Boric acid is introduced into the reactor coolant to control long term reactivity. Forced coolant circulation during normal operation ensures that the boric acid is homogeneously distributed in the reactor coolant system (RCS) so that the boron concentration is practically uniform. Decrease of the boron concentration results in an increase of the reactivity. Causes for decreasing of boron concentration are injection of coolant with less boron content from interfacing systems (external dilution) or separation of the borated reactor coolant into highly concentrated and diluted fractions (inherent dilution). Examples of external dilution are the injection of coolant of reduced boron concentration by the makeup system, and injection of low-boron pump sealing water into the primary system. Inherent dilution can occur after reflux condenser heat transfer or back flow from the secondary system in case of primary-to-secondary leakage accidents.

Operation in the reflux condenser mode over a lengthy period of time could occur in the event of small-break loss-of-coolant accidents (SB-LOCA) concurrent with limited operability of the emergency core cooling (ECC) systems. In such an event the condensate descending down the cold — leg steam generator (SG) tubing into the SG outlet plenum and from there into the pump seal could form slugs of low-boron water. On restoration of natural circulation after refilling of the reactor coolant system such slugs would be transported towards the reactor core. However, on their way to the core, they would be mixed in the cold-leg piping, the reactor pressure vessel (RPV) downcomer and the lower plenum and thus increase in boron concentration.

Restarting of a reactor coolant pump (RCP) after a SB-LOCA or a SG tube rupture (SGTR) is very unlikely to occur as such events can be clearly identified on the basis of measured data and starting of a RCP is an action which would provide several individual actions and therefore some time. Assuming that an inadvertent demineralized water injection into one loop were to occur before starting of the RCP in this loop in spite of the monitoring and measurement of the boron concentration of the water injected into the RCS, a slug of demineralized water moves towards the core inlet after pump start.

Mixing of the diluted slug with the ambient coolant of higher boron content provides the only mitigation mechanism before the slug enters the core.

The main mixing mechanism in case of the low-boron water slug accelerated by the RCP start is turbulent mixing between the fluid flows having different velocities whereas in case of re-establishing the natural circulation after a reflux condensation phase the main mixing mechanism is buoyancy driven turbulent mixing. The density differences between the fluids are due to the temperature and the boron concentration differences.