Effect of pH on cation release from sodalite -based matrices for immobilization of spent chloride salt waste from pyroprocessing

The recovering of energy-producing elements such as uranium and plutonium from spent nuclear fuel, which is in line with Generation IV reactors concept, can be performed by pyro-chemical processing. Molten chloride salt waste arising from pyro-processing is one of the problematic wastes for direct application of vitrification or ceramization. Sodalite, a naturally occurring aluminosilicate mineral containing chlorine, is among those mineral phases under consideration as potential matrix for confinement of spent chloride salt waste coming from pyro-processing. In this work, the effect of pH on metal release from sodalite and sodalite blended with borosilicate glass has been evaluated due the lack of data reported in the literature. To this aim, sodalite samples loaded with simulated chloride salts waste, with and without the addition of borosilicate glass, were leached at 90°C for contact times up to 15 days at three different initial values of pH. Leaching results were compared in terms of normalized releases as a function of pH. SEM analyses were also performed in order to compare the matrix surface before and after leaching. According to this study it is apparent that the retention performances of sodalite-based matrices can be significantly worsened under acidic conditions. Therefore it is fundamental to adopt suitable solutions in order to avoid acidic conditions and to preserve the retention capabilities of the sodalite-based confining matrices.