Phenomena Occurring in the Reactor Coolant System during Severe Core Damage Accidents

The physical and chemical processes occurring within the reactor coolant system (RCS) during normal operation of the reactor are relatively uncomplicated and are reasonably well understood. Phenomena occurring within the RCS even after the pressure vessel has been breached can influence the consequences of the accident if they result in additional releases of radiotoxic materials after the reactor containment has been breached. Paramount among these phenomena is the revaporization of material that had become deposited on the RCS surfaces in the earlier phases of the accident. In contrast to normal operating conditions, severe core damage accidents are characterized by significant temporal and spatial variations in heat and mass fluxes, and by eventual geometrical changes within the RCS. Severe core damage accidents are believed to result typically from the failure of an engineered safety feature (ESF) to respond to a reactor upset condition, either because of failure of the ESF to perform its intended function, or because of its deliberate disablement.