Nathalie Seiler

SIMULATION OF RADIATIVE TRANSFER IN A NUCLEAR REACTOR DURING THE REFLOODING STEP IN A LOCA SITUATION

The Loss Of Coolant Accident (LOCA) is one referenc problem, investigated for the design of Pressurized Water Reactors (PWR) by the “ Institut de radioprotection et de Sûreté Nucléaire” (IRSN) in the frame of its resear ch program on nuclear fuel safety. In case of a break on the primary cooling loop, the co re of the reactor would be damaged due to pressure and water losses and then temperatu re increase, inducing a deformation of the fuel rods. The safety system provides a reflood ing of the reactor core by borated water. During the following transient regime a stro ng evaporation of water is observed carrying a large amount of water droplets, therefor e involving a vapor-droplet medium flowing between the hot rods. One safety criterion is to warrant that the rod temperature does not increase above 1204°C. In the whole heat t ransfer process, radiative transfer cannot be neglected and it has been evaluated as re presenting the same magnitude order than the other transfer modes (see Peak [1] or Wong and Hochveiter [2] for example). A finer evaluation of the radiative transfer contribu t on remains necessary and is the focus of the present work. This has been done in a 2D con figuration for the moment but should be extended to a realistic 3D geometry, and combine d with a global heat and mass transfer simulation using the thermohydraulic Neptu ne CFD code. It requires the capacity to perform accurate and efficient computations for the radiative transfer part (with reasonable computational cost). Owing to the medium of interest (vapor and droplets in a flow), the transfer has to be considered through an absorbing, anisotropically scattering, emitting, non grey medium. Such problem of radiativ e transfer in a 2D geometry has been long investigated with various tools, and with different accuracy levels. The problem can be split into two steps : (i) the compu tation of the radiative properties (which has been done here considering simple additivity of pr perties for pure water droplets obtained with the Mie theory [3], and absorbing pro perties for the vapor using a C-k model [4]); (ii) the solution for the radiative tra nsfer equation itself, for which numerous numerical possibilities are available. Considering the requirement of a solution with a low computational cost, approximate methods have be en preferred. The P1 approximation is well known to provide an efficient method, with satisfactory accuracy. However, its accuracy quickly decreases in the case of non optically thick media. A method derived from this P1 approximation has been chosen : the IDA (Improved Differential Approximation), following Modest [3], with the idea that the intensity may be split into two parts as follows :