Thermal and Structural Analysis of Calandria Vessel of a Phwr during a Severe Accident

Pressurized Heavy Water Reactors (PHWRs) have engineered safety systems to restore the reactor to a safe state in the event of an accident. Most of the safety systems have diverse and redundant features. The purpose is to enhance the availability of these safety systems so that failure of one of them does not lead to a progression of accidents. Besides, the reactors have multiple barriers to arrest the release of activity if a hypothetical accident progresses with the failure of the safety systems. In spite of all these, a low probability accident can be cast beyond the acceptable design basis envelope, which can progress to a severe accident. Severe accidents which arise due to multiple failures of safety systems are beyond design basis accidents and lead to significant core degradation. Whatever be the initiating event, a severe accident occurs when the heat generation rate exceeds the heat removal rate. The fuel is unable to get cooled and gets damaged, leading to its melt down. The severity of such an accident depends on the nature and extent of the core damage during the accident progression. Of course, these accidents have very low probability of occurrence, but certainly they have high risk in terms of consequences to the plant personnel and public. In view of this, most of the new generation reactors are designed with built-in severe accident management strategies. The nuclear utilities also plan to extend or build additional safety features in existing plants in order to enhance the defense-in-depth of these plants so as to cope with such severe accidents. Several scenarios can be postulated, which lead to such core melt down accidents. It can be initiated with a Limited Core Damage Accident (LCDA) which can involve single or multiple channels. For example, a feeder break can result in overheating of the fuel in the affected channel. Or a loss of coolant accident (LOCA) with loss of the Emergency Core Cooling (ECC) system could lead to widespread fuel damage [1]. In both cases, if moderator is available as a secondary heat sink, it can prevent the failure of fuel channels and core degradation. For an accident to proceed to significant core damage, there must be failure of multiple fuel channels. This is possible with the loss of moderator as a heat sink in the case of prolonged Station Black Out (SBO) [2]. The Severe Core Damage Accident (SCDA) may be initiated from an LCDA and progress to significant core damage with the non-availability of moderator as a heat sink. Hence, the early stage of SCDA in heavy water reactors is different from that of a light water reactor in that the presence of a heavy water moderator slows down the progression of core disassembly. In a postulated severe core damage accident in a PHWR, multiple failures of core cooling systems may lead to the collapse of pressure tubes and calandria tubes, which may ultimately relocate inside the calandria vessel forming a terminal debris bed. The debris bed, which may reach high temperatures due to the decay heat, is cooled by the moderator in the calandria. With time, the moderator is evaporated and after some time, a hot dry debris bed is formed. The debris bed transfers heat to the calandria vault water which acts as the ultimate heat sink. However, the questions remain: how long would the vault water be an ultimate heat sink, and what would be the failure mode of the calandria vessel if the heat sink capability of the reactor vault water is lost? In the present study, a numerical analysis is performed to evaluate the thermal loads and the stresses in the calandria vessel following the above accident scenario. The heat transfer from the molten corium pool to the surrounding is assumed to be by a combination of radiation, conduction, and convection from the calandria vessel wall to the vault water. From the temperature distribution in the vessel wall, the transient thermal loads have been evaluated. The strain rate and the vessel failure have been evaluated for the above scenario.