Analyzing the Influence of Coolant Pressure Fluctuation Natural Frequencies on the Vibration Strength of the Main Coolant Pipeline and Steam Generator Header Metal at NPPs with VVER Reactors in the Nominal Operation Mode

The calculation procedures used for vibration strength design of the NPP main coolant pipeline (MCP) are subjected to a comparative analysis. This task is relevant to ensure reliable operation of nuclear power plants equipped with pressurized water reactors. Such zones of the reactor coolant circuit (RCC) as pipeline bends and sections with abrupt widening are prone to the occurrence of vortice, the separation of which produces vibration loads on the NPP equipment. The nominal operation mode of an NPP with a VVER-1000 reactor is analyzed. The calculations were carried out by applying a combination of the method for determining the natural frequencies of coolant pressure fluctuations developed at the NRU MPEI and the finite element method for solving the differential equations used in the relevant theory. Based on the computation results, the possibility of vortices to occur in the zones the MCP bends is demonstrated, and the natural oscillation frequencies of the filled RCC sections are determined. The coolant pressure fluctuation natural frequencies were calculated, and the RCC stressed state resulting from their impact has been determined. It has been shown that the maximum metal stresses are located in the area of the MCP and SG header welds, as well as in the zones of MCP bends and in the place where the MCP is welded to the reactor pressure vessel nozzle. The presented calculation algorithm can be used for determining the vibration stresses under the conditions of anticipated operational occurrences (AOO) and accidents.