Aqueous Durability of Titanate Ceramics Designed to Immobilise Excess Plutonium
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3 Mat. Res. Soc. Symp. Proc. Vol. 608 0 2000 Materials Research Society
Corrosion testing was carried out at 90 'C in deionised water, as described elsewhere12 l. Analysis of inactive elements was carried out by ICP/MS but for Pu, the level of activity was determined by alpha spectrometry carried out on 0.1 mL of leachant evaporated onto a stainless steel planchette. This method allowed the contribution of 24'Am to be separated from that of the Pu', unlike in our previous studiest21 . Results Long-term leaching of Pyrochlore-rich and Zirconolite-rich Formulations The test results for Pu releases for all of the formulations listed in Table 1 are shown in Figure 1. The variation between the Pu release rates after 7 days is about a factor of 20 with the difference reducing to about 4, for the smaller number of samples studied so far, after about 300 days. Overall, the release rates decreased by about one order of magnitude between 3.5 days (i.e. 0 to 7 day period) and 324 days. Data for the effect of impurity addition on U, Pu, Gd, and Hf releases are shown in more detail in Table 3. Figure 2 shows the U release rates measured for all of the sintered formulations. With the exception of Pu97, Pu99 and Pu 101, the U release rates vary over about one and a half orders of magnitude for the 0 to 7 day period and decrease by about one to two orders of magnitude after 150 days. Release rates measured for Gd, for all of the batches listed in Table 1, remained relatively constant between 10-3 and 10-4 g m 2d-1. Gd release results showed a wider variation than for other elements, about an order of magnitude between triplicates, and this has contributed to scatter in the data shown in Table 3. Unfortunately space limitations do not allow for all of the Gd data to be shown in this paper. Hf concentrations were close to the detection limit in most leachants, i.e. 10-7 g.LU, and are generally similar to or lower than those of Pu. (Development work is being carried out to improve the detection limit of the ICP/MS for Hf and Gd.) Although the Pu, Gd and Hf release rates were similar within about one order of magnitude for all of the ceramics listed in Table 1, the U release rate from batches Pu97, Pu99 and Pul 01 has evidently been affected by the additions made to the baseline formulations. The different behaviours for U may indicate that in these batches some of the U is present at higher valence states than in the baseline formulations and as a consequence is more leachable. SEM examination of these ceramics did not identify any obvious U-bearing minor phases arising from the compositional changes relative to the baseline. For batches 68 and 75, which were prepared and tested together, it is possible to compare the effects of impurity additions on the long-term releases of Pu, U and the neutron absorbers (Gd and Hf) from the matrix (see Table 3). Pu release rates, with the exception of those in the last leaching period, are very similar (and the formulation containing the impurities tends to have lower Pu release rates). Overall t
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