Analysis of a boron dilution accident for WWER-440 combining the use of the codes DYN3D and SiTAP

A boron dilution scenario caused by the connection of a previously disconnected primary circuit loop in a Russian WWER­440 type reactor is considered. The scenario is specific for this reactor type because of the existence of Main Isolating Valves in the loops. The additional failure of safety systems during the connection procedure was assumed. The analysis was carried out by the combined use of SiTAP and DYN3D. By the help of the fast running plant simulator code SiTAP several modifications of the scenario were considered. The scenario with the most dangerous consequences was identified and has been analysed using the three­dimensional core model DYN3D including a coolant mixing model for the lower plenum. The boundary conditions for the DYN3D analysis were obtained from SiTAP calculation. Comparing the results of both codes, a similar behaviour of the mean reactor parameters can be observed, but in the 3D analysis local exceeding of safety relevant parameters was obtained. Thus, the point kinetics model is not conservative, but by using SiTAP more realistic time­dependent boundary conditions for the 3D model could be provided than in previous analyses. The strong consequences of the considered scenario suggest the nessecity of additional measures for preventing this type of accidents.