Dynamic analysis and upgrading of reactor cooling systems of VVER 440/312 (PAKS) due to seismic and normal operational loading

In order to requalify the seismic capacity of the reactor coolant system (RCS) as well as the nuclear safety systems (NSS) and components (NSSC), detailed analyses were performed for each of the four VVER-440/213 units. The dynamic behaviour of the reactor coolant system was analyzed and evaluated in detail on the basis of the results of a dynamic calculation (eigenmodes, relative displacements, acceleration response spectra). The findings of the evaluation of the results obtained for the as-built status provided the first upgrading option and indication of arrangement of dampers introduced for stabilizing the steam generators and loops of the RCD. In order to evaluate the efficiency of the adopted dampers and their location, comparisons were performed on the basis of the results of different upgrading concepts considering the available gap (horizontal and vertical) of the dampers as well as the thermal expansion effects of the RCS loops. By evaluating the results obtained for a number of subsequent upgrading alternatives, it was possible to define the final stabilization concept for the reactor coolant systems. The final retrofit alternative of the structures on the reactor coolant systems represents the basis for the upgrading measures as well as the calculation of the final response spectra. (author)