Effect of Gas Entrainment on Liquid Maldistribution in the Freeze Valve Assembly of a Molten Salt Reactor Drain System

Abstract

The drain system functions to transfer fuel salt by gravity during shutdown of molten salt reactors (MSRs). This system has been recognized as a passive approach for enhancing reactor safety, and its performance and importance as a key safety component were confirmed through the Molten Salt Reactor Experiment (MSRE Despite its importance as a major safety feature, previous studies have primarily relied on simplified numerical models, and experimental investigations to evaluate and verify system performance have remained limited. In particular, potential phenomena such as gas entrainment observed in experiments can induce two-phase flow within the drain system, leading to siphon break and degradation of drain performance. These phenomena therefore constitute important considerations in drain system design. This study focused on analyzing the hydraulic behavior of the drain system and evaluating the effects of gas entrainment on liquid maldistribution within the freeze valve assembly as well as on overall drain system performance. A scaled-down mock-up experimental facility was constructed to simulate the hydraulic drain behavior of the MSR, and combined experimental and numerical results were used to characterize the drain behavior under gas entrainment conditions. A drain-time delay of approximately 10% was observed when vortex-induced gas entrainment occurred. When gas entrainment was initiated while the liquid level exceeded 13.2cm of the initial fuel-salt inventory, liquid maldistribution developed and resulted in an insufficient amount of salt remaining within the freeze valve assembly. Liquid maldistribution was quantitatively identified as being caused by siphon break induced by early gas entrainment. The identified hydraulic mechanism was subsequently extended to molten-salt conditions and evaluated through numerical analysis. An advanced freeze valve design was developed to effectively suppress liquid maldistribution. A cross-vane fin structure extending from the flat region of the freeze valve to the tank bottom was experimentally confirmed to be effective in suppressing vortex-induced gas entrainment. An 8 cm-high fin structure restricted the maximum vortex critical height to 8.8 cm and prevented liquid maldistribution under all experimental conditions. In addition, numerical analyses demonstrated that the increased heat-transfer area introduced by the fin structures reduced the valve opening time and consequently decreased the total drain time by more than 60 s, despite the additional pressure loss caused by the fins. This study highlights the significance of vortex-induced gas entrainment in drain systems and demonstrates the effectiveness of its suppression through an improved freeze valve design. The results provide fundamental insight and quantitative data to support the design of drain systems and freeze valves and ultimately contribute to enhanced safety and reliability of future MSR technologies.