Types of Thorium Reactors
As for uranium based reactors, the basic classification of thorium nuclear reactors is based upon the average energy of the neutrons which cause the bulk of the fissions in the reactor core. From this point of view nuclear reactors are divided into two categories:
- Thermal Reactors. Almost all of the current reactors which have been built to date use thermal neutrons to sustain the chain reaction. These reactors contain neutron moderator that slows neutrons from fission until their kinetic energy is more or less in thermal equilibrium with the atoms (E < 1 eV) in the system.
- Fast Neutron Reactors. Fast reactors contains no neutron moderator and use less-moderating primary coolants, because they use fast neutrons (E > 1 keV) to cause fission in their fuel.
In principle, all reactor types may be fuelled with thorium, but some are more suited to thorium fuel than others. As will be shown in further section, uranium 233 has very good parameters in the thermal spectrum. For fast spectrum, thorium is not ideal, because its parameters are between uranium 235 and plutonium 239. It should be pointed out that uranium 233 has a high reproduction factor in the epithermal spectrum, compared with U-235 and Pu-239, making thorium an advantageous fuel for the reduced moderation epithermal LWRs.
According to the World Nuclear Association, there are seven types of reactors that can be designed to use thorium as a nuclear fuel. Six of these have all entered into operational service at some point (usually with uranium fuel). The most promising are for example:
- Heavy water reactors (PHWRs). PHWRs (Pressurized Heavy Water Reactors) generally use natural uranium (0.7% U-235) or slightly enriched uranium oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D2O). Due to the favourable neutron management (very low parasitic capture), caused by online refueling and consequent reduced requirements for control poisons to compensate excess reactivity, an application of thorium is possible and these reactors attain a relatively high conversion factor.
- High-temperature gas-cooled reactors (HTRs). These reactors are well suited for thorium-based fuels in the form of robust ‘TRISO’ coated particles of thorium mixed with plutonium or enriched uranium.
- Molten salt reactors (MSRs, LFTRs). A molten salt reactor (MSR) is a class of generation IV nuclear fission reactor in which the primary nuclear reactor coolant, or even the fuel itself, is a molten salt mixture. Reactors containing molten thorium salt are called liquid fluoride thorium reactors (LFTR).
In general, use of thorium-based fuel in light water reactors is possible, but not so promising. BWR fuel assemblies can be flexibly designed in terms of rods with varying compositions (fissile content), and structural features enabling the fuel to experience more or less moderation (e.g. half-length fuel rods). Viable thorium fuels can be designed for also a PWR, though with less flexibility than for BWRs. Fuel needs to be in heterogeneous arrangements in order to achieve satisfactory fuel burnup.