Modeling scheme of the Safety Injection Tank with Fluidic Device for best estimate calculation of LBLOCA

Abstract Fluidic Device (FD) within the Safety Injection Tank (SIT) of APR1400 plants is designed to prolong the passive injection period and to reduce the bypassed water following a LBLOCA by subdividing the injection phase into high flow and low flow. The present paper is to propose an improved modeling scheme of SIT/FD to best fit the data of SIT blowdown test conducted at Shinkori Unit 3 using a system thermal-hydraulic code, MARS-KS. The potential to nitrogen release during the plant test is then evaluated based on the prediction. And a LBLOCA of APR1400 is calculated using several SIT/FD modeling schemes tested. Through the comparison of those predictions, effect of modeling schemes on cladding thermal response is discussed. Currently, the plant test can be best simulated by the modeling scheme using ‘pipe’ component with dynamic closing of standpipe flow path. The LBLOCA analysis adopting the best modeling scheme showed the increase of the reflood PCT when compared to the case of ‘accumulator’ component model, which was due to the flow rate decrease during transition phase from high flow to low flow and the intrusion of nitrogen gas to the core.