Vapor explosions in light water reactors: A review of theory and modeling

Abstract

A vapor explosion is a physical event in which a hot liquid (fuel) rapidly fragments and transfers its internal energy to a colder, more volatile liquid (coolant); in so doing, the coolant vaporizes at high pressures and expands, doing work on its sorroundings. In present day fission reactors, if complete and prolonged failure of normal and emergency coolant flow occurs, fission product decay heat would cause melting of the reactor materials. In postulated severe accident analyses vapor explosions are considered if this molten “fuel” contacts residual water in-vessel or ex-vessel, because these physical explosions have the potential of contributing to reactor vessel failure and possibly containment failure and release of radioactive fission products. Vapor explosions are also a real concern in industrial processes where a hot fluid can contact a colder volatile fluid, e.g., foundries for aluminum and steel, paper pulping mills, LNG operations. The vapor explosion is commonly divided into four phases of heat transfer: (1) quiescent mixing of fuel and coolant, (2) triggering of the explosion, (3) explosion escalation and propagation, and (4) expansion and work production. This work provides a comprehensive review of vapor explosion theory and modeling in these four areas. Current theories and modeling have led to a better understanding of the overall process, although some specific fundamental issues are either not well understood or require experimental verification of theoretical hypotheses. These key issues include the extent of fuel-coolant mixing under various contact modes, the basic fuel fragmentation mechanism, and the effect of scale on the mixing process coupled to the explosion propagation and efficiency. Current reactor safety concerns with the vapor explosion are reviewed in light of these theories and models.