EFFECT OF BORON AND GADOLINIUM BURNABLE POISONS ON THE HOT-TO-COLD REACTIVITY SWING OF A PRESSURIZED WATER REACTOR ASSEMBLY.

Control requirements for advanced pressurized water reactor designs must be met with heavy loadings of burnable poison rods, the required reactivity holddown typically amounting to 30% or more in a poisoned subassembly. Two apparent choices for poisons are natural boron rods and natural gadolinium rods. Studied and analyzed is the effect of these two poisons on the hot-to-cold reactivity upswing. Compared with an upswing of 2. 9% in a nonpoisoned assembly, the upswing in the gadolinium-poisoned assembly is 3. 0%, and the upswing in the boron-poisoned assembly is 8. 8%. Thus the hot-to-cold control penalty is almost nil for the choice of gadolinium and is considerable for the choice of boron.