Development of a multiscale DLOFC source term analysis framework for pebble bed HTGR

Abstract

DLOFC accidents in HTGRs present potential radiological hazards to both human health and the environment. However, existing source term models often rely on simplified assumptions, introducing significant uncertainties in accident consequence assessments. To enhance simulation reliability, this study develops a multiscale DLOFC source term analysis framework that captures the accident’s physical processes across temporal and spatial dimensions. The framework integrates a suite of high-fidelity, interconnected models to quantitatively characterize radionuclide transport and distribution throughout all key accident phases and reactor regions. The core source term is simulated using a multiphysics-coupled model, while a statistical ensemble of fuel pebble operational histories under steady-state conditions defines the initial accident state, enabling full-core release analysis throughout the accident progression. Radionuclide distribution within the nuclear island prior to depressurization is determined via a one-dimensional, four-zone transport-and-plateout model, combined with a detailed nodalization of the primary loop. Additionally, the framework incorporates the FRG desorption model alongside operational data to simulate radionuclide migration across the core, primary loop, containment, and environment throughout different accident phases. Using the HTR-PM as a reference case, a comprehensive DLOFC source term analysis is conducted. The results indicate that the environmental release fractions for typical nuclides are $3.2\times 10^{-8}$ for the long-lived inert gas Kr-85, $1.4\times 10^{-7}$ for the short-lived inert gas Xe-133, and $6.7\times 10^{-8}$ for the long-lived metallic nuclide Cs-137. Compared to conventional accident analysis models, the radiological releases predicted by the present framework are approximately an order of magnitude lower, which can be attributed to the adoption of a more realistic core fission product transport-release model and a more reasonable fuel performance analysis model. These findings demonstrate that the proposed framework enhances the completeness and accuracy of source term assessment, providing robust technical support for underscoring the inherent safety features of HTGRs.