PNNL

Abstract

Titanate-based ceramics have emerged as the leading candidate for a matrix for disposal of excess weapons-grade plutonium in a deep geologic repository. Concern persists, however, that accumulation of radiation damage will compromise the chemical durability of the titanate matrix. As a first step toward understanding the effects of radiation damage on element release rates, we performed single-pass flow-through (SPFT) experiments with 239Pu- and 238U-bearing ceramics over a solution pH-interval of 2 to 10 at 90?C. The ceramics tested include chemically complex zirconolite (CaBTi2O7), betafite (Ti-rich pyrochlore, ABTi2O7)(A = Ca2+, Gd3+, B = Gd3+, Hf4+, Pu4+, U4/6+), and betafite-rich samples. The 239Pu-bearing specimens contained 7.4 to 12.4 mass% PuO2 and 15.4 to 23.7 mass% UO2. In addition, a 238Pu-bearing (11.8 and 23.9 mass% PuO2 and UO2, respectively) betafite-rich specimen was tested at pH = 2, 90?C. The experiment with the 238Pu-bearing ceramic utilized a platinum-lined Teflon reactor to eliminate radiation damage to the Teflon from direct contact with the ceramic powder. The 239Pu-bearing specimens slowly released Pu to solution (rate = 8.3x10^-6 g m^-2 d^-1), even at pH = 2. At higher pH values, the dissolution rate decreased by a factor of 10 and then increased near pH = 10. Rates based upon release of calcium are typically >10X faster and are inconsistent with rates based on other elements. Compared to results from the 239Pu-bearing materials, the 238Pu-bearing specimen released Pu nearly 400X faster (rate = 3.2x10^-3 g m^-2 d^-1). Release rates of U, Gd, and Hf are also faster from the 238Pu-bearing ceramic compared to the specimen containing 239Pu. Although preliminary, the data can be interpreted to indicate that accumulation of radiation damage may result in faster release of Pu and U to solution than previously suspected.