Post test analysis of ROSA-III double-ended break test RUN 901

The ROSA-III test facility is a volumetrically scaled (1/424) boiling water reactor (BWR) system with an electrically heated core. The purpose of ROSA-III experiment is to study the thermal-hydraulic behavior and the performance of the emergency core cooling system (ECCS) during a postulated loss-of-coolant accident (LOCA) and to provide the data base for the assessment and improvement of reactor safety analysis codes. RUN 901 was a first ROSA-III experiment with the fourth fuel assembly and assumed a 200 % double-ended break at the recirculation pump section line with full ECCS actuation. Post test analyses of RUN 901 were performed with the computer codes RELAP4J and RELAP5/MOD1/001. The system pressure response calculated with the two codes agreed well with the data. The agreement was also good for the core inlet flow behavior until the beginning of the lower plenum flashing (LPF). RELAP4J is a fast running code and calculated well the overall behavior of the mixture level in the core. However, the spray water was accumulated in the upper plenum (UP) due to the inability of the code to calculate counter current flow at the upper tie plate (UTP). While spray water was not accumulated in the upper plenum in the experiment. RELAP5 with an advanced two-phase flow model calculated well the rewetting of the fuel after the LPF and the top-down quench after the uncovery of the whole core. However, the incorporation of a CCFL model and/or the improvement in the interphase drag correlations are necessary to be able to calculate the mixture level behavior more accurately. An appropriate discharge coefficient is also necessary to calculate the break flow accurately with the RELAP5 characteristic analysis break flow model. (author)