Probability and consequences of rapid boron dilution in a PWR: A scoping study

This report documents the results of a scoping study of rapid dilution events in pressurized water reactors. It reviews the subject in broad terms and focuses on one event of most interest. This event could occur during a restart if there is a loss-of-offsite power when the reactor is being deborated. If the volume control tank is filled with water at a low boron concentration then a slug of this water could accumulate in the lower plenum. This would be the result of the trip of the reactor coolant pumps leading to relatively low flow conditions and the restart of the charging pumps on emergency power. The concern is that this diluted slug will rapidly enter the core after a reactor coolant pump is restarted and this could cause a power excursion leading to fuel damage. This problem was studied probabilistically for three plants and the important design features that affect the core damage frequency were identified. This analysis was augmented by an analysis of the mixing of the diluted water with the borated water already present in the vessel. The mixing was found to be significant so that neglect of this mechanism in probabilistic analysis leads to very conservative results. Neutronic calculations for one plant were carried out to understand the effect of nuclear design on the consequences of the event