Cross-Verification of 1D and 3D Models for a VVER-1000 Reactor’s Pressure Chamber Simulated by the KORSAR/CFD Computation Code in the Modes with Asymmetric Loop Operation

The KORSAR/CFD code results from the development of the KORSAR/GP system code certified in 2009 by the Rostekhnadzor (Federal Service for Ecological, Technological, and Nuclear Supervision) as applied to the calculated justification of the safety for VVER reactors. One of the important aspects of the development consists in the introduction of the CFD-module code into functional content for the simulation of spatial turbulent flows in the mixing chambers of reactors using a nested boundary method in the RANS-approximation. The CFD module is combined with a 1D model according to a semi-implicit scheme as a standard code element. Based on the calculations performed for three modes with an asymmetrical equipment operation in the heat-transfer loops of the VVER-1000 reactor, cross-verification has been performed for a 3D model in the CFD approximation, and a quasi-3D-multichannel model of the KORSAR/CFD computation code for the reactor pressure chamber. For cross-verification, the following modes have been chosen: breaking steam pipeline in a steam generator, connecting the main circulation pump while initially operating three pumps at the reactor power of 71% with respect to the nominal value, connecting a pump while initially operating two opposite pumps at the reactor power of 52% with respect to the nominal value. The scenario of the chosen modes is characterized by a decrease in the heat-carrier temperature at the entry into the pressure chamber from a single loop, which leads to an asymmetric increase in the reactor power with respect to the fuel assemblies owing to a negative reactivity effect. It is shown that, because of the artificial increase in the resistance to the downward heat-carrier flow along the channels that represent a pressure chamber in the multichannel calculation scheme, the reproduction of a spatial in-chamber flow pattern obtained using the 3D model of the chamber and its effect exerted on temperature change in the course of the heat-carrier stirring have been gained. The results of calculation using this scheme are in good agreement with data obtained by means of the 3D simulation of the pressure chamber in the CFD approximation in all the considered modes. A sensitivity of the calculation results with respect to changes in the calculation scheme under using the quasi-3D multichannel model of the reactor chamber is demonstrated.