Loss of residual heat removal during PWR mid-loop operation

Loss of residual heat removal (RHR) during a mid-loop operation, i.e., operation of RHR with the primary inventory partially drained after reactor shut-down, has occurred in several PWR plants. The primary causes were a loss-of-AC power or air binding of the RHR pump. Some of these events resulted in boiling of the reactor vessel inventory and suggested the possibility that the coolant inventory would boil away to uncover the core, for a prolonged event. In JAERI, experiments simulating loss-of-RHR events were conducted at the ROSA-IV Large Scale Test Facility (LSTF). Results obtained from an experiment with an opening in the cold leg showed core boiling and, then, core uncoveries, first associated with the loop seal clearing and subsequently due to long-term core boil-off. The influence of the noncondensible gas (air) was found to be significant for the steam condensation behavior especially in the SG U-tubes. This greatly affected the primary pressurization rate and also the loop seal clearing phenomenon. CAHTARE 2 code post-test analysis resulted in qualitative agreement with the data. (author)