An Investigation into the Relationship Between Local and 73 Global Power Excursions in CANDU

Abstract

If a reactor exhibits large neutronic decoupling of one core region from another, it may be able to sustain localized power excursions. If such an excursion is too localized, there may be no in-core detector close enough to detect it promptly. To confirm that CANDU reactors are unlikely to support local power excursions, we selected a credible local reactivity-insertion mechanism in the CANDU 600 and calculated the resulting neutron flux transient with a three-dimensional model. A coupled neutronics-thermohydraulics simulation of the transient was performed. The transient was subdivided into appropriate time steps, and iteration between the neutronics and thermohydraulics calculations was carried out at each step, so that the proper distribution of thermohydraulic feedback reactivity was obtained. The calculation shows that, with credible reactivity insertions, the neutronic characteristics of CANDU reactors do not allow local power excursions to occur without a comparable global power increase. The latter would not escape detection by the protective systems.