Computational analysis for predicting fission product release of PeLUIt-40 under normal operating conditions

PeLUIt (Pembangkit Listrik dan Uap untuk Industri) is an High Temperature Gas Cooled Reactor (HTGR)-based cogeneration reactor designed by the Indonesian National Nuclear Energy Agency of Indonesia (now the Research Organization for Nuclear Energy, BRIN). HTGR uses tri-structural isotropic (TRISO) coated particle fuel, which is the main barrier to the release of fission products. The ability of the fuel to retain fission products both under normal and accident conditions is very important for design licensing. This study was conducted to predict the release of fission products of PeLUIt-40 (PeLUIt with a power of 40 MWt) under normal operating conditions. In this study, the Source Term Analysis Code System (STACY) was used to predict the release of radiologically significant fission products Ag110m, Cs137, I131, and Sr90. OpenMC was used to calculate neutronic parameters such as burnup (Fissions per initial heavy metal atom—FIMA), fast neutron fluence, and fission product inventory. A single pebble model with different irradiation times and temperature variations was used to simulate the fission product release in PeLUIt-40. The time and temperature variations were used to investigate the sensitivity of the fission product release fraction in PeLUIt-40 fuel to these parameters and to estimate the maximum safe fuel temperature during operation. The simulation results showed that the largest release fraction was Ag110m release compared to other radionuclide releases. At the normal operating temperature of 977 °C, the fission product release fractions during one-through-one-out (OTTO) and 5-pass cycles were two orders of magnitude lower than the failure fraction for the high-temperature reactor (HTR)-Module $1.6x{10}^{-4}$ and there was no defective particle during operation. In the OTTO cycle, the maximum fuel temperature that did not cause defective particle was about 1250 °C, but the Ag110m release fraction exceeded $1.6x{10}^{-4}$. The release fraction of all fission products in the OTTO cycle is below $1.6x{10}^{-4}$ when the maximum fuel temperature is 1025 °C. While in the 5-pass cycle, the maximum fuel temperature of about 1200 °C does not cause defective particle, but the release fraction of Ag110m exceeds $1.6x{10}^{-4}$. The release fraction of all fission products in the 5-pass cycle is below $1.6x{10}^{-4}$ when the maximum fuel temperature is 1020°C. The simulation results confirm that the release of fission products in PeLUIt-40 fuel during normal operation is lower than the safety requirement limit of $1.6x{10}^{-4}$ and no defective particle occurs during normal operation. This indicates that the TRISO-coated PeLUIt-40 particles are able to prevent the release of fission products at normal operating temperatures.