Numerical analysis and codes qualification based on SIRIO experiments within the PIACE project

Within the roadmap for the technological development of Generation-IV reactors, the HORIZON2020 EC funded PIACE project has the main objective to support the technology transfer from the research to industry in the area of safety of nuclear installations. In particular, the project supports the development of an innovative decay heat removal system, currently under technology validation in the SIRIO facility. This system is conceived to provide a passive safe long term cooling, as well as to passively control the heat removed from the ultimate heat sink by means of a self-controlled injection of non-condensable gases. In order to test such features, an experimental campaign has been executed on SIRIO facility. A dedicated task has been addressed to the post-test analyses by numerical tools based on the experiments performed. The tests have been simulated by the project participants for a benchmark exercise using system thermalhydraulic codes. This activity has been divided into two phases: a blind phase, and a post-test phase. The present paper illustrates the results achieved during the post-test phase of the SIRIO benchmark exercise. The numerical models have been calibrated to better match the experimental findings, increasing the prediction accuracy and the understanding of both the best modelling practices and the capability of empirical models implemented in the system codes to predict the system behavior. The code qualification activity hereafter presented represents a relevant exercise for evaluating and comparing the predictive capabilities of system thermal-hydraulic codes, in relation to phenomena occurring in Gen-IV passive decay heat removal systems.