PNNL

Abstract

This paper describes a model for determining the stability and associated radionuclide concentrations of colloids that might be present in the nuclear waste package environment from degradation of the nuclear waste forms. The model simplifies radionuclide–colloid behavior by assuming that all colloids can be defined as either smectite clay, a mixed actinide-bearing rare earth-zirconium oxide, iron oxyhydroxide (ferrihydrite {FeOOH}, or uranophane {Ca(UO2)2(SiO3OH)2(H2O)5}. However, for the purposes of predictive stability modeling, the colloids are conceptually represented as montmorillonite, ZrO2, hematite, and meta autunite, respectively. The model uses theoretical calculations and laboratory data to determine the stability of modeled colloids with ionic strength and pH. The true nature of colloid composition and heterogeneity, generation, and flocculation will be extremely complex, involving the formation of numerous types of phases, often depending on the composition of the various waste forms and waste package materials. This model strives to capture the uncertainty of the real system using theoretical models. In this paper, one of the four representative colloids designed to capture the behavior of the spent fuel derived colloids is described in detail.