Thorium Fuel Cycle - Disadvantages As Nuclear Fuel

Disadvantages As 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 233U, 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. Oak Ridge National Laboratory experimented with thorium tetrafluoride as fuel in a molten salt reactor from 1964–1969, which was far easier to both process and separate from contaminants that slow or stop the chain reaction.

In an open fuel cycle (i.e. utilizing 233U in situ), higher burnup is necessary to achieve a favorable neutron economy. Although thorium dioxide performed well at burnups of 170,000 MWd/t and 150,000 MWd/t 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.

In a once-through thorium fuel cycle the residual 233U is long lived radioactive waste.

Another challenge associated with the thorium fuel cycle is the comparatively long interval over which 232Th breeds to 233U. The half-life of 233Pa is about 27 days, which is an order of magnitude longer than the half-life of 239Np. As a result, substantial 233Pa develops in thorium-based fuels. 233Pa is a significant neutron absorber, and although it eventually breeds into fissile 235U, this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.

Alternatively, if solid thorium is used in a closed fuel cycle in which 233U is recycled, remote handling is necessary for fuel fabrication because of the high radiation levels resulting from the decay products of 232U. This is also true of recycled thorium because of the presence of 228Th, which is part of the 232U decay sequence. Further, unlike proven uranium fuel recycling technology (e.g. PUREX), recycling technology for thorium (e.g. THOREX) is only under development.

Although the presence of 232U complicates matters, there are public documents showing that 233U has been used once in a nuclear weapon. The United States tested 233U as part of a bomb core in the MET blast during Operation Teapot in 1955, though with much lower yield than expected.

Though thorium-based fuels produce far less long-lived transuranics than uranium-based fuels, some long-lived actinide products constitute a long term radiological impact, especially 231Pa.

Advocates for liquid core and molten salt reactors such as LFTR claim that these technologies negate thorium's disadvantages present in solid fueled reactors. Since only one liquid core fluoride salt reactor has been built (the ORNL MSRE) and it was not using thorium, it is hard to validate the exact benefits.