In general, a nuclear reactor is a key device of nuclear power plants, nuclear research facilities or nuclear propelled ships. Main purpose of the nuclear reactor is to initiate and control a sustained nuclear chain reaction. Most common nuclear reactors are light water reactors (LWR), that are based on the uranium fuel cycle and use an enriched uranium fuel (~4% of U-235) as a fresh fuel. During the fuel burning, the content of the U-235 decreases and the content of the plutonium increases (up to ~1% of Pu ). Currently, these reactors provides almost 90% of the world’s nuclear electricity generating capacity.
Thorium reactors are based on the thorium fuel cycle and use thorium 232 as a fertile material. During the fuel burning, thorium 232 transforms into a fissile uranium 233. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as thorium 231), which are insufficient to initiate and sustain nuclear chain reaction. Therefore, additional fissile material is necessary to initiate the fuel cycle.
According to proponents, the thorium fuel cycle offers several potential advantages over a uranium fuel cycle, including thorium’s greater abundance, better physical and nuclear properties (e.g. lower capture-to-fission ratio for thermal neutrons), reduced plutonium and actinide production, and better resistance to nuclear weapons proliferation when used in traditional light water reactors.
See also: Thorium vs. Uranium
On the other hand, thorium is “only” a fertile material and the main problem can be directly in the breeding of fissile uranium 233. If 232Th is loaded in the nuclear reactor, the nuclei of 232Th absorb a neutron and become nuclei of 233Th. The half-life of 233Th is approximately 21.8 minutes. 233Th decays (negative beta decay) to 233Pa (protactinium), whose half-life is 26.97 days. 233Pa decays (negative beta decay) to 233U. Therefore, proposed reactor designs must attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.