Preliminary investigation of chlorine speciation in zirconolite glass-ceramics for plutonium residues by analysis of Cl
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.460
Preliminary investigation of chlorine speciation in zirconolite glass-ceramics for plutonium residues by analysis of Cl K-edge XANES Amber R. Mason,1 Stephanie M. Thornber,1 Martin C. Stennett,1 Laura J. Gardner,1 Dirk Lützenkirchen-Hecht,2 and Neil C. Hyatt1 1 Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK
2
Fakultät 4 - Physik, Bergische Universität Wuppertal, Gaußstraße 20, 42097 Wuppertal, Germany
ABSTRACT A zirconolite glass-ceramic material is a candidate wasteform for immobilisation of chlorine contaminated plutonium residues, in which plutonium and chlorine are partitioned to the zirconolite and aluminosilicate glass phase, respectively. A preliminary investigation of chlorine speciation was undertaken by analysis of Cl K-edge X-ray Absorption Near Edge Spectroscopy (XANES), to understand the incorporation mechanism. Cl was found to be speciated as the Cl- anion within the glass phase, according to the characteristic chemical shift of the X-ray absorption edge. By comparison with Cl K-edge XANES data acquired from reference compounds, the local environment of the Cl- anion is most closely approximated by the mineral marialite, in which Cl is co-ordinate to 4 x Na and/or Ca atoms.
INTRODUCTION A significant fraction of the UK plutonium stockpile, predicted to reach 140 tons at the end of nuclear fuel reprocessing, is known to be unsuitable for reuse as (U,Pu)O2 mixed oxide (MOX) nuclear fuel.1-3 A zirconolite glass-ceramic wasteform is under development for the immobilisation and disposal of contaminated plutonium and plutonium residues as a waste.1,4-7 In this wasteform, the plutonium is targeted for immobilisation by solid solution in the zirconolite phase, for which there is high confidence in long term performance, given the evidence available from natural analogues and investigation of prototype and surrogate materials. The contaminants and impurities are targeted for immobilisation within the accessory aluminosilicate glass
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phase, which should therefore have a high solubility for these elements. Some of the waste plutonium inventory is contaminated by chlorine resulting from degradation of the polyvinyl chloride (PVC) liner used within the can-in-can storage package.8 It is thought that thermal and radiation induced degradation of PVC resulted in the release of HCl which was subsequently adsorbed onto the surface of the PuO 2. Chlorine is known to have a low solubility in silicate glasses, typically < 3 wt%,9-11 above which phase separation of a chloride salt (such as NaCl) is observed. Incorporation of Pu within such a salt, would afford a highly soluble instant release fracti
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