RELAPS/MOD3 analysis of a ROSA-IV/LSTF loss-of-RHR experiment with a 5% cold leg break

An analysis was performed using the RELAP5/MOD3 version 3, I ,2 code for a loss of residual heat removal event during midloop operation after reactor shutdown in a Westinghouse-type pressurized water reactor. An event during a main coolant pump shaft seal removal operation was analyzed using the data of a ROSA-IV LSTF experiment that simulated the event with a 5ʹ% cold leg break. The predicted time when boiling started in the core and the peak primary system pressure showed good agreements with the measured data. The late loop seal clearing, however, was calculated because the steam condensation in the U-tubes started early. A large mass error appeared in the transient calculations because of numerical truncation errors. The influence of the mass error was insignificant on the primary pressure prediction in this analysis