Optimizing dissolution and chemical processing of fast reactor MOX fuel

Generation IV reactors are being developed to enhance the efficiency and sustainability of nuclear electricity production. SCK CEN is involved in the development of Lead-Cooled Fast Reactors (LFRs) which use liquid lead or lead-bismuth eutectic as coolant. This coolant enables high-temperature operation without pressurization and offers an enhanced neutron economy leading to a better fuel utilization. A current trend is the development of Small Modular Reactors (SMRs), which are safer, more flexible, and scalable. The EAGLES consortium, including SCK CEN, is working on LEANDREA, an SMR-LFR demonstrator planned for 2035 in Mol. It will use MOX fuel with high plutonium content (30% PuO₂). A swift recovery of plutonium and uranium from spent SMR-LFR fuel and recycling into new fuel is essential as plutonium is a synthetic element. The high Pu content presents some challenges for the aqueous reprocessing of spent FR MOX fuel of which the most important are slow dissolution in nitric acid, increased insoluble Pu containing residues, radiolysis of solvents, criticality risks. Furthermore, SCK CEN has performed experiments in the 1980s on fast reactor MOX fuels e.g. the FARFADET experiments which aimed to study the fuel restructuring, cladding corrosion and thermal behavior at the beginning of life. Due to the termination of the fast reactor program, the post-irradiation examination (PIE) was, however, never completed. The fuel from these experiments remains stored at SCK CEN and currently no disposal strategy exists. In the context of LEANDRA and to reduce waste liability for historical fuels, the back-end of the nuclear fuel cycle for these high Pu containing MOX fuels will be studied. A nitric acid dissolution method for the MOX fuel will be developed. Since MOX fuels with high Pu content are difficult to dissolve, development of an electrochemically assisted or ultrasonic aided dissolution method might be needed. For the recycling of Pu and U from used fuel, the PUREX process is the most widely used and industrially mature method. The aim of the PhD study is to develop a CHON compliant separation process. The degradation products will interfere less with the separation process and during the second-generation MOX fuel fabrication they will be burned off during calcination and/or sintering stages and will not affect the purity of the recycled U and/or Pu product.

The student will develop advanced dissolution methods for difficult-to-dissolve MOX fuels and develop a CHON compliant solvent extraction process for recycling of Pu and U from historical and future high Pu MOX fuels. Radiochemical analysis and measurement uncertainty analysis will be performed by the student as well as UV/Vis spectroscopy, pycnometry and microscopy.