The Preparation and Application of Thorium-based Nuclear Fuels

Thorium is currently produced primarily as an impure by product of the mining and processing of the rare earth phosphate mineral monazite. Thorium concentrates are currently purified industrially by solvent extraction with PC-88a, but this extractant cannot separate uranium and iron from thorium. In this work mixtures of PC-88a and HDEHP were investigated for the mutual separation of uranium, thorium and iron. The extracted complexes were identified. U and Fe were extracted by cation exchange, while Th was extracted by a mixed cation exchange/solvation mechanism. It was found that three contact stages could extract > 99% of the thorium. A flowsheet was proposed.
The first modern use of thorium as a nuclear fuel is most likely to be as an oxide fuel within Generation III+ nuclear reactors. In this work a uranium-plutonium mixed oxide was investigated as a fissile driver for thorium in the Enhanced CANDU 6 reactor, as an alternative to the proposed UK CANMOX fuel for irradiation of the UK plutonium inventory. A large number of fuel concepts were considered, and several were analysed by Monte Carlo simulation. It was found that U-Pu-Th fuels could offer transmutation of the plutonium, irradiate UK reprocessed uranium and give improved coolant void reactivities, while irradiating thorium and converting it to fissile 233U.
Thorium and uranium may be recovered from spent nuclear fuel by the Acid THOREX process, which uses TBP solvent extraction. However, TBP has a number of disadvantages. In this work several alternative solvent extraction systems were investigated for the separation of Th, U, Fe and Zr. PC-88a was mixed with ten other extractants as potential synergists, extracting from hydrochloric, nitric and sulfuric acids. Several promising systems were identified based on distribution ratios and separation factors.