Leach testing is an important tool for the investigation of chemical durability of radioactive waste forms. The data obtained from these tests allows for the selection of the most durable ceramics for the immobilization of weapons-grade plutonium. Ceramics based on cubic zirconia, (Zr,Gd,Pu)O2, doped with 10 wt.% 239Pu, zircon, (Zr,Pu)SiO4, doped with separately 5-6 and 10 wt.% 239Pu, pyrochlore, (Ca,Pu,Gd,Hf,U) 2Ti2O7, doped with 10 wt.% 239Pu were studied using MCC-1 leach test. Ceramic samples were placed in test vessels with deionized water and set temperatures of 25°C and 90°C in an oven for 28 days. The leaching solution was removed and replaced with fresh deionized water after 3 and 14 days from the start of the test. Only data obtained after 14 and 28 days were used for the interpretation. No saturation of leach solution with Pu was observed in any experiments. It was found that the (without correction for porosity) normalized Pu mass loss (in g/m2) after 14 and 28 days was:
• for the zirconia ceramic - 2 × 10-2 to 4 × 10-2 at 90°C and 8 × 10-3 to 9 × 10-3 at 25 °C
• for zircon doped with 5-6 wt.% of Pu - 7.0 × 10-3 to 8.2 × 10-3 at 90°C and 1.0 × 10-3 to 1.2 × 10-3 at 25 °C
• for zircon doped with 10 wt.% of Pu - 0.2 to 0.2 at 90°C and 3.0 × 10-2 to 4.0 × 10-2 at 25 °C
• for pyrochlore - 1.0 × 10-3 to 1.0 × 10-3 at 90°C and 2.0 × 10-3 to 3.0 × 10-3 at 25 °C.
It was shown that the high leach rate for zircon based ceramic doped with 10 wt.% Pu was caused by the presence of the separated inclusions of PuO2 phases observed in ceramic matrix. The results obtained so far allow us to conclude that ceramics based on zircon, zirconia and pyrochlore are characterized by similar chemical resistance to leaching in deionized water. Based on its low porosity, zircon may be most durable ceramic host phase for Pu.
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Nikolaeva, E.V., Burakov, B.E. Investigation of Pu-Doped Ceramics Using Modified MCC-1 Leach Test. MRS Online Proceedings Library 713, 1118 (2001). https://doi.org/10.1557/PROC-713-JJ11.18