Safe external reactivity perturbation limits in IBR-2

Abstract

For the IBR-2 pulsed reactor safe reactivity perturbation limits have been determined by mathematical simulation of reactor dynamics in the plane “input reactivity level–input speed”. From power transients calculated using a reactor model with experimental power feedback parameters safe reactivity perturbation limits corresponding to different years of reactor operation have been obtained.It has been shown that in the course of reactor operation continuous narrowing of the region of permissible reactivity perturbations accompanied by significant fluctuations of the boundaries of the region takes place. The safety limit for some reactivity perturbations has decreased over 3 times in 20years of reactor operation.Such reduction of the region of permissible reactivity fluctuations is connected with gradual degradation of the fast power feedback. It has been demonstrated that the effect is most likely due to increase in the positive feedback component because of changes in thermo-mechanical deformation of fuel elements in the course of power generation.It has been noted that an authorized automated power control system secures safe operation of the reactor even in the case of continuous narrowing of the safety limit.The investigation has been performed at the Frank Laboratory of Neutron Physics, JINR.