Development of radionuclide inventory and fission product release calculation model and its application to HTR-PM

The safety of High-Temperature Gas-cooled Reactors (HTGRs) critically depends on understanding the radionuclide inventory and Fission Products (FPs) release behavior, which are fundamental for radiological protection and source term determination in reactor licensing. This study presents a novel method that combines well-established codes (ORIGEN2.2, NECP–MCX, V.S.O.P. (99/11), and STACY) to perform coupled calculations for neutronics, thermal hydraulics, fuel depletion, and fission product releases. An elaborate simulation code, Fission Products Inventory and Release Rate Calculation System (FIRCS) has been developed to track several fictitious tracer pebbles across a user-defined grid. The concept of mock tracers is introduced for equilibrium core and release scenarios. Neutron flux and fuel temperature distributions are derived from the Multiphysics code VSOP. ORIGEN2.2 then simulates flux irradiation at each grid point, utilizing burnup-dependent neutron cross-section libraries generated by NECP–MCX for each core pass. The code tracks radionuclides, temperatures, and Particle Failure Fraction (PFF) for the entire flow history of each tracer. This data is used to calculate release rates for individual tracers in STACY. In HTGR cyclic simulation, these tracers are sequentially introduced into the core with each cycle and a recirculation matrix is computed based on the quantity, pass number, and position of tracers in the core. The matrix is used to retrieve the Concentration and Release Rate (CRR) of radionuclides from these tracers which is then utilized to calculate CRR for the entire core. The estimate converges towards accurate estimates as the number of tracers increases. Thermal decay power, discharge inventory, and photon emission spectra are also calculated for spent fuel. Over a period of 50 days, the accumulated decay power for 40,000 spent fuel pebbles is determined to be 27.4 kW. This work delves deeper into the methodological details and its first application to a 250 MW(t) HTR-PM design. Results are presented for the equilibrium core, including radionuclide inventory and release rates of key fission products. Iodine-131, Cesium-137, Strontium-90, and Silver-110 m have activities of 2.5 × 10¹⁷ Bq, 2 × 10¹⁶ Bq, 1.6 × 10¹⁶ Bq, and 3.5 × 10¹⁴ Bq, respectively. Among these radionuclides, Iodine-131 exhibits the highest release rate, followed by Cesium-137, Silver-110 m, and Strontium-90. The calculations in this study have been validated against published data, demonstrating the reliability of the results presented in this work. The application of this methodology to a 250 MW(t) HTR-PM design demonstrates its potential for informing future core design decisions and safety assessments in HTGR development.