Stand-Alone Containment Analysis of the Phébus FPT Tests with the ASTEC and the MELCOR Codes - The FPT-0 Test

During the last 40 years, several efforts have been carried out to investigate the different phenomena occurring during a Severe Accident (SA) in a Nuclear Power Plant (NPP). Such efforts have been supported with the execution of different experimental campaigns investigating only specific phenomena or the coupling among two or more of them. The integral Phébus tests were probably one of the most important experiences in this field. In these tests, the degradation of a PWR fuel bundle was investigated employing different control rod materials and burn-up levels in strongly or weakly oxidizing conditions. Such 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 SA in a real NPP. Two of the main SA codes developed also basing on the findings of the Phébus tests were the ASTEC and the MELCOR codes. In the latest years, these two codes were furthermore expanded to implement the more recent findings after the termination of the Phébus experimental campaign. Therefore, a continuous verification and validation work is necessary to check that the new improvements introduced in such codes allow also a better prediction of these Phébus tests. Therefore, the aim of the present work is to re-analyze the Phébus FPT-0 test employing the up-to-date ASTEC and MELCOR codes. The analysis focuses only on the stand-alone containment aspects of this test, and three different spatial nodalizations of the containment vessel have been developed, showing that at least 15/20 Control Volumes (CVs) are necessary for the spatial schematization to predict thermal-hydraulics and the aerosol behavior. Then, the paper summarizes the main thermal-hydraulic results and presents different sensitivity analyses carried out on the aerosols and Fission Products (FPs) behavior. When possible, a comparison among the results obtained in this work and by different authors in previous works is also performed. This paper is part of a series of publications covering the four executed Phébus tests employing a solid PWR fuel bundle: FPT-0, FPT-1, FPT-2, and FPT-3.