Reactivity Transient Due to Malfunction of a Mechanical Zone Controller in the CANDU-3 Reactor

Abstract

The three dimensional neutronic kinetics code CERBERUS was used to simulate the time dependent neutron transients due to the insertion of a localized positive reactivity perturbation in the CANDU 3 reactor. The perturbation was caused by the postulated malfunction of a stainless steel zone control absorber which was assumed to drop out of the core from the fully inserted position. Since the event was classed as a loss of regulation, the reactor regulating system was not invoked. The transients were calculated for three different postulated drop speeds of the absorber. The action of the shutdown system (SDS) was not explicitly simulated. But the time of trip was calculated. The reactor, channel and bundle powers at that time were obtained from the transients. The maximum positive reactivity on complete removal of the absorber from the core was 20.64 mk for the three transients (corresponding to the three different drop speeds). The reactor and channel power increases before shutdown were about 10% and 15% above the nominal (or steady state) values respectively. The results confirmed that the mechanical zone controller concept introduces relatively benign failure modes, easily mitigated by the existing CANDU reactivity control system.