BEPU analysis of a passive decay heat removal system with RELAP5/MOD3.3 and RELAP5-3D

Passive safety systems are currently implemented or under consideration in several advanced nuclear plants. In the Best-Estimate Plus Uncertainty (BEPU) approach it is fundamental to qualify Best-Estimate thermal-hydraulic system codes and models for the phenomena typical of passive systems and to quantify the uncertainty of calculations. In the present paper, through RELAP5/MOD3.3 and RELAP5-3D codes, thermal-hydraulic analyses were carried out and the obtained results compared against the experimental data collected in the experimental test facility PERSEO. For the present analysis, Test 7 Part 2, conducted at the nominal primary pressure of 7 MPa, has been selected. An accuracy evaluation has been performed on the reference calculation carried out with the two codes. For the accuracy evaluation both a qualitative and quantitative approach have been considered. The uncertainty of both calculations has been quantified by means of the probabilistic method of propagation of input uncertainties. Uncertain input parameters have been selected considering initial and boundary conditions, physical parameters and model uncertainties. The Uncertainty Analysis has been carried out through the RAVEN framework developed by Idaho National Laboratory. Additionally, regression and correlation indices have been used to characterize the importance of the selected uncertain input parameters for the selected Figure Of Merit. The effect of sampling methodologies on the results has also been evaluated. •RELAP5 and RELAP5-3D nodalizations of the PERSEO facility have been developed.•The code accuracy has been evaluated qualitatively and quantitatively.•An uncertainty analysis has been carried out adopting the RAVEN framework.•Correlation and regression indexes have been compared.•The effect of the uncertain parameters sampling has been analyzed.