Development and application of axially variable strength control rods for power maneuvering of pressurized water reactors

Abstract

Axially variable strength control rods (AVSCRs) are suggested and developed as a solution to problems related with the variation of the axial power distribution during the power maneuvering of PWRs and to accomplish the power maneuvering with only control rods and without reactivity compensation by boron concentration change. AVSCRs are control rods that, unlike conventional control rods, have axially non-uniform strength. In a typical nuclear power plant, reactor power change is caused by variation of reactivity. Two primary mechanisms for reactivity changes are control rods and soluble boron. Moderator temperature control is an auxiliary means. During the power maneuvering of a nuclear power plant, the reactor core is in a transient state and the axial power distribution should be maintained within some prescribed range in order to prevent a xenon oscillation. However, there are difficulties involved in regulating axial power distribution within the prescribed range during reactivity change using conventional mechanisms. AVSCRs are suggested as a method of ameliorating these difficulties. Control rods are classified into two types: `multi-purpose control rod` and `regulating control rod`. Two multi-purpose control rod banks (AVSCR1, AVSCR2) are newly developed, while conventional control rods are adopted as regulating control rod banks to minimize design change of PWRs. The newly developed AVSCRs are axially three sectioned control rods, and the worth shapes of these AVSCRs are optimized to obtain appropriate moving characteristics for power maneuvering. As analytic objective functions do not exist in the optimization of the AVSCRs, the simulation optimization methodology is used. Then, an operation strategy employing developed AVSCRs for the power maneuvering is developed. This strategy consists of simple logics and no use of reactivity compensation by boron is considered. Finally, the AVSCRs are applied to power maneuvering with a typical 100-50-10...