Dynamic Model of the VVER-1000 Reactor for Seismic and LB LOCA Evaluation

The assessment of seismic and dynamic strength for the VVER-1000 reactor with emphasis on fuel assemblies interaction is carried out based on experience for Ukrainian NPPs. Evaluation of fuel assembly structural response to externally applied forces is based on NUREG-800 “Standard Review Plan”, item 4.2 “Design of fuel systems”. The analysis considered two types of events: Safety Shutdown Earthquake (SSE) and Large break Loss Of Coolant Accident (LB LOCA). As LB LOCA for the VVER-1000 – “Maximum design accident” Recirculation Line Break is considered. For the analysis of the initial dynamics, the 3D CFD model (ANSYS CFX) of the VVER-1000 was adapted to account for the two-phase flow. To take into account the transience of the phase transition during depressurization, a special model of interphase exchange is proposed, which takes into account not only the nonequilibrium temperature, but also the features of the pressure change by analogy with cavitation. For full dynamic analysis, it is necessary to develop an analytical and/or FEM model of the reactor, which will include reactor elements, their masses stiffness and inertia characteristics. The developed model is validated against experimental data of natural frequencies of fuel assembles, for uncompressed and compressed working position, also with and without fluid influence. The seismic analysis consider the available response spectra for NPP site with 3-component seismic impact of SSE level. Another important issue is to analyze reactor elements interaction and in case of need, to account for the non-linear behavior of the model. The seismic and LOCA impact at the location of fixation of the fuel assemblies (lower and upper support plates) is calculated by taking into account the dynamic response of the reactor system as a whole. After demonstrating the seismic/dynamic stability of the fuel assemblies and the reactor internals, a justified conclusion is provided on preserving the safety factor for fuel assembles at both loadings.