A Numerical Investigation of Conjugated Heat-loss Mechanism Depending on Gas Type and Surface Emissivity of a Metal Containment Vessel in a Water-cooled Small Modular Reactor During Normal Operation

Abstract

Recently, small modular reactors (SMRs) have been designed in the form of replacing a conventional containment building made of concrete with a metal containment vessel (MCV). This trend is primarily attributed to potential advantages such as the application of modularity, which simplifies the manufacturing of MCV. In addition, the MCV improves the integrity of the reactor pressure vessel (RPV) by promoting effective external cooling mainly via a high thermal conductivity of the metal in the accident situation. However, under normal operation, the high thermal conductivity causes unfavorable effects, i.e., inevitably more heat losses. To address this issue, the gap between the MCV and the RPV is treated with stagnant gas or vacuumed weakly, effectively suppressing conductive and convective heat transfer. Nevertheless, thermal radiation remains an unmitigated heat transfer mechanism within the gap during normal operation. In addition, the absence of insulation materials surrounding the RPV is expected to make the influence of radiation considerable, even at relatively low core temperatures of the integral pressurized water reactor (IPWR) system. Therefore, the primary objective of this thesis is to investigate the heat loss mechanism within the IPWR-type SMR under various gas-filling conditions using computational fluid dynamics. Based on the simulation result, thermal radiation shielding (TRS), which has an emissivity of one-tenth that of the MCV surface, was suggested to reduce radiative heat loss, and its effectiveness was evaluated. The results consistently revealed that thermal radiation was the dominant contributor to the total heat loss (48–97%), while the conjugated effects of the gas candidates on natural convection and thermal radiation varied according to their thermal and radiative properties, including absorption coefficient. The TRS showed a remarkable insulation performance, resulting in a substantial reduction in the total heat loss (56–70%), except for carbon dioxide (13%). Consequently, the arrangement of TRS can be promoted to enhance the thermal efficiency of SMR designs by suppressing heat loss through the MCV.