Performance analysis for containment of radionuclides within the category a surface disposal system: Near field model calculations and additional argumentation

In the framework of the license application for the construction and operation of a surface repository for category A waste at Dessel (Belgium), a performance analysis of its containment capacity was performed. The objectives are threefold: • to quantify the expected performance of the disposal system as a whole and of its components/safety functions; • to demonstrate that the disposal system and its system, structure or components (SSCs)/safety functions are robust against reasonably foreseeable disturbances; • to demonstrate that, until the end of phase V of the repository’s lifetime, the performance of the disposal system is proportional to the residual risk associated with the waste. The expected performance of the disposal system and its components is based on the phenomenological expected evolution and the containment strategy, detailed in the long-term safety concept. A comprehensive and systematic approach is applied, in which for every system, structure or component with a main role for a containment safety function, the expected performance is quantitatively and/or qualitatively substantiated. The following lines of argumentation are used: • compliance with conformity criteria derived from design requirements and that will be verified during implementation to provide reasonable assurance that the SSC can perform its intended safety function(s). A typical example is the low hydraulic conductivity value of key concrete components and their resilience against internal and external degradation factors; • a set of performance indicators, calculated from near field models simulating the migration of radionuclides through the different system components and towards the environment. These indicators include mass-based (e.g. activity in compartments), flux-based (e.g. activity flux between compartments) as well as residence time-based (e.g. containment factors) indicators; • other (semi-)quantitative indicators building on the processes on which the containment characteristics rest, or derived from supporting models, used for components that are not explicitly represented in near field models. Robustness of the disposal system’s containment capacity was analysed by comparing the expected performance with the residual performance of the system subjected to 5 types of threats. These threats correspond to (grouped) effects caused by initiating FEPs, leading to earlier and/or extreme degradations, radionuclide complexation, and combinations thereof. Additionally, effects on containment caused by human intrusion leading to a bypass of affected SSC’s are considered. The risks associated with the waste radiotoxicity decline over time. So does the level of overall containment performance due to expected and possible degradation processes and events (translated into alternative evolution scenarios). The third part of the performance analysis therefore aims to demonstrate that the residual containment capacity offered by the disposal system remains commensurate with the residual risks associated with the waste by a sufficient margin. Numerical measures for the delay of release, spreading of release, total amount of the released radiotoxicity, containment and decay within the disposal system were developed which allowed to compare the different scenarios in a quantative manner.