DRACCAR: A multi-physics code for computational analysis of multi-rod ballooning, coolability and fuel relocation during LOCA transients. Part Two: Overview of modeling capabilities for LOCA

Abstract

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).

An old issue and a new challenge for nuclear reactor safety

“…All of this left room to lack of directions for progress in the area and lack of common understanding within the technological community. Furthermore, nuclear fuel failure mechanisms characterized more recently and related modeling [13,14], confirm inadequacy of connection between LBLOCA scenario and licensing-design requirements.…”

Parametric effects on the flow redistribution in ballooned bundles evaluated by magnetic resonance velocimetry

“…These MRV experiments will complement the analysis of its main experimental campaign in the thermalhydraulics axis named COAL1, which consists of reflooding experiments in a partially ballooned bundle containing 49 elements with electrically-heated fuel rods [24]. Furthermore, the present study provides valuable data for the validation of numerical codes like IRSN's DRACCAR [25,26] and, more specifically, corroborate its pre-simulations of the COAL experiments [27,28]. Although we used specific geometries and test conditions for a hypothetical LOCA in a French PWR, the results herein presented provide important insights for any applications with tube bundles, like heat exchangers or fluid mixers.…”

Velocity field and flow redistribution in a ballooned 7×7 fuel bundle measured by magnetic resonance velocimetry

“…With the main experiment in the thermal-hydraulics axis, which is named COAL 1, it is expected to obtain measurements during reflooding experiments in a partially ballooned 49-element bundle with electrically-heated fuel rods [14]. In fact, IRSN's code named DRACCAR [15,16] was used to perform pre-simulations of the COAL experiments without validated models of flow redistribution [17,18]. Hence, the results of this experimental work will be used to develop and validate such a model.…”