PuO₂ and Fuel Residues

Building on research undertaken by the Fellow as a PDRA in DIAMOND, we propose to investigate the incorporation of Pu and Am in cement materials relevant to the encapsulation and disposal of radioactive wastes (in particular, PCM). Predicting the release of these radionuclides from the GDF is a key factor in developing a robust safety case for the long-term storage of nuclear waste. This requires an understanding of their interactions with the main sorbing components of the cement backfill, including ordinary Portland cement and the binder phase, Calcium-Silicate-Hydrate gel (C-S-H), and also a comprehension of possible transport pathways through the engineered backfill cement. While it is expected that reducing conditions will prevail in the repository, such that actinide species are expected to be present in their reduced, insoluble forms, the initial oxidation state and pH of PCM wastes is poorly defined, largely due to their encapsulation with organic waste products and their acid derivatives¹.

This study will investigate the sorption and incorporation mechanisms of Pu and Am onto engineered barrier cement materials, and the subsequent transport processes under groundwater flow. A mechanistic understanding of Pu and Am immobilisation by cement barrier materials will be determined by investigating the effects of initial pH and redox state on incorporation, using aqueous geochemical and radioanalytical techniques (LSC, ICP) to monitor solution chemistry. The Ca:Si ratio will be varied to determine the role of C-S-H binder phases on immobilisation, and experiments will be conducted under controlled CO₂/carbonate conditions to develop an understanding of carbonation on backfill material-Pu/Am interactions. Information of the distribution and coordination environment of sorbed species will be determined using digital autoradiography and μ-XRF techniques, coupled with μ-XCT (X-ray Computed Tomography) techniques at the Argonne National Laboratory (USA) and the Diamond Light Source (UK) to derive high resolution, detailed datasets, which can be used to develop the first verified conceptual and numerical models of Pu and Am sorption and transport in engineered barrier cement.