Corrosion tests with uranium- and plutonium-loaded ceramic waste forms.

Abstract

Tests were conducted with ceramic waste form (CWF) materials that contained small amounts of uranium and plutonium to study their release behavior as the CWF corroded. Materials made using the hot isostatic press (HIP) and pressureless consolidation (PC) methods were examined and tested. Four different materials were made using the HIP method with two salts having different U:Pu mole ratios and two zeolite reagents having different residual water contents. Tests with the four HIP U,Pu-loaded CWF materials were conducted at 90 and 120 C, at CWF-to-water mass ratios of 1:10 and 1:20, and for durations between 7 and 365 days. Materials made using two PC processing conditions were also tested. Tests with the two PC U,Pu-loaded CWF materials were conducted at 90 and 120 C, at a CWF-to-water mass ratio of 1:10, and for durations between 7 and 182 days. The releases of matrix elements, U, and Pu in tests conducted under different test conditions and with different materials are compared to evaluate the effects of composition and processing conditions on the release behavior of U and Pu and the chemical durabilities of the different materials. The distributions of released elements among the fractions that were dissolved, in colloidal form in the solution, and fixed to test vessel walls were measured and compared. Characterization of Pu-bearing colloidal particles recovered from the test solutions using solids analysis techniques are also reported. The principal findings from this study are: (1) The release of U and Pu is about 10X less than the release of Si and 50X less than the release of B under all test conditions. This implies that U and Pu are in a phase that is less soluble than the sodalite and binder glass matrix. (2) Almost all of the plutonium that is released from U,Pu-loaded CWF is present either as colloidal-sized particles in the size range between 5 and 100 nm in the test solution (about 15% of the total) or becomes fixed on stainless steel test vessel walls (about 85% of the total). Work is in progress to determine the nature of the material fixed to the steel. (3) The Pu in the colloidal particles is present as the dioxide PuO{sub 2} or a mixed oxide (U,Pu)O{sub 2}. These particles are of dimensions and composition similar to the (U,Pu)O{sub 2} particles that are present as inclusions in the CWF. They are often found associated with larger (100-nm size) aluminosilicate colloids. (4) The degradation behaviors of U,Pu-loaded CWF processed by HIP and PC are the same with regard to the release of Si, U, and Pu. One difference is that B is released faster from the HIP CWF materials than from the PC CWF materials. (5) The presence of U and Pu does not affect the durability of CWF materials.