Characterization of Alteration Phases on HLW Glasses After 15 Years of PCT Leaching

We have collected an extensive data base of leaching data for numerous high-level nuclear waste (HLW) glass compositions using the PCT leaching procedure (deionized water, 90°C, S/V = 2000 m -1 ) for test durations of up to fifteen years. In a previous paper, statistical tools were used to evaluate the solution data and to classify several qualitatively different types of leaching behavior. In particular, it was found that there is a clear segregation between glasses with alteration rates that remain low and those that show resumptions of alteration. In this paper, we focus on the surface characterization of samples of leached glass recovered from these tests. From the subset of 98 glass compositions used in the development of the reference glass formulation for the immobilization of West-Valley nuclear waste, leached samples of 22 glasses were characterized. When no resumption of alteration occurs, the residual rate is approximately constant and non-zero. The existence of a residual rate appears to be linked to the slow growth of a phyllosilicate (smectite-type) phase containing Fe, Mg, and K. A nontronite phase is identified by XRD while SEM-EDS gives an average composition of (Na,K) 0.8 (Fe,Mg) 1.3 (Al,Si) 4 O 10 . The occurrence of a resumption of alteration is associated with the precipitation of zeolite-type aluminosilicate phases, identified by XRD as phillipsite, with a typical composition (K,Na,Ca 0.5 ) 1-2 (Si, Al) 8 O 16 .6H 2 O, as measured by SEM-EDS. In such cases, the glass is still covered with the underlying nontronite phase of the same average composition as identified for samples that have not exhibited a rate increase. Both our earlier evaluation of the leachate concentrations and the present results from characterization of the alteration phases highlight the important effects of the glass composition, to the extent of producing qualitatively different alteration behavior.