Abstract
In the context of the simulation of the Severe Accidents (SA) in Light Water Reactors (LWR), we are interested on the in-core corium pool propagation transient in order to evaluate the corium relocation in the vessel lower head. The goal is to characterize the corium and debris flows from the core to accurately evaluate the corium pool propagation transient in the lower head and so the associated risk of vessel failure. In the case of LWR with heavy reflector, to evaluate the corium relocation into the lower head, we have to study the risk associated with focusing effect and the possibility to stabilize laterally the corium in core with a flooded down-comer. It is necessary to characterize the core degradation and the stratification of the corium pool that is formed in core. We assume that the core degradation until the corium pool formation and the corium pool propagation could be modeled separately. In this document, we present a simplified geometrical model (0D model) for the in-core corium propagation transient. A degraded core with a formed corium pool is used as an initial state. This state can be obtained from a simulation computed with an integral code. This model does not use a grid for the core as integral codes do. Geometrical shapes and 0D models are associated with the corium pool and the other components of the degraded core (debris, heavy reflector, core plate…). During the transient, these shapes evolve taking into account the thermal and stratification behavior of the corium pool and the melting of the core surrounding components. Some results corresponding to the corium pool propagation in core transients obtained with this model on a LWR with a heavy reflector are given and compared to grid approach of the integral codes MAAP4.
Highlights
In the context of Light Water Reactors (LWR) Severe Accidents (SA) analysis and management strategies evaluation, a key element is the phenomenology associated with a corium pool that can be formed after the loss of the primary coolant and the induced core degradation
Core degradation [10,11,12] is a combination of complex physical phenomena which could result in the corium pool formation in core and after to its propagation and its relocation in lower head
From the TMI observations, we assume that the core is composed of the following degraded components: the upper plate, the core support plate, the heavy reflector, the corium pool, an empty volume and the core debris
Summary
In the context of Light Water Reactors (LWR) Severe Accidents (SA) analysis and management strategies evaluation, a key element is the phenomenology associated with a corium pool that can be formed after the loss of the primary coolant and the induced core degradation. For LWR with heavy reflector, another modelling issue that justifies the study of corium pool propagation in core is to know the mode of corium transfer from the core to the vessel (through reflector, or core support plate or both) It has consequences on the relocation of the corium in the lower head (instant, duration, mass, composition) and it is linked to the evaluation of focusing effect in the corium pool in core. To take into account the complex physical phenomena, and to manage the high level of uncertainties and the various time and spatial scales involved during the corium pool propagation in a SA, the Commissariat à l’Énergie Atomique (CEA) has developed a coupled physical and statistical framework named PROCOR (stands for “PROpagation of CORium”) This tool is based on a simplified physical modelling and the experience gained on the LEONAR code [14].
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