SAFE-CLEAR: Integrated Safety Assessment Framework based on Advanced Numerical Modeling for Clearance of Decommissioning Radioactive Waste

Abstract

This dissertation develops and validates SAFE-CLEAR (Safety Assessment Framework for Exempt waste CLEARance), an integrated national safety assessment framework for clearance of nuclear facility decommissioning waste. The framework is designed to overcome methodological limitations and regulatory blind spots in the current RESRAD code family, including fragmented scenario-specific tools, outdated physical modelling, and the absence of linked scenarios across the waste management chain. SAFE-CLEAR comprises three advanced core modules that are consistently integrated into a integrated assessment platform. First, the external exposure module replaces simplified dose conversion factor corrections with a point-kernel integration scheme combined with probabilistic sampling. Benchmarking against MCNP and MicroShield® demonstrates that this module prevents the hazardous underestimation observed in existing models under complex three-dimensional source geometries and heterogeneous multi-layer shielding, while maintaining MCNP-level credibility with reasonable conservatism. Second, the groundwater transport module couples 1D and 2D advection–dispersion analysis to resolve physical inconsistencies in RESRAD-Onsite, such as neglect of dispersion and a single-path 1D flow assumption. Second, the groundwater transport module couples one-dimensional and two-dimensional advection–dispersion analysis in a way that retains the conservative assumptions embedded in RESRAD-Onsite but explicitly corrects the physical errors originating from its simplified formulations. Dispersion, multidimensional flow paths, and mass balance are treated in a physically consistent manner, removing non-physical error such as illogical overestimation of released activity and dose. In doing so, the module preserves the conservative frame of the original RESRAD approach, while replacing its oversimplified transport representation with a hydrologically and radiologically coherent description of plume evolution. Third, the multi-phase carbon-14 release module links pH- dependent geochemical reactions and isotopic exchange with gas–aqueous transport. This overcomes the empirical compartment approach in RESRAD that, for example, unrealistically blocks gaseous migration in cover layers. The module quantitatively captures dynamic phase partitioning and pathway shifts with pH, significantly improving the reliability of 14C safety assessments without unnecessary over-conservatism. Finally, Integrated scenario analyses using SAFE-CLEAR identify and quantify key missing links, including activity concentration in by-products (e.g., dust and slag), landfill behavior of residues, and pH-dependent 14C migration in alkaline disposal environments. Compared with RESRAD-Onsite, SAFE-CLEAR enables higher annual disposal capacities for landfill scenarios while satisfying regulatory dose criteria, and demonstrates that excluding the landfill pathway under a conditional clearance strategy can increase nuclide-specific clearance levels by orders of magnitude. Overall, SAFE-CLEAR functions not only as a dose calculation code but as a decision-support tool that enhances regulatory completeness and provides a technically robust basis for flexible, efficient, and safely conservative management of decommissioning waste.