SB LOCA thermal-hydraulic analyses for Krsko Full Scope Simulator validation

The Krsko nuclear power plant (NPP), which is a two-loop pressurized water reactor, Westinghouse type, before modernization in 2000 obtained plant specific full scope simulator. The purpose of the presented analyses was to perform Small Break Loss of Coolant Accident (SBLOCA) reference calculations for KFSS validation in 2004. In addition, the thermal-hydraulic response of the reactor coolant system (RCS) was studied in detail. For the thermal-hydraulic analysis the RELAP5/MOD3.3 code and input model delivered from Krsko NPP were used. The RELAP5 calculated reference results showed that the plant system response to breaks with small break area is slower compared to breaks with larger break area. The comparison of the KFSS data with calculated results suggest that the simulator validation testing in the year 2004 for this kind of accident was successful. Nevertheless, when comparing the physical phenomena and processes, the RELAP5/MOD3.3 predicted smaller core uncovery compared to the KFSS measurement. One reason is different core cycles. Finally, this finding suggests that even for simulator reference calculations the quantification of model uncertainties would be useful. (author)