Abstract At the heart of the UKâ€™s fuel reprocessing capabilities lies the Thermal Oxide Reprocessing Plant (THORP) at Sellafield which since its opening in 1994, has reprocessed spent fuel stocks generated both domestically and from abroad. Spent nuclear fuel (predominantly consisting of UO2 pellets) is initially placed into interim storage ponds to cool before chemical reprocessing can separate the fuel into its component parts. However, with reprocessing operations halting in November 2018, the main aqueous storage facilities at Sellafield, the Fuel Handling Plant (FHP), AGR Storage and THORP receipt and storage (TR&S) ponds have now transitioned from a temporary cooling pond to a long-term holding facility. Unprocessed spent fuel will now be retained within its stainless steel cladding and held under aqueous alkaline conditions until a more permanent solution can be devised. Due to these uncertainties, it is now more important than ever to understand the mechanisms of corrosion of spent fuel rods under these conditions in order to monitor the release of radioactive material into the surrounding pond waters. The mechanism for the dissolution of uranium spent fuel under neutral and alkaline aqueous conditions has been well described, with H2O2 generated from gamma induced radiolytic degradation of water being the primary oxidative species. However, far fewer studies have investigated the effect of other constituents such as nitrate and chloride within the THORP ponds on the kinetics of this reaction and the synergistic reaction of the most omnipresent inorganic species, NaNO3, NaCl, Na2SO4 and Na2HPO4, with the spent fuel surface has yet to be well described. This thesis is focussed on the radiation induced chemical behaviour of water and indigenous dissolved aqueous salt species adsorbed onto the surface of spent uranium oxide fuels, under gamma irradiation. Studies herein have examined: (1) Aqueous speciation of [H2O2] and the dissolution mechanism of [U] induced by reaction radiolytically produced intermediated of H2O, NaNO3, NaCl, Na2SO4 and Na2HPO4. (2) Quantification of corrosion products formed at the uraniumâ€“aqueous interface. The post-irradiation characterisation of the liquors has enabled spectroscopic determination of [H2O2] at discreet dose intervals, with the concentration as a function of absorbed dose being recorded for systems including NaNO3, Na2SO4, NaHPO4 and NaCl. Of these species, sodium nitrate was the only one to both increase the evolution of solution peroxide and consequently elicit formation of secondary uranium oxidation product phases. Peroxide and hydroxide mineral species such as studtite and schoepite were detected on the surface of irradiated uranium samples. It can therefore be concluded that although the other aqueous species are not inconsequential, nitrate is the greatest determinator of dissolution of the uranium fuel matrix, whereas a combination of nitrate and phosphate may act to retard the oxidative corrosion of uranium oxide in spent fuel. The key findings of this research can be summarised as: (1) Deionised water; produces low [H2O2] but high [U] (2) Sodium nitrate; results in high [H2O2] and high [U]. Therefore, this species is a corrosion accelerator. (3) Sodium chloride; resulted in no enhancement to [H2O2] and no enhancement to [U]. (4) Sodium sulphate; produced a small enhancement to [H2O2] production but no enhancement to [U]. (5) Sodium phosphate; resulted in a small reduction in [H2O2] and a large reduction in [U]. Therefore, this species is a corrosion inhibitor.
|Date of Award||1 Aug 2021|
- The University of Manchester
|Supervisor||Francis Livens (Supervisor) & Louise Natrajan (Supervisor)|
- spent nuclear fuel