Development and verification of control rod performance analysis code COTTON

Abstract [Background] The control rod is mainly composed of neutron absorber, stainless steel cladding and absorber spring. During reactor operation, control rods modulate the reactor’s power output by absorbing neutrons through the absorber, thereby ensuring its safe operation. [Purpose] This work aims to evaluate the irradiation performance of control rod under long term operation in the reactor, including the temperature, diameter and inner pressure and so on. [Methods] First of all, a set of basic theoretical models, such as thermal analysis model, mechanical calculation model, internal pressure model and spring model, was established to analyze the radiation behavior of control rods with COTTON code. In particular, the integral thermal conductivity method and the mechanical transfer matrix algorithm were established for heat transfer and stress-strain calculation respectively. And then, the method of time discretization and space discretization were used to complete the software development, and the multidimensional analysis of the control rod at different times and different positions in its life cycle was realized. Finally, the validation assessment was conducted using pool-side inspection data from nuclear power plants by China Nuclear Power Technology Research Institute. [Results] Based on the operation example of nuclear power plant, the results show that the predicted values meet the design requirements. The temperature of the absorber and the cladding temperature are lower than the melting point, especially the temperature margin of the absorber is 393.9℃. The predicted value (P) of cladding diameter is in good agreement with the experiment data (M), the mean value of M/P is 0.992, and the standard deviation of M/P is 0.003. [Conclusions] The COTTON code has integrated comprehensive functionality for analyzing control rod performance, offering technical support for both the mechanical design and operational processes.