Alpha-Spectroscopy and Leach Testing of Synroc Doped with Actinide Elements
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K. P. HART, E. R. VANCE, R. STANOJEVIC and R. A. DAY Australian Nuclear Science and Technology Organisation, PMB 1, Menai 2234, Australia ABSTRACT
Synroc specimens doped individually with - 1 wt% of Np or Pu have been studied, after leaching for >2,500 days, using cr-spectroscopy and SEM. In the last leaching period, the leachant was replaced on either a daily or monthly basis. When the leachant was replaced on a daily basis the release rate of Np was similar to that measured when the samples were first leached, whereas for the monthly replacement the release rates of Np or Pu were about a factor of 20 to 30 lower than that for daily replacement. These findings agree generally with the results obtained from surface examination of the samples which showed that the thickness of actinidedepleted surface layer of anatase was 0.3 gm for monthly replacement but only 0.1 ýtm when the leachant was replaced frequently. Overall, the results suggest that release of actinides from Synroc are controlled by solubility limiting effects at the surface area to volume ratios employed in MCC-1 tests. INTRODUCTION
Ceramic waste forms, including Synroc and its constituent phases, are being increasingly 12 examined for use, not only for disposal of HLW but also actinide-rich waste streams ' . is environments and aqueous forms waste ceramic between interactions However, the study of often complicated by very low absolute leach rates and the possible formation of surface films and/or secondary alteration phases. A good understanding of the releases of important nuclides, especially actinides, from these waste forms is critical to understanding and predicting their longterm performance in a geologic environment. The work herein has been carried out to show how
a range of techniques, namely leaching, a-spectroscopy and SEM examinations of the leached surfaces, can help to elucidate the factors controlling releases of actinide elements from Synroc. Samples used in this study had been leached in long-term experiments with very long times used between leachant replacement. Previous work3 had tentatively interpreted results from these samples as suggesting that releases of actinide elements were independent of the leach period duration. Consequently, at the end of the long-term experiments two different leachant replacement regimes were investigated to assess the effect of leach period duration on the release of actinide elements from these Synroc samples. EXPERIMENT Synroc samples containing simulated PW-4b waste and individually doped with either
237Np or Pu" at levels of 1.14 and 0.76 wt%, respectively, were prepared as detailed 24 # The Pu used in these experiments contained - 4 wt% °pu (half-life = 6500 years) and - I wt%
241
Pu, (half-life 239 addition to 239pu. Furthermore, 24Am had ingrown from 241pu. As the ax-energies of Pu and it is not easy to distinguish between them by cz-spectrometry; consequently, the reported Pu Pu + 240Pu. leach rate is based on ... =24 13 years) in 0Pu overlap,
173 Mat. Res. Soc. Symp. Proc. Vol. 556 ©
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