Assessment of homogeneity in geopolymer waste form containing ion exchange resin using laser-induced breakdown spectroscopy (LIBS) and statistical analysis

Abstract

The immobilization of spent ion exchange resins (IERs) is essential for disposal, with solidification being a representative method for immobilization. The homogeneous distribution of IER within the waste form is a key criterion for assessing its suitability for disposal. However, a quantitative method for evaluating the homogeneity of waste forms is currently lacking. In this study, a methodology was devised to assess the homogeneity of waste forms containing spent IER by creating and analyzing both homogeneous and heterogeneous samples. The compressive strength and leaching tests were conducted, followed by the measurement of nuclide intensities (non-radioactive Cs or Sr) adsorbed on IER using LIBS. Subsequently, various statistical analyses were applied to the measured LIBS intensity data. The impact of heterogeneity on compressive strength and leachability was limited. Compressive strength and leaching tests indicated that only samples exhibiting extreme heterogeneity yielded distinguishable low strength and increased leachability compared to homogeneous samples. The statistical analysis of LIBS measurement data confirmed that homogeneity assessment can be conducted through formal normality test (Shapiro-Wilk test) and calculation of the coefficient of variation. In statistical analysis, heterogeneity can be confirmed starting from heterogeneous sample that exhibits one level lower heterogeneity than the level that could impact performance. Due to this difference, a comprehensive assessment of the homogeneity of waste form should be tested by various performance indicators such as compressive strength, leaching characteristics.