Development of a Safety Assessment for the Brazilian Near-Surface Repository in the Site Selection Phase

The internationally accepted approach for developing a safety case focuses on the integration of repository design and site characteristics and the safety demonstration through the elaboration of safety assessments and it may relate to a given stage of project development. This paper describes a safety assessment for the Brazilian Near-surface Repository for Low and Intermediate-Level Radioactive Waste at the current site selection stage. Given the lack of data in terms of isotopic inventory, detailed deposition system and site characteristics, the main objective was to indicate which radionuclides are most significant, the most important pathways and input parameters for the current knowledge of the repository design and possible site characteristics in order to meet the requirements of the Brazilian nuclear regulatory standards. This analysis was based on the methodology established by the ISAM reports (Improvement of Safety Assessment Methodologies for Near Surface Disposal Facilities). A conservative reference scenario was assumed based on the review of international experiences of repository safety assessments and safety cases, as well as information about possible sites for the repository. The reference scenario includes the release of radionuclides from the deposition zone (source term), radon release and dust suspension; transport of radionuclides through the deposition zone, geosphere (unsaturated and saturated zones) and biosphere (atmosphere and soil); and estimation of the dose evolution over time for a rural resident farmer scenario, considering the pathways for ingestion (water and food produced locally), inhalation (radon and air particles) and direct external exposure, both in onsite and offsite areas. RESRAD-OFFSITE code version 4.0 was used to implement the assumed conceptual model. A sensitivity analysis was carried out considering a singleparameter variation (deterministic analysis) and probabilistic simulation with multiple parameters, identifying which are the most sensitive parameters for different pathways. The results showed the evolution of the total specific dose over time obtained for the reference scenario. Three specific peak doses were observed: the first at the beginning of the simulation time (right after the repository closure), of short duration (due to Ra-226); the second of lower magnitude, around 1,000 years (related to the I-129), and the third, of greater magnitude and duration, between 4,000 and 19,000 years (related to the process of erosion of the cover layer). Carried out at the early stages of the development of a repository project, such analysis can be useful to support decision makers, to assess needs for further analyses and to build confidence. (author)