Thorium nuclear reactor

The use of thorium would reduce the quantities of dangerous nuclear waste, and avoid the generation of plutonium, which is problematic with the use of enriched uranium. There is a great deal of interest in utilising thorium as a fuel for nuclear reactors, especially in India, which has no uranium resources, but abundant thorium.

Thorium reactors

Thorium (Z = 90) is a radioactive element of the actinide series. It is a potential fuel for nuclear reactors. Thorium is more abundant in the Earth’s crust than uranium, and is found in monazite deposits. Monacite is thorium diphosphate Th(PO4)2, and thorium can be extracted through a process involving nitric acid. Thorium-232 can be transmuted to U-233 by bombardment with neutrons. U-233 could be used as a fissile fuel for power generation. Technical difficulties have so far prevented the use of this alternative to U-235, but India has large reserves of thorium, and is carrying out a programme to develop it.

Thorium is weakly radioactive and has seven naturally occurring isotopes, all of which are unstable to very varying degrees (half-lives vary from 25.5 hours for Th-231 to 14 billion years for Th-232!). Th-232 is by far the most abundant thorium isotope in the crust. It is 3 or 4 times as abundant as uranium.

23290Th + 10n

23390Th + γ

β-

23391Pa

β-

23292U

When U-233 undergoes fission, it emits neutrons, which can impact another thorium-232 nucleus, causing the decay to start again. The chain reaction would be self-sustaining at a critical mass and geometry. The cycle is similar to that in fast-breeder reactors, which produces highly-fissile Pu-239 from low-fissile U-238. However, thorium is more abundant than uranium, and offers a more sustainable supply, especially since the fissile U-233 it uses can be ‘bred’ from natural ore thorium. Most uranium reactors are ‘burner’ type, and its fuel is enriched natural uranium ore.

Thorium also has the advantage that it can be mixed with U-238, and therefore has no weapons-grade potential, which is not the case with U-235 enriched fuel. It also has a higher neutron yield, produces fewer long-lived transuranium elements, and makes better-performing reactor cores. Thorium needs to be neutron irradiated before it can be used as a fuel, and this presents greater technological challenges, which is why it has not been adopted extensively to date.

For an account on the Indian Advanced Heavy Water Reactor (AHWR) project, designed to utilise Thorium-232: BARC Report on AHWR development programme