Stand-alone containment analysis of the Phébus FPT-0 test with the ASTEC V2.1 and the MELCOR v2.2 codes

During the last 40 years, several efforts have been carried out to investigate the different phenomena occurring during a severe accident in a Nuclear Power Plant (NPP). Within this framework, the execution of different experimental campaigns, investigating only specific phenomena or the coupling among two or more phenomena, has been one of the main activity and the integral Phébus FP tests were probably the most important experiences in this field. In these tests, the degradation of a PWR fuel bundle and the related phenomena in the primary circuit and in the containment system were investigated, employing different control rod materials and fuel burn-up levels in strongly or weakly oxidizing conditions. Such Phébus integral tests were of fundamental importance to understand the key aspects of each phenomena and to develop numerical codes capable to simulate the evolution of a severe accident in a real NPP. Two of the main codes international employed (ASTEC and MELCOR) for severe accident analysis were intensively benchmarked basing on the findings of the different Phébus FP tests. In the latest years, these two codes were furthermore improved, to implement the more recent research findings after the termination of the Phébus experimental campaign, as the results obtained in the SARNET projects. Therefore, a continuous verification and validation work is still needed for the codes to check that the new improvements introduced in such codes really allow a better prediction of the Phébus tests and of the other tests forming the validation test matrix. The aim of the present paper is to re-analyze the first Phébus FPT-0 test employing the latest ASTEC V2.1 and MELCOR V2.2 code versions. The performed analysis focuses on the thermal-hydraulics /aerosol coupling, and only the stand-alone containment aspects of the test have been investigated. Three different spatial nodalizations of the Phébus containment vessel have been employed, showing that at least 15/20 control volumes are necessary for the vessel spatial schematization to correctly predict thermal-hydraulics and aerosol behavior. Furthermore, the paper summarizes the main thermal-hydraulic results and presents the different sensitivity analyses carried out on the iodine and aerosols behavior. When possible, a comparison among the results obtained during this work and by different authors in previous works is also performed, to highlight the improvements in the physical models implemented in the two codes.