Studies of core response in local boron dilution accidents

During the past few years major attention has been given to analyzing the possibilities and potential consequences of a slug of diluted water entering the core of the WWER-440 reactors in Loviisa. Suitable countermeasures have already been implemented to reduce the probability of external inhomogeneous dilution. Inhomogeneous dilution scenarios with their high reactivity disturbance potential were not included in the original safe analyses. The presentation concentrates on typical dilution scenarios. Results of potential consequences in the core are presented for three typical dilution scenarios for the hot reactor condition. The calculations have been performed using the 3-dimensional dynamic core model HEXTRAN coupled with the circuit model SMABRE. The principles of the new preventive automation recently implemented at Loviisa NPS are also described. (7 refs., 6 figs.)