| dc.description.abstract | SECTION I: Economic Analysis of Extended Operating Cycles in Existing LWRs The generic economic aspects of extending operating cycles in LWRs are examined to assess the factors associated with cycle lengths at or near the limit of technical feasibility, based on current NRC mandated burnup limits. These factors are broken into 2 basic categories, Fuel Cycle Economic Factors and Operations and Maintenance (O&M) Economic Factors. Results are evaluated relative to current practice: 18 calendar month cycles refueling 72 of 193 assemblies each shutdown for the case study PWR and 24 month cycles refueling 255 of 764 assemblies each shutdown for the case study BWR, both with a 6% Forced Outage Rate (FOR) and 49 day Refueling Outage (RFO). Tallying all of the realizable factors, it is evident that large fuel cycle costs will be incurred as a result of cycle length extension. Thus, large savings in O&M must be realized to make ultra-long cycles economically attractive. Quantifying these factors, it is shown that cycle length extension to 48 calendar months (with a RFO length of 42 days and FOR of 3%) incurs a significant deficit for the case study BWR (-$8.9M/yr.) and cycle length extension to 41.4 calendar months (with a RFO length of 42 days and FOR of 3%) yields a profit of approximately $1.OM/yr. for the case study PWR. A simple model is also constructed and applied to find the economically optimum cycle length. This model employs only five basic factors: increased fuel costs, increased spent fuel storage costs, savings from avoided refueling outages, savings from a reduced forced outage rate and replacement energy savings; all others are considered either constant regardless of length of cycle extension, or insignificant. This model shows that multi-batch fuel management is more profitable than single batch management for cycle lengths shorter than ultra-long cycles, i.e. 63 calendar months for the case study BWR and 48 calendar months for the case study PWR. The most profitable strategy at which to operate both of these plants was found to be at or near current practice: n=3, 24 calendar month cycles for the case study BWR and n=3, 18 calendar months cycles for the case study PWR. The economically optimum strategy predicted for the case study PWR violated current burnup limits, suggesting a need to re-evaluate these limits as a means of improving plant economics. Additionally, since these current practice strategies that were evaluated using this model were awarded the same operational benefits as the extended operating cycles and were found to be less costly, investing in improving the operations of current nuclear power plants is a more economically viable option than cycle length extension. Parametric studies are performed using this model to vary important parameters such as replacement energy costs, carrying charge rate, unit enrichment costs, and operational parameters. Increasing replacement power costs not only increases the cost of extending cycle length, but also increases the optimum cycle length. For increased carrying charge rate, cost increases, while optimum cycle length decreases. Additionally, the sensitivity of cost to carrying charge rate increases with cycle length. Lower enrichment costs not only decrease the cost of a particular operating strategy, but also significantly increase the optimum cycle length. Finally, the sensitivity of the cost of an operating strategy to its respective (cont.) operational parameters (FOR, RFO) decreases with increasing cycle length; optimum cycle length also increases with poorer operational characteristics. The significant effect of unit enrichment costs on an operating cycle's total cost shows that innovations in enrichment technologies are essential for making extended operating cycles economically competitive. The increase in the volume of spent nuclear fuel that is generated by extending operating cycle length is also an area that requires further consideration in order to make this strategy more attractive. Extending burnup limits to realize the full economic potential of long cycle operation, especially in the case study PWR, is also an area deserving future investigation. SECTION II: Fuel Performance Analysis of Extended Operating Cycles in Existing LWRs 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). | en_US |
