Georges Repetto

Analyses of the Phébus FPT3 Experiment Using the Severe Accident Codes ATHLET-CD, ICARE/CATHARE, and MELCOR

Abstract The aim of the Phébus Fission Product (FP) experimental program is to study the degradation phenomena and the behavior of the FPs released in the reactor coolant system and the containment building. The program consists of four in-pile bundle tests (FPT0, FPT1, FPT2, and FPT3), performed under different conditions concerning the thermal hydraulics and the environment of fuel rods, in particular, the amount of steam (strongly or weakly oxidizing atmosphere). The last test of this program, FPT3, was performed in November 2004 in Cadarache. During the FPT3 experiment, for the first time, boron carbide (B4C) was used as the absorber material instead of Ag-In-Cd, which was used in all the previous tests. Boron carbide is used in western-type pressurized water reactors, the EPR, boiling water reactors, and the VVER; consequently, assessing the effects of B4C on the main degradation phenomena and on gas release, as well as its impact on FP behavior is very important. This paper describes results from the Phébus FPT3 experiment, summarizes the test code modeling used in the different code applications, and reports the code results comparing some important experimental parameters, in particular regarding B4C control rod behavior. The severe accident codes used in these studies are Analysis of Thermal-Hydraulics of LEaks and Transients with Core Degradation (ATHLET-CD), ICARE/CATHARE, and MELCOR. The first part is an overview of the experimental results (boundary conditions, temperature evolutions, hydrogen and carbon compound releases coming from the oxidation of the Zircaloy claddings and the B4C absorber, and bundle degradation). The second part summarizes the code modeling used in the different code applications, in particular, those regarding absorber rod degradation and the oxidation process. The third part summarizes the code results comparing some important experimental parameters [thermal behavior, gas releases (H2, CO, CO2), and bundle degradation]. The conclusion focuses on the capabilities of the severe accident codes to simulate control rod behavior in a fuel rod assembly during the course of a severe accident transient.

Code Simulation of Quenching of a High Temperature Debris Bed: Model Improvement and Validation With Experimental Results

The loss of coolant accidents with core degradation e.g. TMI-2 and Fukushima demonstrated that the nuclear safety analysis has to cover accident sequences involving a late reflood activation in order to develop appropriate and reliable mitigation strategies for both, existing and advanced reactors. The reflood (injection of water) is possible if one or several water sources become available during the accident. In a late phase of accident, no well-defined coolant paths would exist and a large part of the core would resemble to a debris bed e.g. particles with characteristic length-scale: 1 to 5 mm, as observed in TMI-2. The French “Institut de Radioprotection et de Surete Nucleaire” (IRSN) is developing experimental programs (PEARL and PRELUDE) and simulation tools (ICARE-CATHARE and ASTEC) to study and optimize the severe accident management strategy and to assess the probabilities to stop the progress of in-vessel core degradation at a late stage of an accident. The purpose of this paper is to propose a consistent thermo-hydraulic model of reflood of severely damaged reactor core for ICARE-CATHARE code. The comparison of the calculations with PRELUDE experimental results is presented. It is shown that the quench front exhibits either a 1D behavior or a 2D one, depending on injection rate or bed characteristics. The PRELUDE data cover a rather large range of variation of parameters for which the developed model appears to be quite predictive.Copyright