Preparation and Characterization of 238Pu-Ceramics for Radiation Damage Experiments

The results from this initial characterization of the 238Pu- and 239Pu-bearing ceramics showed that the target phase assemblage was achieved in all but one material, 238Pu-zirconolite baseline. This 238Pu-zirconolite baseline material appears to have been prepared incorrectly with a 14 mass% excess of Pu (9.6 mass% actual vs. 8.4 mass% target; 4.8 mole% actual vs. 4.1 mole% target). It is not surprising that PuO2 was found to be one of the dominant phases. The densities of these materials compared well with the theoretical densities given by Stewart, Vance, and Ball [14]. For all but three of the materials, the average density was >94% of theoretical. Of the three, one was 238Pu-zirconolite baseline (108%) that contained unreacted PuO2. In our MCC leach testing, the normalized elemental mass losses from the various ceramic specimens depended on the elemental ceramic constituent, the Pu isotope, and the ceramic. Of the primary constituents, Al and Ca were the most easily released. Plutonium and U were the next most susceptible to release. In general, the Hf had the lowest releases during the tests. The Gd and Ti releases varied, depending on the ceramic and the Pu isotope in the ceramic. The Mo, which was added as a trace constituent to monitor the stability of the crystalline structure, exhibited consistently high-normalized elemental releases. The amount of Pu leached depended the most on the Pu isotope in the ceramic with more Pu released from the 238Pu specimens than from the 239Pu specimens, independent of ceramic type. Interestingly, the Mo releases were typically higher for the 239Pu specimens than for the 238Pu specimens. The higher Pu release from the 238Pu specimens is not yet understood; the consistency between the ICP/MS- and GEA-measured Pu releases from the 238Pu specimens eliminates analytical problems as an explanation. With respect to the effects of ceramic type, the three baseline ceramics often had the highest releases. The highest Pu releases were from the pyrochlore-rich and zirconolite-rich baselines. The highest U releases were from the pyrochlore-rich and zirconolite-rich baselines and zirconolite. Values for the pH of the leachates were deemed unreliable because of the length of time between the end of the test and the measurement. This resulted in pH values that were consistent with CO2-saturated water at ambient conditions. However, initial calculations with EQ3/6 suggest that the measured pH values are to be expected, given the solution concentrations and the assumptions that went into the calculations