All reactor designs take advantage of favourable inherent characteristics and compensate for unfavourable ones. Because of a very short reactor period for some postulated accidents, the Pressurized Water Reactor (PWR) design requires, and possesses, large negative values of fuel temperature and moderator temperature reactivity coefficients to ensure that rod ejection accidents can be compensated, and to stabilize reactivity transients from the operating state, which would otherwise be fairly rapid. In contrast, the CANDU design does not require strong negative feedback, given the small values of the reactivity coefficients around the operating point and the low reactivity worth of the control devices, both individually and collectively. Even for positive reactivity insertions near prompt critical, the rate of increase in reactor power in a CANDU reactor is inherently limited by its relatively long prompt neutron lifetime (about 40 times longer than that in a PWR), so that the reactor period is much longer and the rate of rise in power and enthalpy is much slower. Consequently, control and shutdown mechanisms are a practical and effective means for reducing total reactivity in the CANDU reactor.
Although there are many international initiatives to align nuclear regulations and hence eliminate nation-specific requirements, many are still design-specific. The regulators who deal primarily with Light Water Reactor (LWR) designs tend to embed the LWR requirement of negative reactor reactivity coefficients in their regulations, whereby the regulations become very design-specific. In contrast, regulators who deal with various reactor designs typically favour a more technology-neutral approach — as in International Atomic Energy Agency (IAEA) standards, stating the safety goals to be achieved rather than defining reactivity coefficients.
This paper presents a comparison of reactivity coefficients between typical modern LWRs and CANDUs. It discusses the relative importance of the reactivity coefficients in reactor safety, identifies major design differences and their influence on the type and value of the reactivity coefficients, and explains key features of the reactor operation and reactor behaviour in transients.