Potential Indoor Worker Exposure From Handling Area Leakage: Example Event Sequence Frequency Analysis

The U.S. Department of Energy (DOE) is currently considering design options for the facilities that will handle spent nuclear fuel and high-level radioactive waste at the potential nuclear waste repository at Yucca Mountain, Nevada. The license application must demonstrate compliance with the performance objectives of 10 CFR Part 63, which include occupational dose limits from 10 CFR Part 20. If DOE submits a license application under 10 CFR Part 63, the U.S. Nuclear Regulatory Commission (NRC) will conduct a risk-informed, performance-based review of the DOE license application and its preclosure safety analysis, in which in-depth technical evaluations are focused on technical areas that are significant to preclosure safety and risk. As part of pre-licensing activities, the Center for Nuclear Waste Regulatory Analyses (CNWRA) developed the Preclosure Safety Analysis Tool software to aid in the regulatory review of a DOE license application and support any independent confirmatory assessments that may be needed. Recent DOE information indicates a primarily canister-based handling approach that includes the wet transfer of individual assemblies where Heating, Ventilation, and Air Conditioning (HVAC) systems may be relied on to provide confinement and limit the spread of any airborne radioactive material from handling operations. Workers may be involved inmore » manual and remote operations in handling transportation casks, canisters, waste packages, or bare spent nuclear fuel assemblies inside facility buildings. As part of routine operations within these facilities, radioactive material may potentially become airborne if canisters are opened or bare fuel assemblies are handled. Leakage of contaminated air from the handling area into adjacent occupied areas, therefore, represents a potential radiological exposure pathway for indoor workers. The objective of this paper is to demonstrate modeling capabilities that can be used by the regulator to estimate frequencies of potential event sequences. A hypothetical case is presented for failure of the HVAC exhaust system to provide confinement for contaminated air from otherwise normal operations. This paper presents an example calculation of frequencies for a potential event sequence involving HVAC system failure during otherwise routine wet transfer operations of spent nuclear fuel assemblies from an open container. For the simplified HVAC exhaust system model, the calculation indicated that the potential event sequence may or may not be a Category 1 event sequence, in light of current uncertainties (e.g., final HVAC system design and duration of facility operations). Categorization of potential event sequences is important because different regulatory requirements and performance objectives are specified based on the categorization of event sequences. A companion paper presents a dose calculation methodology and example calculations of indoor worker consequences for the posed example event sequence. Together, the two companion papers demonstrate capabilities for performing confirmatory calculations of frequency and consequence, which may assist the assessment of worker safety during a risk-informed regulatory review of a potential DOE license application.« less