Chemical Criteria for Waste Group Classification: a Model Approach

The purpose of this report is to develop and substantiate the criteria allowing the classification of waste groups for low- and intermediate-level waste (L/ILW). The criteria are primarily based on the chemical properties of the waste and their potential consequences. The emerging waste groups are relevant for:

• the consequence analyses supporting the safety case for the general licence application (RBG) (Nagra 2024a)
• the assessment of gas-related impacts (Nagra 2024b)

The report focuses on the following geochemical aspects of the cementitious near-field:

• classification of waste package types (AGTs) into waste groups
• model-based determination of a local pH value within AGTs. The local pH serves as the basis for assignment of corrosion rates to metallic materials contributing to gas production.
• calculation of the concentrations of complexing ligands in the near-field pore water. These concentrations serve as the basis for an assessment of the impact of complexing ligands on radionuclides sorption.

The methodology followed in this report is based on a so-called mixing tank approach with a homogenised fully saturated near-field as done for the abstraction of the dose calculations (Nagra 2024a). It is to be noted that partially saturated conditions within the L/ILW near-field dominate the time of the period of consideration as shown by Papafotiou et al. (2016) and explained in Martin et al. (2023). However, as thermodynamic calculations can only be performed for fully saturated conditions and as these conditions are a conservative abstraction of the repository, a full mixing tank assumption is considered for the waste group classification, which is a conservative scenario.

Compared to the 2014 version of the same report for SGT Stage 2 (Cloet et al. 2014), all assumptions about the composition of the L/ILW refer to the MIRAM ("Model Inventory of Radioactive Materials") RBG database (Nagra 2023a) instead of MIRAM 14 (Nagra 2014a). Furthermore, the data are presented as a function of AGT, instead of waste sorts. AGT’s are representing the composition of each individual waste type whereas in the waste sorts, similar AGT’s have been merged. The previous classification in terms of waste sorts is no longer used for RBG.

The degradation rates for organic materials are reported in Guillemot et al. (2023b), the definition of the source term and release rates of 14C containing waste types are documented in Nagra (2023b), and the assignment of the corrosion rates to metallic materials based on the pH values calculated here are reported in (Nagra 2023c). Therefore, these data will not be presented in the scope of this report anymore in comparison to the previous study by Cloet et al. (2014).