Study On The Excess Of Structural Design Flow In Primary Coolant System Of PWR Nuclear Power Plant

Controlled nuclear chain reaction is maintained in the reactor of nuclear power plant to produce energy, which is ultimately converted to electric power. To ensure long-term safe and stable operation of reactor, the produced heat in the reactor must be continuously conducted by primary coolant system to maintain the fuel elements beyond the highest operating temperature, which requires the primary pump to provide sufficient coolant flow. Meanwhile, excess of design flow has a negative impact to the normal operation of reactor vessel and fuel assembly. Consequently, the primary coolant flow should be limited to a proper level. For this purpose, the practical and effective methods need to be introduced for the measurement, as well as the relevant appraisal system. Those measures are contributing to nuclear power safety.The abnormal condition of excess of structural design flow in primary coolant system has taken place more than once in nuclear power plants under construction in China. Through research analysis on the measurement principles of primary coolant flow, this article attempts to explore the root cause analysis, thereout measures and methods could be undertaken to achieve the ultimate aim of solving this problem.According to the research on methods and uncertainty of measurement of the coolant flow, it is concluded that the large flow measurement uncertainty has an adverse contribution to the abnormal condition of excess of structural design flow. High-precision measurements and reducing the number of transmission link can improve the accuracy of flow measurement.Base on the analysis of the primary coolant resistance characteristics and pumping head of primary pump, material causes of the primary flow exceeding the original Structural design limit as follows. Firstly, there is uncertainty in the resistance characteristics calculation of reactor core, reactor structure and steam generator, which may cause the actual resistance of equipment smaller than the design values. Secondly, the pumping head of primary pump and other characteristics with uncertainty, and the actual value is larger than designed. In consideration of the above two factors, the problem of actual operation flow exceeding the original mechanical design limit could take place.This article provides new guide lines of the structural design flow in primary coolant system:75558 m3/h for core and 25186 m3/h for loop, which are based on the analysis of statistics, the experience feedback from similar plants and the structural design flow of Generation III nuclear plant models (such as EPR, AP1000).This article also analyzes the equipment structure integrity and the plant safety during the plant operating under the new structural design flow rules (75558 m3/h for core mechanical design flow and 25186 m3/h for loop design flow).and mentioned parameters could guarantee the structural integrity analysis and safety evaluation of unit.