Power coefficient of reactivity in CANDU 6 Reactors

The Power Coefficient of Reactivity (PCR) measures the change in reactor core reactivity per unit change in reactor power and is an integral quantity which captures the contributions of the fuel temperature, coolant void and coolant temperature reactivity feedbacks. All nuclear reactor designs provide a balance between the inherent nuclear characteristics and the engineered reactivity control features, to ensure that changes in reactivity in all operating conditions are maintained within a safe range. The CANDU reactor design takes advantage of the inherent nuclear characteristics of small reactivity coefficient, minimal excess reactivity and very long prompt neutron lifetime to mitigate the magnitude of the demand on the engineered systems for controlling reactivity. In particular, CANDU reactors have always taken advantage of the small value of the PCR associated with its design characteristics, such that the overall design of the reactor does not depend on the sign of the PCR. This is a contrast to other reactor design concepts which are dependent on a PCR which is both large and negative in the design of their engineered systems for controlling reactivity. It will be demonstrated that during a Loss of Regulation Control (LORC) event, the impact of having a positive power coefficient, or of hypothesizing a PCR larger than that estimated for CANDU, has no significant impact on the reactor safety. Since the CANDU 6 PCR is small, its role in the operation or safety of the reactor is not significant.