THORIUM FUEL CYCLE
THORIUM FUEL CYCLE
- The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, 232 Th, as the fertile material. In me reactor, 232 Th is transmuted into the fissile artificial uranium isotope 233 U which is the nuclear fuel.
- Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as 231 Th), which are insufficient to initiate a nuclear chain reaction.
- Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, 232 Th absorbs neutrons eventually to produce 233 U.
- This parallels the process in uranium reactors whereby fertile 238U absorbs neutrons to form fissile 239 Pu.
- Depending on the design of the reactor and fuel cycle, the 233 U generated either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.
- The thorium fuel cycle claims several potential advantages over a uranium fuel cycle, including thorium’s greater abundance, superior physical and nuclear properties, better resistance to nuclear weapons proliferation, and reduced plutonium and actinide production.
Advantages of using Thorium as a nuclear fuel
- Thorium is estimated to be about three to four times more abundant than uranium in the Earth’s crust, although present knowledge of reserves is limited. Current demand for thorium has been satisfied as a by-product of rare-earth extraction from monazite sands.
- Also, unlike uranium, mined thorium consists of a single isotope (232 Th). Consequently, it is useful in thermal reactors without the need for isotope separation.
- Thorium-based fuels also display favourable physical and chemical properties which improve reactor and repository performance.
- Compared to the predominant reactor fuel, uranium dioxide (UO2), thorium dioxide (ThO2) has a higher melting point, higher thermal conductivity, and lower coefficient of thermal expansion. Thorium dioxide also exhibits greater chemical stability and, unlike uranium dioxide, does not further oxidize.
- The long term radiological hazard of conventional uranium-based used nuclear fuel is dominated by plutonium and other minor actinides, after which long-lived fission products become significant contributors again. THORIUM FUEL CYCLE
Disadvantages of using Thorium as a nuclear fuel
- There are several challenges to the application of thorium as a nuclear fuel, particularly for solid fuel reactors.
- Unlike uranium, natural thorium contains no fissile isotopes; fissile material, generally 233 U, 235U, or plutonium, must be added to achieve criticality. This, along with the high sintering temperature necessary to make thorium-dioxide fuel, complicates fuel fabrication.
- In an open fuel cycle (i.e. utilizing 233 U in situ), higher burn up is necessary to achieve a favourable neutron economy. Although thorium dioxide performed well at burn ups at Fort St. Vrain Generating Station and AVR respectively, challenges complicate achieving this in light water reactors (LWR), which compose the vast majority of existing power reactors.
- Another challenge associated with a once-through thorium fuel cycle is the comparatively long interval over which 232 Th breeds to 233 U. The half-life of 233 Pa is about 27 days, which is an order of magnitude longer than the half-life of 239 Np.
- As a result, substantial 233 Pa develops in thorium-based fuels. 233 Pa is a significant neutron absorber, and although it eventually breeds into fissile 235 U, this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.