Plutonium Migration during the Leaching of Cemented Radioactive Waste Sludges
Centre for Radiochemistry Research, School of Chemistry, The University of Manchester, Manchester M13 9PL, UK
National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park, Warrington WA3 6AE, UK
Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA
Radiochemistry Unit, Department of Chemistry, the University of Helsinki, A.I. Virtasen Aukio 1 (PL 55), 00014 Helsinki, Finland
School of Earth and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, the University of Manchester, Manchester M13 9PL, UK
Author to whom correspondence should be addressed.
Present address: Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, UK.
Present address: AWE, Aldermaston, Berkshire, RG7 4PR, UK.
Geosciences 2019, 9(1), 31; https://doi.org/10.3390/geosciences9010031
Received: 30 November 2018 / Revised: 21 December 2018 / Accepted: 24 December 2018 / Published: 8 January 2019
(This article belongs to the Special Issue The Development and Use of Synchrotron Radiation Techniques for the Geological Disposal of Radioactive Wastes)
One of the most challenging components of the UK nuclear legacy is Magnox sludge, arising from the corrosion of Mg alloy-clad irradiated metallic U fuel that has been stored in high pH ponds. The sludges mainly comprise Mg hydroxide and carbonate phases, contaminated with fission products and actinides, including Pu. Cementation and deep geological disposal is one option for the long-term management of this material, but there is a need to understand how Pu may be leached from the waste, if it is exposed to groundwater. Here, we show that cemented Mg(OH)2 powder prepared with Pu(IV)aq is altered on contact with water to produce a visibly altered ‘leached zone’, which penetrates several hundred microns into the sample. In turn, this zone shows slow leaching of Pu, with long-term leaching rates between 1.8–4.4 × 10−5% of total Pu per day. Synchrotron micro-focus X-ray fluorescence mapping identified decreased Pu concentration within the ‘leached zone’. A comparison of micro-focus X-ray absorption spectroscopy (µ-XAS) spectra collected across both leached and unleached samples showed little variation, and indicated that Pu was present in a similar oxidation state and coordination environment. Fitting of the XANES spectra between single oxidation state standards and EXAFS modeling showed that Pu was present as a mixture of Pu(IV) and Pu(V). The change in Pu oxidation from the stock solution suggests that partial Pu oxidation occurred during sample ageing. Similarity in the XAS spectra from all samples, with different local chemistries, indicated that the Pu oxidation state was not perturbed by macro-scale variations in cement chemistry, surface oxidation, sample aging, or the leaching treatment. These experiments have demonstrated the potential for leaching of Pu from cementitious waste forms, and its underlying significance requires further investigation.