Study on the integrity of prolonged steam oxidized clad column quenched at various reflood rates

Abstract

An unmitigated loss of coolant flow accident degrades the decay heat removal from the reactor core. Because of this situation, the temperature of the clad column escalates. The steam surrounding the cladding causes an exothermic oxidation reaction between the cladding surface and steam at elevated temperatures typically above 700 °C. Under such a scenario, the fire fighting water injects into the core to reduce the accident severity and arrest the clad temperature escalation. Usually, violent heat exchanges happen between the clad column and cold water during the late reflood quenching event. Furthermore, the expansion and subsequent shrinkage of the clad column could be structurally constrained. Thus, the oxidized clad column may be exposed to unchecked thermal shock during the reflood quenching of a severely heated core, causing clad failure or fracture.In this study, a 2x2 square lattice arrangement of the zircaloy-4 cluster is selected for the experiment. This is to study the quenching behaviour of the cluster prolonged heated (1150⁰ C- 1250⁰C) clad column in steam media, simulating late phase injection of DEC-A scenario. The bottom reflooding is carried out at flow rates between 3 and 10 L per minute, typical to a light water reactor SAMG action. This study explores the qualitative structural integrity of the clad column. The analytical hydrogen generation is also estimated for all cases using the Baker and Just correlation. The experimentally measured transient hydrogen generation rate agrees well with the analytical prediction. The maximum amount of equivalent cladding reacted is 27 % during heating. A renewed enhanced oxidation leads to a drastic shoot-up in the hydrogen concentration during the reflood quenching. Nevertheless, the maximum hydrogen shoot-up remains the same irrespective of the reflood rates. The clad column in each test case has failed during reflooding.