Proposing of a new fitting and iteration method (FIM) to correct measured reactor core reactivity

China experimental fast reactor (CEFR) achieved its first criticality in 21st July 2010. In CEFR physics startup test, an external neutron source (252Cf) with initial intense of 109 n/s has been placed in the core center. When using period method or inverse kinetic method to measure reactivity, the relatively strong neutron source will lead to over-estimation of reactivity measured, meanwhile, dead-time of neutron detection system (pulse mode) will lead to under-estimation of reactivity measured. To solve these problems that actually encountered in the measurement of reactivity during CEFR physics startup test stage, a new fitting and iteration method (FIM) has been proposed to correct measured reactivity, and to determine dead-time of detection system. The basic algorithm and the calculation flow are described in the paper, and based on this method a computing code is developed. Two categories of data are used to verify this method, including dummy analytic data and actually measured data. The calculation results indicate that, according to recorded neutron count sequence, the FIM method can successfully calculate accurate reactivity with consideration on the impact of external neutron source effect and can obtain relatively accurate dead-time value of detection system; both of the results show the proposed fitting and iteration method is effective and practicable.

► We study and solve the problem of reactivity correction encountered in physics start-up test of CEFR. ► A new fitting and iteration method (FIM) is proposed to do external source effect correction and dead-time effect correction on measured reactivity. ► The basic algorithm and the calculation flow of FIM method are presented in the paper. ► Through verification work with several groups of dummy data and actually measured data, the FIM method is proved to be effective and practicable.