Studies of mixed convection effect in water cooled small modular reactor with natural circulation

Abstract

After the Fukushima accident, there is an increased interest in a nuclear reactor’s reliance on passive safety systems. Most of the existing passive systems utilize buoyancy force to drive the cooling flow. Previous research works have found that heat transfer can be deteriorated when the driving force of a system is shifted from external force (forced convection) to self generated buoyancy force or a combination of both (natural or mixed convection). Recently nuclear reactor development is focusing more on a Small Modular Reactor (SMR) design, and SMRs can adopt passive safety system more easily due to increased cooling water volume to power ratio. Thus, an operating heat transfer regime analysis for water cooled reactors is performed to see the effect of size reduction and natural circulation mode to the reactor`s operating regime. The analysis results show that Pressurize Water Reactor (PWR) has a tendency to fall to Deteriorated Turbulent Heat Transfer (DTHT) regime under such circumstances. Therefore, there is an urge need to update the existing system analysis code by including the mixed convection regimes as the development of SMRs and passive systems are growing further. For this purpose, a proper mixed convection turbulent heat transfer correlation for water system is needed. A numerical analysis is performed using ν2-f turbulence model to see the performance of the existing heat transfer correlation. The numerical analysis results show that the selected correlation may not predict well in high length to diameter ratio (L/D ≥ 50) which implies that further experimental investigation and development of better correlation maybe necessary for future SMR development.