FUEL PERFORMANCE ANALYSIS OF EXTENDED OPERATING CYCLES IN EXISTING LWRs

Abstract

An integral part of a technical analysis of a core design, fuel performance is especially important for extended operating cycles since the consequences of failed fuel are greater for this operating strategy than for current practice. This stems mainly from the fact that extended cycles offer a unique benefit by running longer without interruption; poor fuel performance, i.e. failed fuel, would degrade this benefit. The issues in this research are assessed only at the steady-state level, as a foundation for the consideration of Anticipated Operational Occurrences (AOOs) and transient conditions, which are certain to present greater challenges to nuclear fuel performance due to their more severe conditions. Even at this preliminary steady state level, extended cycle operation is found to exacerbate several fuel performance issues, resulting mainly from the fact that some fuel in an extended operating cycle is operated at higher powers over part of the core life and does not have the benefit of shuffling. In order to accurately quantify the fuel performance effects of extended cycle operation, a pseudo or "envelope" pin is created, which represents the operating characteristics of the highest power fuel rod in the core at a given pin burnup interval. This envelope pin was created for both extended cycle and current practice, so that extended cycle results could be compared to both existing licensing limits and current practice. While this approach is somewhat conservative, it is the simplest way to evaluate fuel performance in an extended cycle core where the location of the limiting fuel rod changes often and operates at higher powers for prolonged periods of time. The US Nuclear Regulatory Commission's Standard Review Plan's Sections 4.2 and 4.4 are used as the basis for the criteria that should be evaluated in this report, since these are the relevant sections of the document that prescribes the licensing limits and criteria for nuclear fuel design. From this document, ten steady state fuel performance issues are identified: (1) stress and strain, (2) fatigue cycling, (3) fretting, (4) waterside corrosion, (5) axial growth and rod bowing, (6) rod internal pressure, (7) primary hydriding, (8) cladding collapse, (9) cladding overheating, and (10) fuel centerline melt. Of these ten issues, (7) and (8) were found to be not uniquely affected by extended cycle operation. While (9) and (10) are found to not be concerns for extended cycle operation, the higher powers at which extended operating cycles can operate degrade some of the margin for transient effects, which is more of a significant concern for (9). (1) and (5) are predicted to be worse for both BWRs and PWRs when compared to current practice, and (4) and (6) are projected to present greater challenges for PWRs. Additionally, (2) is the only factor that is predicted to actually be better for extended cycle operation in both the BWR and PWR while (4) was predicted to have less of an effect in BWRs, given the comparable operating powers and shorter in-core residence time for the extended cycle case. The effects of the proposed new operating strategy on (3) were uncertain. Of all ten issues, (5) seemed to be the most problematic, as no solution was readily available. Solutions to other issues included improved assembly grid design (3), water chemistry control (4), annular fuel pellets (6), and, potentially, increasing the number of fuel rods per assembly (1,4,6,10).