Gallium Passive Decay Heat Removal Systems Design and Evaluation for IVR-ERVCS of APR 1400 and PDHRS of UCFR-100

Abstract

The passive decay heat removal system is one of the important concepts of nuclear power plants. Because the principle of the system is based on laws of physics such as gravity or natural convection, it is able to function without electric power or actuation by control equipments. In this paper, the liquid metal, gallium-cooled passive decay heat removal systems of pressurized water reactor (PWR), APR 1400 and Ultra-long-life-Core fast Reactor, UCFR-100 were suggested and simulated using MARS-Ga code. The experimental study to investigate the natural convective heat transfer was also conducted. Although MARS-LMR code was originally intended for a safety analysis of liquid metal-cooled fast reactor such as a sodium-cooled fast reactor, gallium properties were newly added to this code as so-called MARS-Ga code which is applicable for gallium cooled systems. The implementation of the properties for liquid metals in MARS-LMR code used the soft-sphere model based on Monte Carlo calculations for particles interacting with pair potentials, and the transport properties such as surface tension, thermal conductivity, and dynamic viscosity for the liquid and vapor state were also included. In the experimental analysis, the natural convection heat transfer of liquid gallium was investigated in the rectangular loop which consists of an indirect heating block test section, a condenser, and the 1/2 inch SS 316L tubes as well as the orifice for measuring mass flowrate. The average Nusselt number of liquid gallium was measured within the heat flux range of 6.17×103 ? 5.07×104 W/m2. The mass flow rate for the natural convection of liquid gallium depending on power level was also compared by using CFD and MARS-Ga code. In the numerical analyses, the evaluations of the gallium-water in-vessel corium retention through external reactor vessel cooling system (IVR-ERVCS) in APR 1400 and gallium-cooled passive decay heat removal system (PDHRS) in UCFR-100 were performed using MARS-Ga code. The attractive properties such as the low melting point, the high boiling point, and no reaction with water ensure that gallium can play an important role in nuclear safety as an alternative coolant in PWRs and SFRs. In the gallium-water IVR-ERVCS, the generated decay heat is transferred to liquid gallium through the reactor pressure vessel and then removed from the water pool as a heat sink. The numerical analysis results showed that the temperature range of the liquid gallium is much lower than its boiling point and confirm the natural convection under a medium break loss of coolant accident (MBLOCA) and large break loss of coolant accident (LBLOCA). Because liquid gallium in this system didn’t have a phase change, unlike water, the gallium-water IVR-ERVCS can provide stable and reliable cooling capability. Sensitivity studies were also performed by changing several parameters such as the initial temperature of liquid gallium and water pool inventory, and their results indicated that the working time of the gallium-water IVR-ERVCS depends on the inventory of the water pool using MARS-Ga code. UCFR-100 is a 260MWth / 100MWe sodium-cooled fast reactor which requires no on-site refueling. UCFR-100 is a pool type reactor including the metallic fuels, intermediate heat exchangers, steam generators, and gallium-cooled PDHRS unlike the existing designs of sodium fast reactors. The safety analysis was performed for loss of flow (LOF) due to the pumping failure of primary pumps using MARS-Ga code. As a result, it confirmed that the liquid gallium can work properly as a boundary material between sodium and atmosphere for steady state and transient situation in UCFR-100.