Variables impacting sorption of Pu to the CILVA-immobilization matrix: Results of screening for active factors in experimental design

In the frame of the safe disposal of nuclear waste, ONDRAF/NIRAS has submitted a license application for the building 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. Sorption data for Pu to cementitious matrices are available (Wang, 2009), and used in the safety case for near surface disposal, but uncertainties persist concerning the sorption values of Pu specifically on the CILVA-mortar. 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 more representative sorption values. This experimental program is divided in two phases. A first phase focuses on the sorption on fresh CILVA mortar. A second phase focuses on oxidized CILVA mortar. This report is part of the first phase (fresh CILVA mortar). At the start of the experimental program, the pore water of the fresh CILVA matrix was characterized, in order to define the prerequisite chemical conditions and boundaries in which the experimental set-up should fit. Preliminary tests were then carried out to define the analytical and practical boundary conditions for the experiments. Based on these data and previously gained knowledge on the conditions affecting radionuclide (RN) sorption (and Pu-sorption in particular), an experimental matrix was defined to screen for variables affecting Pu sorption to the fresh CILVA matrix. The aim of this first part of the experimental program is to define which factors would affect sorption of Pu to the fresh CILVA immobilization matrix. The different factors are variables related to the CILVA pore water composition on one hand (concentrations of Ca2+, Cl-, SO42-, S2-, K+ and OH- (pH)), and variables characteristic for batch sorption experiments on the other hand ([Pu], S/L and equilibration time). From these 9 variables, only [Ca2+] was found to be an active factor, and as such affecting Pu-sorption. In a next step (not included in this report), the extent of the impact of the [Ca2+], together with the effect of [ISA], will be the subject of a more elaborate experimental set-up, following a central composite design.