Impact of Ca2+ and isosaccharinic acid (ISA) on sorption of Pu to the CILVA-immobilization matrix

In the frame of the safe disposal of nuclear waste, ONDRAF/NIRAS has submitted a license application for the exploitation of a near surface facility for the disposal of short-lived low and intermediate level waste (category A (cAt) waste). A significant part of the cAt waste, including Pu-contaminated cAt waste, has been conditioned in the CILVA-installation of Belgoprocess. In this respect, ONDRAF/NIRAS has launched an experimental program to determine sorption values for Pu specifically to the CILVA-mortar, in order to obtain representative sorption values. This experimental program is divided in two phases: one on sorption of Pu to fresh CILVA mortar (reducing conditions), and a second one on sorption to oxidized CILVA mortar. This report is part of the first phase, and describes the design and outcome of the experimental plan to assess and predict the effect of [Ca2+] and [ISA] on Pu-sorption to fresh CILVA mortar, following a preliminary screening study (Wouters et al., 2022a). In the experimental design, the concentrations of Ca2+ and ISA were varied between two extremes: 3.1*10-4 M to 8.1*10-3 M for Ca2+ and 1.5*10-6 M to 6.7*10-1 M for ISA. First, an ANOVA was performed, showing that an increase of the concentration of ISA and of Ca2+ (separately and combined) resulted in a decrease of Pu sorption to the CILVA matrix. In order to enable the most conservative prediction for Pu-sorption to the CILVA matrix in the presence of the interacting factors [ISA] and [Ca2+], a confidence region was calculated for the intrinsic [Ca2+] in the supernatant of the CILVA suspension, based on measurements of the [Ca2+] in the equilibrated CILVA suspension. Subsequently, the impact of the ISA concentration on Pu sorption was predicted for the most conservative scenario, being at the highest end of the confidence region of the [Ca2+] ([Ca2+]=3.09 mM). The subsequent prediction shows a clear decreasing trend of the Rd while increasing [ISA] from 10-5 M to 10-2 M, and an increase of Rd at [ISA] above approximately 10-2 M. At the lower estimate of the 95% prediction interval, predictions fall slightly below a log Rd value of 3.5, in the vicinity of 3.45.