations use established chemistry and chemical engineering techniques.
The molten salt reactor exhibits a strong negative temperature coefficient of reactivity, reflecting changes in heat withdrawal. This allows load-following and fail-safe operation. Overheating slows the reactor down.
Liquid fuel allows various fuel cycle options depending on reactor configuration. A molten salt reactor can be an actinide burner to destroy waste from other reactors; as a thorium to uranium-233 breeder; and also in other roles. Some of these options are less available when using solid fuels. Start-up and initial operation of industrial sized reactors can use legacy waste from decommissioned weapons and from civilian pressurized water reactors. In centuries to come, as legacy waste becomes less available, uranium-233 bred from thorium emerges as a standard fuel.
Unavoidable accumulation of unwanted fission products in solid fuel elements has presented very costly problems that are now apparent at, for example, Hinkley C. This next large UK rector project represents a likely and expensive end to half a century of a first nuclear era with solid-fuelled water-cooled reactor technology. In molten salt reactors, neutron interaction with the fuel creates more-or-less routine and manageable tasks for chemical engineering as predicted by Eugene Wigner. A change to liquid fuelled reactors allows a second nuclear era, without these costly problems, where reactors can be engineered to harness affordable energy that is carbon-free, sustainable, and cheap.
Third, Why thorium?
The only fissionable material in useful quantity as an energy source that occurs naturally in the earth’s crust is uranium 235 that constitutes 0.7% of natural uranium deposits, which are mainly uranium 238. Both uranium-238 and thorium-232 are ‘fertile’, that is, can transmute by absorbing a neutron to become respectively plutonium-239 and uranium-233, a process referred to as ‘breeding’. Both are fissile, that is, they can sustain a chain reaction in a reactor. Both can be fuels for nuclear fission reactors with fast spectrum molten salt reactors more efficient at burn-up than conventional solid-fuelled reactors.
Thorium is about four times more abundant in the earth’s crust than uranium and extracted as a single isotope, thorium 232. Thorium 232 and uranium 238, both naturally occurring, are described as ‘fertile’ because they capture neutrons and transmute to heavier fissionable material, uranium 238 transmuting to plutonium and thorium 232 transmuting to uranium 233, both fissionable. The process involved is referred to as ‘breeding’.
Thorium has detrimental effects on the neutron economy when used in solid fuel elements. But it will eventually emerge as a best option if more appropriate technology, when fissile material carried in molten salts becomes industry-standard, providing a sustainable energy resource for several millennia.
What of proliferation risk with thorium fuelled reactors?
In terms of material acquisition, processing, and weaponisation, history indicates that uranium 233 bred from thorium has not been a weapons technology of choice. Over time uranium 233 brings with it intrinsic difficulties in the natural radioactive decay chain. It has not seen a role as a desirable route for diversion or development of weapons-usable material. Denaturing fuel salt with added natural uranium is effective but reduces benefits from a favourable isotopic make-up of the uranium 233 decay chain.
Why is Molten Salt Reactor technology not yet industry-standard?
Funding, or investment, is the key. Also, molten salt reactor technology fell victim to perceived military requirements during the cold war period and presented as an unattractive alternative technology. Developing new fuel cycles, new containments and new reactor structures, in what is such a heavily regulated environment, is not straightforward for investors wanting quick returns.
New UK government policy in 2016 envisages the emergence of distinctive innovative nuclear fission technology by development of a UK Small Modular Reactor. If the selected design for this initiative is a liquid-fuelled reactor fast-tracked by government, that is safe, sustainable, and competitive in price with fossil fuel, then this significantly brightens prospects for a second nuclear fission era.
Further reading from MSR vendors:
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