Implementation of advanced PRIME fuel features

The PRIME fuel features consist of three enhancements to the 17x17 optimized fuel assembly (OFA) fuel design: low tin Zirlo grid strap material, reinforced dashpot for the guide thimble tubes, and a lower pressure drop bottom nozzle (PRIME BN).

The PRIME fuel was used at Southern Nuclear’s Farley-1 and -2, in Columbia, Ala., and Vogtle-1 and -2, in Waynesboro, Ga. The units are all Westinghouse PWRs.

Innovation: With an eye toward innovative approaches, the PWR fuel engineering team achieved gains in fuel performance margins by leveraging improvements in nuclear materials (low tin Zirlo) and ongoing research and development of bottom nozzle flow and debris capture.

The grid strap implementation at Farley and Vogtle represents the first reloads of this grid material design in a Westinghouse nuclear steam supply system fuel design (17x17 OFA). Implementation of this change leveraged data and evaluations from previous lead-test-assembly programs, as well as from successful reload implementation in other reactor designs (Siemens and Combustion Engineering plants).

The PRIME BN implements the debris resistance and debris capture features of Westinghouse’s advanced debris filter bottom nozzle (ADFBN) to prevent inter-fuel assembly debris migration. The PRIME BN also uses an updated flow hole design implemented in 17NGF (next generation fuel) assemblies to improve pressure drop and increase flow through the assembly. Thus, this bottom nozzle design combines the benefits of two previous bottom nozzle improvements for a more robust bottom nozzle design.

The reinforced dashpot in the guide thimble tubes has successfully operated in other Westinghouse fuel designs. The implementation at Farley and Vogtle, however, is the first use of this variation in the 17x17 OFA fuel design.

Safety: The design functions of the fuel assemblies are to serve as the primary fission product barrier, to allow full control rod insertion within the credited rod drop time, and to maintain a coolable geometry.

The fuel rod matrix and cladding provide the primary fission product barrier. The PRIME mid and IFM (intermediate flow mixer) grids use low tin ZIRLO material, which has a lower tin content than the previously used ZIRLO material. The lower tin content results in less growth at high burnup and improves corrosion resistance and grid-to-rod fretting margin. In addition, the PRIME BN fuel design enhances debris resistance performance. This change provides a more robust barrier to fuel failure.

The fuel assembly skeleton is primarily responsible for maintaining a coolable geometry. The bottom nozzle serves as the bottom structural element of the fuel assembly and directs the coolant flow distribution to the fuel assembly bundle region. The PRIME BN decreases flow resistance that allows increased flow through the fuel assembly. This increased flow has a positive impact on the margin to departure from nucleate boiling (DNB). DNB analysis is one of the key safety analyses performed each cycle.

Maintaining sufficient DNB margin prevents excessive clad temperature during safety analysis transients, specifically Condition II faults.

The top nozzle and guide tubes allow for control rod insertion. The reinforced dashpot design is a more robust guide tube design that improves dimensional stability at higher burnup. This more robust design is less prone to guide tube and fuel assembly bowing with increased burnup, thus improving margin to incomplete control rod insertion. This change also benefits fuel handling at higher burnup.

Cost savings: For nuclear power generation to remain competitive, the industry is developing various initiatives to increase cycle lengths and increase fuel average burnups. Increased cycle lengths have the potential to reduce outage lengths and/or frequency that reduces overall outage costs including replacement power cost. Increased fuel average burnups allow for better fuel utilization and optimization. This can lead to reduced upfront fuel purchase costs as well as savings in dry cask storage.

The improvement in fuel performance margins gained using PRIME fuel features can be leveraged as part of these cost-saving initiatives. In addition, the PRIME fuel features provide improvements to fuel performance by reducing risk of grid-to-rod fretting failures, debris fretting failures, and assembly damage due to assembly bow and distortion. Fuel failure and fuel damage often require core redesign during the outage that results in a less optimized core loading pattern and less optimized use of the fuel, thus impacting fuel costs. In addition, preventing fuel failures results in $4 million-$6 million in inspection and root cause costs that are saved for every fuel cycle that operates defect-free. The PRIME fuel features provide mitigation against such costs.

Productivity/efficiency improvements: The PRIME fuel features provide improvements to fuel performance by reducing risk of grid-to-rod fretting failures, debris fretting failures, and assembly damage due to assembly bow and distortion. Fuel failure and fuel damage can significantly impact engineer workload and productivity during the cycle as well as additional fuel management and handling during outages to perform fuel inspections. Also, failed or damaged fuel can often require core redesign during the outage that requires significant employee time and hours. The PRIME fuel features provide mitigation against these employee and contractor burdens.

Transferability: The PRIME fuel features have been implemented on the 17 OFA design at the Farley and Vogtle units. The successful implementation by Southern Nuclear is fully transferable to all units using the 17 OFA fuel design and is expected to become the new standard for this fuel design. These changes are also transferable to other Westinghouse NSSS designs such as the 17 RFA (robust fuel assembly) design.

Communications: Implementation of the PRIME fuel features at the Farley and Vogtle units utilized the nuclear fuel design change process, which was developed to ensure successful management of fuel design changes. This process was used for effective communication of PRIME fuel impacts to the plant, site personnel, and engineering groups.

Vision and leadership: With the implementation of the PRIME fuel features, Southern Nuclear’s PWR fuel engineering better positions the nuclear power industry for operation well into the future by challenging the status quo and implementing innovative approaches to fuel design. The PRIME features were evaluated based on the returns and costs to the business now, and on how viable the solution would be for the entire industry in the face of new technology, competition from new industry entrants, or other disruptors well into the future. By enhancing fuel reliability and performance, Southern Nuclear and the industry can refocus resources away from expensive, overly conservative, historical solutions toward more strategically advantageous technologies and programs.

Victoria Fitz is the principal engineer of Southern Nuclear’s PWR fuel engineering, Bradley Balltrip is an engineer in Southern Nuclear’s PWR fuel engineering, Matthew Leonard is the lead engineer in Southern Nuclear’s PWR fuel engineering, Matthew Lynch is an engineer for the Vogtle plant’s reactor engineering, and Raymond Flanery is a senior engineer for Southern Nuclear’s PWR fuel engineering.

Contributing to the Prime fuel project were Jennifer Baker, manager of Southern Nuclear’s PWR fuel engineering; Michael Boone, fuel hardware solutions product manager for Westinghouse; and Jonathan Chavers, director of nuclear fuel and analysis for Southern Nuclear.

This article was adapted from Southern Nuclear’s submission to the Nuclear Energy Institute’s Top Industry Practice Awards (TIP) program.