CATHARE2 analysis on the loss of residual heat removal system during mid-loop operation : pressurizer and SGI outlet plenum manways open

The present study is to analyze the BETHSY test 6.9c using CATHARE2 v1.3u. BETHSY test 6.9c simulates plant conditions following loss of residual heat removal system under mid-loop operation. The configuration is that the pressurizer and steam generator outlet plenum manways are opened as vent paths in order to protect the system from overpressurization by removing the steam generated in the core. Most of the important physical phenomena are observed in the experiment have been predicted reasonably by the CATHARE2 code. Since the differential pressure between the pressurizer and the surge line is overestimated, the peak pressure in the upper plenum is predicted higher than the experimental value by 11 kPa and occurrence is delayed by 210s. Also earlier core uncovery is predicted, mainly due to overprediction of the manway flows. The analysis results are demonstrated that opening of the pressurizer and the steam generator outlet plenum manways is effective to prevent the core uncovery by only gravity feed injection. Although some disagreements found in detailed phenomena, the prediction of the overall system behavior by the code does not deviate from the experimental results unacceptably. The core bypass flowrate is found to be very sensitive to mass distribution in the core and the system behaviors are strongly affected by phase separation modeling under low pressure and particularly stratified flow condition. the main purpose of the present study is to understand physical phenomena under the accident and to assess the capability of CATHARE2 prediction for enhancement of reliability in actual plant analyses. (author). 11 refs., 3 tabs., 41 figs