Analysis of heat-loss mechanisms with various gases associated with the surface emissivity of a metal containment vessel in a water-cooled small modular reactor

In various small modular reactor (SMR) designs currently under development, the conventional concrete containment building has been replaced by a metal containment vessel (MCV). In these systems, the gap between the MCV and the reactor pressure vessel is filled with gas or vacuumed weakly, effectively suppressing conduction and convection heat transfer. However, thermal radiation remains the major mode of heat transfer during normal operation. The objective of this study was to investigate the heat-transfer mechanisms in integral pressurized water reactor (IPWR)-type SMRs under various gas-filled conditions using computational fluid dynamics. The use of thermal radiation shielding (TRS) with a much lower emissivity material than the MCV surface was also evaluated. The results showed that thermal radiation was always the dominant contributor to heat loss (48–97%), while the conjugated effects of the gas candidates on natural convection and thermal radiation varied depending on their thermal and radiative properties, including absorption coefficient. The TRS showed an excellent insulation performance, with a reduction in the total heat loss of 56–70% under the relatively low temperatures of the IPWR system, except for carbon dioxide (13%). Consequently, TRS can be utilized to enhance the thermal efficiency of SMR designs by suppressing the heat loss through the MCV. •Quantitatively analyzed the final heat loss in a metal containment vessel under the different gap conditions using CFD code.•Confirmed that the dominant heat transfer mechanism in the gap is thermal radiation by analyzing conjugated heat transfer.•Confirmed that similar total heat losses among the non-absorbing candidates, indicating the replaceability of weak vacuum by gases.•Demonstrated that heat losses were effectively mitigated with the use of a low-emissivity coating such as an aluminum layer.•Showed that the effect of thermal radiation shielding was limited by the radiation absorption coefficient in the case of carbon dioxide.