A review of freeze valves and freeze seals: Design, multiphysics modeling, and reliability

Freeze valves and freeze seals constitute a distinctive class of high-temperature flow-control and leakage-mitigation devices in which the working fluid itself is deliberately solidified to create a sealing barrier. Although the concept first emerged in liquid-metal reactor technology, especially in sodium systems, it has since reappeared in molten-salt reactors, passive shutdown devices, and concentrated solar power applications in increasingly diverse forms. This review restructures the literature around three questions: where the concept originated and how it evolved, what principal device families now exist, and which physical mechanisms govern their performance and remaining limitations. The survey covers annular stem freeze seals, flattened-pipe freeze valves, freeze flanges, passive melt-triggered shutdown elements, fin-enhanced concepts, hybrid thermally managed valves, and recent system-level reliability studies. Across the literature, valve behavior is shown to depend not only on local freezing and thawing conditions but also on wall conduction, natural convection, radiation, materials behavior, auxiliary heating and cooling systems, and broader loop integration. The synthesis identifies surface-directed melting, orientation-dependent natural convection, wall conduction, and auxiliary thermal management as the principal factors governing plug stability and opening performance, although several of the reported trends remain configuration-specific or primarily numerical. It also reveals persistent gaps in unified design criteria, fully coupled multiphysics modeling, and long-duration reliability assessment under reactor-relevant conditions.