THE BROWNS FERRY NUCLEAR PLANT, beside the Wheeler Reservoir near Alabama, USA, is the Tennessee Valley Authority’s largest site. The plant comprises three General Electric boiling water reactors (BWRs) with Mark I containment. Browns Ferry 1 entered commercial operation in December 1973, followed by unit 2 in June 1974 and unit 3 in July 1976. All three units have been granted 20-year operating licence extensions by the US Nuclear Regulatory Commission (NRC) allowing them to remain in service until the 2030s.

In 2017, TVA decided to move forward with an extended power uprate (EPU) at Browns Ferry as part of a substantial investment in its generation resources aimed at meeting the energy needs of the Tennessee Valley area in future decades. The $475 million project was completed this summer, adding 465MW of low-carbon electricity to TVA’s portfolio.

Project scope

The Browns Ferry upgrade project involved:

  • Developing a licence amendment request (LAR) for submission to the US Nuclear Regulatory Commission, receipt of safety evaluation and licence amendment approval;
  • Design, implementation, and packaging of all related modifications and upgrades to the plant;
  • Resolution of all technical and administrative issues;
  • Updating training manuals, simulator scenarios, procedures and design and licensing basis documentation;
  • Providing operational and technical training;
  • Optimising plant operations at upgrade conditions by examining scenarios in the simulator;
  • Startup testing key parameters.

The schedule was designed to have all the modifications and upgrades implemented, testing performed, and the station ready to operate at uprated capacity coming out of the 2018 and spring 2019 refuelling outages. Startup testing was performed on all three units at incremental power levels to ensure accuracy of analytical predictions over a two-month period. This result was that the three units each generated 155MW more power on 22 June 2019 – 465MW in total.

Licensing

The Browns Ferry uprate required frequent communication with the US NRC. Regular and routine meetings resulted in improved communications that facilitated timely completion of NRC-required reviews throughout the project and during power ascension. Members of the EPU project team worked through licensing activities to develop the licence amendment request and obtain NRC safety evaluation and licence amendment approval.

The project team commissioned the nuclear fuel vendor to develop a fuels uprate safety analysis report; the nuclear steam supply system vendor to develop a power uprate safety analysis report; and TVA subject matter experts to develop the environmental assessment. The safety analyses provided the technical basis for the uprate licence amendment request, which was assembled by the project team in accordance with RS-001, NRC Review Standard for Extended Power Uprates.

Specific structures, systems, components were evaluated for uprated conditions; these evaluations were documented in task reports, and comprehensively reviewed by the regulator.

After the request was submitted, the NRC completed a two-month sufficiency review to ensure it had enough information to perform its assessment. During the 21-month review the project team presented several technically complex issues such as the steam dryer analysis, emergency core cooling capability and nuclear fuel analyses utilising NRC audits and public meetings.

Overall, the dedicated project team — which included site, corporate, and vendor partners — answered more than 200 requests for additional information (RAIs) during the NRC review process. The NRC approved TVA’s request in August 2017.

Upgrades

Equipment upgrades were implemented to maintain and improve design and operational margins and eliminate several reactor trip single-point vulnerabilities. These included: steam dryer replacement; modifications to the high-pressure turbine to accommodate additional steam flow; and upgrades to the moisture separator internals to improve efficiency. The feedwater pumps and heaters were modified to accommodate increased pressure and flow. The generator was also rerated and upgraded.

A number of other modifications were carried out to further improve margins:

  • Increased boron enrichment in the standby liquid control system to significantly reduce suppression pool temperature post anticipated transients without Scram (ATWS).
  • Installed integral flow diffusers in condensate demineralisers and RWCU demineralisers, improving reactor water chemistry.
  • Converted the main generator hydrogen makeup system to automatic to reduce operator workload.
  • Installed visco-elastic dampeners in main steam lines to reduce turbine stop valve vibration and premature failure of stop valve position instrumentation.
  • Relocated main condenser vacuum instrumentation to provide consistent condenser vacuum indication and protection.
  • Reduced low condenser vacuum alarm and trip setpoints to provide increased operational margin.

There were no significant technical changes applied to subsequent units based on lessons learned from previous uprates, with the exception of reactor vessel pressure electro hydraulic control system tuning. However, TVA notes that there were several benchmarks performed on the first unit uprated (Browns Ferry 3) designed to facilitate implementation and testing efficiency improvements on the subsequent units. TVA believes the best benchmark example is that only the first unit had instrumentation applied to the replacement steam dryer and the main steam lines to monitor acoustic vibration. In succeeding units, instruments were used only on the main steam lines. Data from the first unit were used to develop the relationship between the main steam line and dryer instrument measurements.

Training operators

“When you make changes to the plant, one of the biggest issues to overcome is how these changes will affect the people who operate and maintain it,” says TVA.

It worked with licensed operators early on in the process, letting them operate a simulator and provide feedback. This helped them identify challenges up front and be confident about the operation of the plant. This also helped TVA identify better processes that could be used across the industry.

There was collaborative training for the uprate and site engineering teams before the uprate took place. Operational and design margins that were affected were documented in a formally reviewed and approved document and expected margins were revalidated during power ascension.

Operators performed simulator runs at each power ascension test plateau to validate the startup test procedure and to gather key plant parameter data. The simulator data at each test plateau provided a second method for extrapolating predicted margins for subsequent test plateaus during the actual test.

Simulator scenarios for plant operations, transients and accidents were revalidated with EPU project licensed reactor operators prior to shift crew training. Specifically, the project team developed enhancements to ATWS emergency procedure guideline responses, which resulted in increased margin to respond to ATWS with core instability. This has subsequently been adapted as best practice by the BWR Owners Group for revisions to EPG Revision 4.

TVA says that, “Industry operating experience and benchmarking, with particular focus on stations that implemented extended power uprates, was extensively utilised during the identification and development of the modifications to support EPU operation. This contributed to the successful implementation and operational performance of these modifications.”

Main Image: Steam dryer replacement (Photo: TVA)