Thermal Hydraulic and Mechanical Aspects of In-Vessel Retention of Core Debris

Several advanced water reactor designs incorporate provisions to manage the escaped core debris outside the reactor vessel. This type of ex-vessel management approach involves a difficult task of core debris cooling, direct containment heating and steam explosion control inside the containment building. If one can secure the integrity of the lower head by maintaining coolability within the reactor, core meltdown accidents will be arrested within the vessel at a significant advantage over the ex-vessel measures. This paper examines potential options for in-vessel retention for large PWRs as functions of time spent for the wetting of the external wall of the lower head. Several important options were evaluated utilizing detailed thermohydraulic and mechanical analysis models that have been developed and benchmarked against experimental data. Results elucidate that the risk of lower head failure by either creep rupture or quench fracture increases rapidly with the amount of time spent till external wall wetting. To expedite rapid wetting of the lower head without installation of major pumping systems and large inventory of cooling water, gap structures have been proposed inside and outside the reactor vessel lower head. The external gap structure, designated as COASISO, is shown to be an effective tool for in-vessel retention of core melt for both new and operating reactors during a hypothetical severe accident. (authors)