Radionuclides Solubilities in Canadian Crystalline Rock and Sedimentary Environments

Abstract

The maximum concentration of a radionuclide within or near a failed container for used nuclear fuel will be limited by the radionuclide solubility. Radionuclides solubilities are calculated by geochemical modelling using thermodynamic data under relevant geochemical conditions.

The NWMO (Nuclear Waste Management Organization) has recently updated the databases of radionuclides solubilities for Canadian crystalline rock and sedimentary environments. This paper presents radionuclides solubility calculations in Canadian crystalline rock and sedimentary environments. The radionuclide solubility limits were calculated in a reference groundwater CR- 10 (Ca-Na-Cl type with an ionic strength of 0.24 mol/kgw) which simulates the potential groundwater geochemical conditions at the repository depth of Canadian crystalline rock, and in a reference groundwater SR-290-PW (Na-Ca-Cl type with an ionic strength of 7.2 mol/kgw) which simulates the potential groundwater geochemical conditions at the repository depth of Canadian sedimentary rocks. The radionuclide solubility calculations considered three scenarios: 1) groundwater directly enters the container without interacting with the bentonite buffer or the container materials; 2) groundwater interacts with the carbon-steel container prior to contacting the used nuclear fuel waste inside the container; and 3) groundwater interacts with both bentonite buffer and carbon-steel container prior to contacting the used nuclear fuel waste inside the container. The solubility calculation in CR-10 was conducted with the ThermoChimie SIT thermodynamic database (version 10a). The solubility calculation in SR-290-PW was conducted using a Pitzer database DGR-Pitzer-TDB which has been developed based on the Yucca Mountain Pitzer database data0.ypf.R2 and modified with additional information from the THEREDA database and the available Pitzer data from the literature.