Assessment of Cobalt Adjuster Effect on CANDU reactors

A CANDU reactor has 21 stainless steel adjuster rods of four types in three rows of seven. The adjuster rods are used to provide positive reactivity by withdrawing them from the core. They are used primarily to compensate transient increases in xenon concentration, or to compensate for reactivity decrease due to fuel burnup during periods when fuelling machines are not available. Also, they play a role of the spatial control of power. The design basis for reactivity worth of the system is to balance the reactivity decrease due to the increase in xenon during 30 minutes following a shutdown from full power. The stainless steel adjuster rods have been replaced with cobalt adjuster assemblies in a CANDU reactor, such as Gentilly - 2 and Embalse. The assemblies have to be neutronically equivalent to stainless steel adjuster rods and have a minimum effect on the safety and operating of reactors. The neutronic equivalence means that an identical reactivity worth and the effect on the power and flux distribution of the core are same for both stainless steel and a cobalt adjusters. In this study, we calculate the incremental cross section of cobalt adjuster rods with irradiation and evaluate the reactivity worth of cobalt adjuster rods using RFSP code