Approach for Rod Burst Ratio Evaluation during Loss-Of-Coolant Accident with DRACCAR software

Abstract

In the frame of the “Reduction of Radiological Consequences of design basis and extension accidents” (R2CA) European Union’s project, the French Institute for Radiation and Nuclear Safety (IRSN) has developed an approach to evaluate more realistically the rod burst ratio (RBR) during loss-of-coolant accidents (LOCA) and the associated radiological releases. The approach is based on the IRSN DRACCAR and ASTEC software. The DRACCAR model consists in a reactor coolant system thermalhydraulic model coupled to a core model using 3D thermal hydraulics and describing each fuel assembly by several 2D (r,z) lumped rods. This model can capture distinctive fuel assembly responses by coupling thermal hydraulics to thermomechanic. Radiological consequences can then be evaluated by a chain between DRACCAR and ASTEC software. Some highlights on DRACCAR core model are provided by discussing results obtained on a PWR intermediate break loss-of-coolant accident scenario. Several burst criteria are tested on this demonstrative case through a sensitivity study. In this work, the influence of burst criterion selection on the rod burst ratio prediction was highlighted and the rod burst prediction varied strongly with the difference between predicted cladding temperatures and burst temperature criterion. Therefore, the methodology should be improved by managing uncertainties quantification associated to LOCA phenomena as in the Best-Estimate Plus Uncertainty core coolability analysis. Finally, some prospects for the DRACCAR software are identified to bring some advances of the presented approach.