A unique molten salt reactor feature – The freeze valve system: Design, operating experience, and reliability

Abstract

Reliable mechanical valves that can withstand the corrosive and high-temperature conditions in Molten Salt Reactors (MSRs) have not yet been demonstrated. In their place, freeze valves (sometimes called freeze plugs) represent a unique nuclear design solution for isolating salt flow during operations. Additionally, in some cases, they are intended to perform safety-related functions. As implied by the name, operation of a freeze valve depends upon the controlled phase change of salt, and the ability of increased reactor heat from transients to induce modulation of the “valve” has frequently been equated to safety function passivity. In this article, a review of literature pertaining to freeze valve design, performance, and thermodynamic analyses is presented first. The Molten Salt Reactor Experiment (MSRE) was the first MSR to implement the use of freeze valves and is the only liquid-fueled MSR with significant operating experience; yet, the success of the MSRE freeze valve to thaw on demand or remain frozen during power operation depended on two subsystems comprising power-operated sensors, mechanical valves, and other active components. Accordingly, as executed at the MSRE, the freeze valve system was not a fully passive safety feature. Performance of original Process Hazards Analysis (PHA) studies on an MSRE safety-related freeze valve design indicates failure of multiple individual active components could result in the impairment of the freeze valve function, such that the freeze valve itself is a system of components – rather than a single component. Fault Tree Analysis (FTA) based on PHA results provides estimated failure rates for two important failure modes of freeze valves. The results suggest that designers of freeze valve systems may face a trade-off between the reliability of the freeze valve to thaw on demand and the reliability of the freeze valve to remain frozen during normal reactor operations; however, a sensitivity study showed that the likelihood of a spurious thawing of the MSRE freeze valve system could have been reduced without significantly affecting the likelihood of a failure to thaw.

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