Boron dilution reactivity transients, an overview of efforts

The US NRC staff reviewed several studies concerning rapid boron dilution during shutdown and SBLOCA events. Probabilistic analysis indicates that a rapid boron dilution event has a low frequency of occurrence. However, several potential mechanisms could lead to such an event. No three-dimensional (nuclear and hydraulic) analyses have been performed in recent years; also, analyses such as those reported in the paper are subject to considerable uncertainty, particularly regarding mixing. Consequently, these analyses must be viewed as conservative analyses. It is believed that the events reported can be prevented by the use of appropriate procedures which anticipate the possibility of dilution in various recognized situations and prevent it, or prevent the inappropriate starting of pumps until suitable mixing procedures are carried out