Computational predictions concerning ballooning of multiple fuel pin bundles during a loss of coolant accident with a final reflooding phase are now more than ever of interest in the framework of light water reactor nuclear safety. To carry out these studies, two difficulties have to be overcome. First, the modeling has to take into account many coupled phenomena such as heat transfer (heat generation, radiation, convection and conduction), hydraulics (multidimensional 2-phase flow, blockage), mechanics (thermal expansion, creep, embrittlement) and chemistry (oxidation, hydriding). Secondly, there are only a few experimental investigations that can help to validate such complex coupled modeling. Over several years, IRSN has developed the 3D computational tool DRACCAR to investigate rod bundle strain during LOCA transients including prediction of the reflooding phase. DRACCAR code is dedicated to study complex configurations such as the deformation and possible contact between neighboring rods and the associated blockage of thermalhydraulic channels in the ballooned zone of the fuel assembly. Modeling efforts have been devoted to the assessment of the coolability of deformed geometries by coupling the thermo-mechanical behavior of the fuel assembly to the thermalhydraulics. The physical modeling available in the current version of DRACCAR V2.3.1 as well as its flexibility are depicted. As a conclusion, some prospects regarding the development of the future version DRACCAR V3 are provided, in particular accounting for the knowledge acquired through IRSN R&D project PERFROI.

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Computational predictions concerning ballooning of multiple fuel pin bundles during a loss of coolant accident with a final reflooding phase are now more than ever of interest in the framework of light water reactor nuclear safety. To carry out these studies, two difficulties have to be overcome. First, the modeling has to take into account many coupled phenomena such as heat transfer (heat generation, radiation, convection and conduction), hydraulics (multidimensional 2-phase flow, blockage), mechanics (thermal expansion, creep, embrittlement) and chemistry (oxidation, hydriding). Secondly, there are only a few experimental investigations that can help to validate such complex coupled modeling. Over several years, IRSN has developed the 3D computational tool DRACCAR to investigate rod bundle strain during LOCA transients including prediction of the reflooding phase. The DRACCAR code is dedicated to study complex configurations such as the deformation and possible contact between neighboring rods and the associated blockage of thermalhydraulic channels in the ballooned zone of the fuel assembly. To accompany the development of DRACCAR, efforts have been devoted to the validation of the coupling between the thermo-mechanics and thermalhydraulic models -including reflooding -through a comparison to integral experiments dedicated to LOCA. The DRACCAR capabilities and validation status are depicted for the version DRACCAR V2.3.1. DRACCAR provides an interesting insight on LOCA by simulating multi-rod and fluid interaction which cannot be investigated with a classical single rod approach. As a conclusion, some prospects regarding the development and validation of the future version DRACCAR V3 are mentioned. In particular significant evolutions are expected regarding the cladding rupture prediction, the contact simulation and the assessment of the coolability of deformed geometries. These evolutions will be based on the knowledge acquired through the R&D project PERFROI, a project dedicated to LOCA, launched by IRSN in association to other partners and supported by the French National Research Agency (ANR).

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