Plutonium Migration during the Leaching of Cemented Radioactive Waste Sludges

Kathleen A Law,Nicholas D Bryan,Luke O’Brien,Darrell Knight,Stephen Parry,Steven M Heald,Gareth T W Law,William R Bower,Francis R Livens,Adam J Fuller,Sarah L Heath

doi:10.3390/geosciences9010031

Kathleen A Law, Nicholas D Bryan + Show 9 more

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https://doi.org/10.3390/geosciences9010031

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Abstract

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.

Highlights

The UK’s first generation of civil nuclear reactors were fueled with natural uranium metal, clad in an Mg-based alloy (Magnox) [1]
As the Mg-rich cement may serve to lessen Pu release into the backfill and geosphere, here, we evaluate the performance of this material in Pu leach tests, making use of micro-focus X-ray absorption spectroscopy to determine Pu location and speciation in the cement during aging and leaching
The Mg(OH)2 -containing cements used in this study leach readily on contact with water, and results show that the addition of Mg(OH)2 can affect permeability and leaching behavior

Summary

Introduction

The UK’s first generation of civil nuclear reactors were fueled with natural uranium metal, clad in an Mg-based alloy (Magnox) [1]. In some of the facilities at the Sellafield site, Magnox fuel has been stored for several decades, and the fuel elements are extensively corroded [2] This has led to the accumulation of Mg-rich fine particulate sludges in the fuel ponds, which are mainly composed of brucite (Mg(OH)2 ), contaminated with spent fuel particles, associated actinides, and fission products [2,3]. This sludge is sufficiently radioactive to be classified as intermediate level radioactive waste (ILW; in the UK, ILW is defined as radioactive waste containing over 4 GBq tonne−1 alpha activity or 12 GBq tonne−1 beta/gamma activity, but for which the heat output need not be considered in the design of storage or disposal facilities). The total volume of sludge is around 5000 m3, and it contains a wide range of fission and activation products, including plutonium, which is released due to the corrosion of the fuel cladding [2]

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