AGING EFFECTS ON CURIUM-DOPED TITANATE CERAMIC CONTAINING SODIUM-BEARING HIGH-LEVEL NUCLEAR WASTE

Curium-doped titanate ceramic containing sodium-rich high-level nuclear waste showed a gradual decrease in density up to a dose of 8.5 x 10(17) alpha decays.g-1. After that, the rate of density change increased apparently because of crack formation. Optical microscopy showed cracks > 0.1 mm long and > 1-mu-m wide after a dose of 7.9 x 10(17) alpha decays.g-1. Leach tests suggested that the dissolution-control phases for sodium and cesium changed from freudenbergite and hollandite, respectively, to intergranular phases after significant cracking. Aging also enhanced strontium losses, relative to calcium, indicating that strontium may also be partitioned to the intergranular phases. After the fresh surfaces produced by cracking were exposed to leachant, and the dissolution of soluble intergranular surfaces was complete, the leaching of nonradioactive elements from the samples having a dose of 12.3 x 10(17) alpha decays.g-1 was limited by the following dissolution-control phases: freudenbergite (Na), hollandite (Cs and Ba), perovskite and/or zirconolite (Sr and Ca), and alloys (Mo). The leaching behavior of the nonradioactive indicator elements revealed that chemical durability was reduced by two main factors: (1) increasing the effective surface area by crack formation and (2) decreasing the stability of the actinide-host phases by alpha-recoil damage. In combination these factors increased longer-term (> 7 days) leach rates of sodium and cesium, and strontium and calcium by 1 and 2 orders of magnitude, respectively. In spite of deterioration of the actinide-host phases, the curium leach rate after a dose of 12.3 x 10(17) alpha decays.g-1 decreased by 2 orders of magnitude, possibly as a result of precipitation of curium hydrolysis products.