During a Loss of Coolant Accident (LOCA) in a Pressurized Water Reactor, partial or complete drying of fuel assemblies may take place. In these conditions, the fuel temperature increases and may lead to substantial deformation of the fuel rod cladding and a partial blockage of the fluid sub-channels. These regions could significantly affect the cooling capacity of the nuclear core during the reflooding phase by the emergency core cooling systems. Understanding the cooling process and the thermal-hydraulic characteristics of the flow in these deformed regions is decisive to guarantee nuclear safety. Looking to provide valuable experimental data to validate existing and new models, this work is an experimental study on the thermal hydraulics during the cooling of a vertical tube with an internal steam-droplets flow, representing the LOCA conditions at sub-channel scale. The effect of the blockage ratio on the tube temperature, heat dissipation, wall rewetting, and droplets characteristics is evaluated by testing three configurations (0%, 61% and 90%). Optical techniques were used for a comprehensive characterization of the process, being them infrared thermography, phase-Doppler analyzer and three-color laser induced fluorescence thermometry. In general, wall rewetting in the test section occurs from bottom to top, although there is a discontinuity in the rewetting front with the 90% blockage ratio configuration. The droplets diameter reduces downstream of the test section because of evaporation. Droplets breakup was specifically observed with 90% blockage ratio. In all the cases, the droplets temperature was approximately the same up-and downstream of the test section, which indicates they are in nearly thermal equilibrium state and, therefore, representative of a LOCA situation.

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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During a loss of coolant accident (LOCA), blocked sub-channels may appear due to the swelling of the fuel rods' cladding, which results in flow redistribution during the reflooding phase. For this reason, special attention has been paid to the effect of fuel rods ballooning on the thermal-hydraulics in LOCA conditions. Due to the practically impossible physical or optical access to blocked sub-channels, no experiment so far has performed precise threecomponent velocity field measurements in the presence of ballooned regions. In this study, we used magnetic resonance velocimetry (MRV) to obtain three-component velocity fields of water flow within two 7x7 fuel rods bundles built mainly in plastic, one regular and one containing sixteen ballooned fuel rods with 90% blockage ratio and 240 mm blockage length. We present herein results with 50 lpm water flow rate. With the regular bundle, the performance of spacer grids' mixing vanes to homogenize the flow was notable. With the ballooned bundle, we observed transverse velocities upstream of the ballooned zone that are as intense as the bulk mean velocity. Furthermore, there are substantial decreases in the axial velocity within blocked sub-channels up-and downstream of the ballooned zone, reaching near-zero and even negative values downstream, indicating flow recirculation. Although the flow is highly affected by the ballooned zone, the mixing spacer grid placed downstream remarkably homogenized the flow and effects of the flow redistribution disappeared. Finally, with the present ballooned bundle configuration, about 90% of the flow that should pass through blocked sub-channel deviates towards less resistant regions, which suggests a predominant geometric effect on the flow redistribution.

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).