Validation of coupled neutron-kinetic/thermal-hydraulic codes for VVERs

Three-dimensional hexagonal reactor dynamic codes have been developed for VVER type reactors and coupled with different thermal hydraulic system codes. Under the auspices of the EU's Phare programme, these codes have been validated against real plant transients by the participants from several countries. Data measured during tests in the Balakovo-4 VVER-1000 and the Loviisa-1 VVER-440 have been analysed by the codes. Critical boron concentrations, 3-D core power distributions, time behaviour of total fission power, coolant temperatures and pressures were the main parameters utilised in the validation. Good accuracy of the results was generally achieved, comparable to the measurement accuracy. The confidence in the results of the different code systems has increased and the consequences of certain model changes could be evaluated.