Optimization of the axial power shape in pressurized water reactors

Abstract

Analytical and numerical methods have been applied to find the optimum axial power profile in a PWR with respect to uranium utilization. The preferred shape was found to have a large central region of uniform power density, with a roughly cosinusoidal.profile near the ends of the assembly. Reactivity and fissile enrichment distributions which yield the optimum profile were determined, and a 3-region design was developed which gives essentially the same power profile as the continuously varying optimum composition.

State of the art computational methods, LEOPARD and PDQ-7, were used to evaluate the beginning-of-life and burnup history behavior of a series of three-zone assembly designs, all of which had a large central zone followed by a shorter region of higher enrichment, and with a still thinner blanket of depleted uranium fuel pellets at the outer periphery. It was found that if annular fuel pellets were used in the higher enrichment zone, a design was created which not only had the best uranium savings (2.8% more energy from the same amount of natural. uranium, compared to a conventional, uniform, unblanketed design), but also had a power shape with a lower peak-to-average power ratio (by 16.5%) than the reference case, and which held its power shape very nearly constant over life. This contrasted with the designs without part length annular fuel, which tended to burn into an end-peaked power distribution, and with blanket-only designs, which had a poorer peak-to-average power ratio than the reference udblanketed case.