ANALYSES OF BEYOND DESIGN BASIS ACCIDENT (BDBA) “HOMOGENEOUS” BORON DILUTION SCENARIOS
24th Symposium of AER on VVER Reactor Physics and Reactor Safety (2014, Sochi, Russia)
Reactor dynamics and safety analysis
ANALYSES OF BEYOND DESIGN BASIS ACCIDENT (BDBA) „HOMOGENEOUS” BORON DILUTION SCENARIOS
András Keresztúri, György Hegyi, Csaba Maráczy, István Trosztel
MTA EK, Centre for Energy Research, Hungarian Academy of Sciences Reactor Analysis Department
H-1525 Budapest 114, P.O. Box 49, Hungary firstname.lastname@example.org
Homogeneous boron dilution scenarios were analyzed by using the coupled KIKO3D- ATHLET code. The scenarios were named as homogeneous because of the very slow dilution caused by a rupture in a heat exchanger of the make up system. In case of the presented BDBA scenarios, it was supposed that after 30 minutes the operator doesn’t stop the dilution in spite of the prescriptions. Without the presented analyses, significant contribution of the „homogeneous” boron dilution to the CDF had to be assumed in the PSA analyses. The most relevant reactor states were certain shut down ones at low initial temperature and pressure. The analyses have shown that these events don’t lead to core degradation, although large amount of steam can be generated in the primary and secondary loops in absence of any reactivity protection. In spite of the very slow dilution, the time dependent boron concentration is becoming inhomogeneous. A reactivity initiated ATWS event occurs, where the reactor is becoming critical from time to time, demonstrating the strong self-regulating ability of the VVER-440 core also in case when the power rise is starting from low temperature and pressure. In spite of the dilution, the long term average of the power is stabilizing because of the adverse effects of the boron-free primary coolant discharge through the pressurizer relief valve and the decreasing moderator density.