Dynamic Modeling of a BWR Control Rod Insertion System for Seismic Analysis and its Experimental Validation

The assessment of the seismic scrammability, which means the control rod insertability during a seismic event, is one of the most important design tasks for ensuring the seismic safety of nuclear power plants in Japan. This paper discusses the dynamic modeling of the control rod insertion behavior of a boiling water reactor (BWR) during an earthquake. A dynamic model of a control rod insertion system for BWR was developed based on multi-body dynamics. The coupled vibration behavior of the fuel assemblies in the fluid was modeled as an inertial coupling system. The effect of the interaction force between the control rod and the fuel assemblies was considered in a three-dimensional contact analysis. The hydraulic control unit and the control rod drive, which provide the control rod with drive force, were modeled in the concentrated parameter system. The model parameters, such as the friction coefficient between the control rod and the fuel assembly and the discharge coefficient of the scram piping, were obtained by conducting experiments. The validity of the model was confirmed by comparing the analytical results with the experimental ones. First, the validity of the fuel assembly model was verified through a comparison with the vibration testing in an underwater condition. It was confirmed that the calculation results for the frequency response of the fuel assembly were in good agreement with the experimental ones. Second, the validity of the modeling method of the drive system consisting of the hydraulic control unit and the control rod drive was verified through a comparison with the scram testing under non-vibration condition. The calculation results for the time history of the control rod insertion, the accumulator pressure, and the flow through the scram piping were in good agreement with the experimental ones. Finally, the validity of the modeling method of the whole system consisting of the fuel assemblies, the control rod, and the drive system was verified through a comparison with the scram testing under vibration condition. The calculation results for the time history of the control rod insertion stroke and the time delay of the insertion motion during an earthquake were in good agreement with the experimental ones. The results of these comparisons show that the developed analysis model can simulate the control rod insertion behavior during an earthquake.