Assessing low power reactivity levels in subcritical CANDU reactors

A new technique has been developed for monitoring slow reactivity changes in Canadian deuterium uranium (CANDU) nuclear reactors during low power operation, following a sustained period of high power operation. The power doubling halving worth (PDHW) test tracks slow reactivity changes by evaluating the reactivity perturbation required by the power regulating system to halve and double reactor power over time. During low power operation of a CANDU reactor, the PDHW test is used to monitor the decay of the photoneutron precursors, so that the reactor power can be lowered using a preset amount of reactivity. The PDHW test is described in this paper and is validated by using computer simulations and operating data from a CANDU reactor.