Thermal–hydraulic phenomenology analysis and RELAP5/MOD3.3 code assessment for ATLAS 1% upper head SBLOCA test

In 2002, vessel head wall thinning that was identified at the Davis Besse reactor in the US shaded the lights on the importance of SB-LOCA’s in the upper head of RPV. The lower rates of coolant discharge through the break and primary circuit depressurization are the main characteristics in which SB-LOCA’s differ from LB-LOCA’s, and throughout the transient, three distinct and independent occurrences of core heat-up’s can potentially take place. SB-LOCA’s are troublesome scenarios for the development of EOP’s and modelling capabilities to adequately predict the overall T-H system response. This thesis contributes to the nuclear reactor safety from the thermal-hydraulic perspective, and it aims to develop and enhance a computational model of the ATLAS IET facility for the T-H system code RELAP5/MOD3.3. By applying the UPC-ANT Uncertainty Analysis methodology for code assessment, thermal-hydraulic phenomenology analysis is performed to analyze the model adequacy and accuracy to reproduce the most relevant T-H phenomena involved in Test B5.1 at the ATLAS facility, which is an SB-LOCA in the upper head. The work presented in the thesis is within the framework of UPC participation to the OECD/NEA ATLAS joint project 2. A variety of verification procedures are applied which focus on the statistical characteristics of sampled populations, the validity and robustness of the developed ATLAS-RELAP5 model by performing “null-transient” calculations. The implemented code assessment procedure is based on the adequacy of both the base-case and BEPU analysis calculations. It is concluded that the base-case calculation reproduces all the relevant T-H phenomena, either totally or partially. In essence, core heat-up and uncovery takes place ealier as compared to the experiment, about 900 seconds, due to quick clearing and refilling of loop-seals; thus, later events are accelerated in the base-case calculation. The BEPU analysis is generally adequate to perturb the relevant T-H phenome