The scaling effect analysis of LSTF/ATLAS test facilities on 1% Reactor Pressure Vessel Top Head Break Loss of Coolant Accident via OECD-ATLAS project

Abstract

In 2002, one of the serious incidents happened at Davis Besse NPP, where a SBLOCA occurred at the top head of RPV. This was examined in test facilities for the first time in 2005 using LSTF of Westinghouse design. KAERI, the Operating Agency of ATLAS project, decided to conduct B5.1 test, which reflects the accident scenario of SB-PV-07 LSTF. It is the first test of its kind reflecting the impact of top head break in APR1400 design. The analysis of this study is broken into two parts. In the first part, we analyze B5.1 test in pre-and post-test calculations using RELAP5/MOD3.3 system code. Pre-test calculation showed a high discrepancy of RELAP5 simulation against the experiment. Whereas, in post-test calculation, six sensitivity cases were implemented to improve the simulation results. Discharge coefficient at break, break line modeling, environmental heat loss on secondary side, Core Exit Temperature set point and CCFL at Fuel Alignment Plate showed a large impact in improving the simulation results in terms of total integrated mass from break and timings of accident managements. Sensitivity cases that were implemented around downcomer level could not improve LSC timing as opposed to experiment. From the results, LSC was not predicted well by RELAP5 code. The second part of the analysis looked at comparing test results of ATLAS and LSTF with consideration of scaling parameters. The two tests predict the scenario differently, ATLAS showed a longer transient compared to scaled LSTF. RELAP5 code was used to simulate LSTF. It generally captured the sequence of event, however, it overpredicted the upperhead level. For fair comparison between LSTF and ATLAS facilities, scaling parameters were reviewed and re-modeled for upper head, outlet plenum, and CRGT elevation. ATLAS modifications towards LSTF showed relatively a negligible impact of scaling parameters and importance of design differences between the two facilities, while the design of CRGT has a large impact on changing ATLAS result towards LSTF. It was concluded that the design difference between Westinghouse and APR1400 determines the sequence of the transient.

Date of Award	Dec 2020
Original language	American English