PHD - Co-funded by the EPSRC and the Lloyd's Register Foundation for 4 years as part of the "Next Generation Nuclear" Centre for Doctoral Training.
Recycling of spent nuclear fuel is an important step in the operation of closed nuclear fuel cycles. Reprocessed uranium and plutonium may be used as uranium oxide or mixed oxide (MOX) fuel in a range of reactor designs including light water reactors (LWRs) or breeder reactors – so recycling the bulk of the spent fuel material and significantly reducing waste output.
In the UK, spent nuclear fuel is currently reprocessed at Sellafield, Cumbria, using the Plutonium URanium EXtraction process (PUREX). The initial step in this process is the dissolution of spent fuel rods in high concentration nitric acid, ~7-8 mol dm-3. Through a series of solvent extraction steps, the dissolved U and Pu are then chemically separated into either an aqueous HNO3 stream or a non-aqueous tri-n-butyl phosphate (TBP) and odourless kerosene (OK) stream. The separated uranium and plutonium are recycled as new fuels whilst the remaining highly radioactive liquid containing, amongst other things, fission products and minor actinides (neptunium, americium and curium) is calcined into glass and stored pending disposal as high level waste.
As a result of dilution, and also dissolved fission product/minor actinide induced radiolysis of HNO3, the concentration of HNO3 in the aqueous stream may vary through the PUREX flowsheet, to a minimum of ~1 mol dm-3.
Thus, an understanding of the chemistry of HNO3 is vital to the safe and efficient recycling of spent fuel. Its acidity notwithstanding, nitric acid is considered to be effectively inert (its acidity notwithstanding) under a range of conditions in non-nuclear-related processes. However, the intense ionising radiation field and high concentrations of HNO3 present in reprocessing streams give lie to this assumption during spent fuel recycle. The influence of the former on the latter gives rise to significant in-process concentrations of redox-active nitrous acid from nitric; the following chemical reactions of this radiolytically generated HNO2 then giving rise to a range of similarly active nitrogen-oxygen species such as NO2, N2O4 and NO. Re-examination of the literature pertaining to nitric / nitrous chemistry and its applications across the fuel cycle reveals a number of unresolved issues such as
- the means to reliably sense the concentration of nitrous, and
- the possible role of NO (a species hitherto relatively unstudied in the context of nuclear process chemistry) in key reactions during spent fuel reprocessing.
Thus, by using a combination of experimental, in situ quantitation of key species and kinetic / process modelling, we will seek to address these knowledge gaps, generating new insights into nitric / nitrous / NOx chemistry across the fuel cycle, so underpinning a deeper understanding of associated reaction mechanisms, process chemistries and thus greater system control and safety.
Application procedure
To apply, send a copy of your CV to Professor Colin Boxall, Engineering Department, Lancaster University, Lancaster LA1 4YR, or by email to c.boxall@lancaster.ac.uk. Informal enquiries to this address are also very welcome.