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Title: Plutonium Migration during the Leaching of Cemented Radioactive Waste Sludges

Journal Article · · Geosciences
 [1];  [1];  [2];  [1];  [3]; ORCiD logo [1];  [4];  [1];  [5];  [6];  [1]
  1. Univ. of Manchester (United Kingdom)
  2. Univ. of Manchester (United Kingdom); National Nuclear Laboratory, Warrington (United Kingdom)
  3. Argonne National Lab. (ANL), Argonne, IL (United States)
  4. National Nuclear Laboratory, Warrington (United Kingdom)
  5. University of Helsinki (Finland)
  6. Univ. of Manchester (United Kingdom); University of Helsinki (Finland)

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.

Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
AC02-06CH11357
OSTI ID:
1491807
Journal Information:
Geosciences, Vol. 9, Issue 1; ISSN 2076-3263
Publisher:
MDPICopyright Statement
Country of Publication:
United States
Language:
English

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Figures / Tables (10)