TVA completed the $1.9 billion restart of the 1,100-MW Browns Ferry Unit 1 in 2007. That restart project provided the opportunity to incorporate state-of-the-art materials and radiation-reduction techniques to ensure that Unit 1 would be an industry-leading low-ALARA-exposure plant when it returned to service. The reductions achieved were significant.
Tennessee Valley Authority (TVA) became a nuclear utility when the Browns Ferry Nuclear Plant (BFNP) Unit 1 entered service on August 1, 1974. By March 1977, the plant had grown to three 1,000-MWe units, making it the largest nuclear plant in the world at the time. The plant, named after a ferry that once operated nearby, is located on an 840-acre site beside Wheeler Reservoir on the Tennessee River near Athens, Alabama (Figure 1).
 |
| 1. Full service. With the 2007 restart of Unit 1, Tennessee Valley Authority’s three-unit Browns Ferry Nuclear Plant is now back in full service. Source: TVA |
Following the infamous 1985 plant fire, TVA shut down all three units, indefinitely. After completing necessary repairs and plant safety upgrades, Units 2 and 3 reentered service during 1991 and 1995, respectively. Unit 1 remained in long-term shutdown until a $1.9 billion restart project was approved by TVA’s Board of Directors in 2002. Unit 1 reentered service in 2007. That same year, the American Nuclear Society recognized the unit with its Utility Achievement Award for Outstanding Improvement in Performance, “in recognition of the most extensive restart effort in the nuclear industry.”
BFNP features three General Electric boiling water reactors (BWR). After restart, the BWR/4 reactors produced about 1,100 MWe, after completing Nuclear Regulatory Commission (NRC)–approved “stretch” uprates on each unit. Today, BFNP has applications pending for a 14.3% thermal uprate for each of the three units. For more on the history of BWR design evolution, see “The Evolution of the ESBWR” in POWER’ s online archives at http://www.powermag.com.
As part of the Unit 1 restart project, state-of-the-art materials and techniques were to be incorporated to ensure that it would be an ALARA (as low as reasonably achievable) radiation exposure plant when it returned to service. The remainder of this article discusses the program approach and specific improvements made to components and systems to achieve an industry-leading low ALARA exposure rate.
Early Program Planning
The minimum requirement for ALARA exposure for the Unit 1 restart project was to match the radiation levels of the other two BFNP units. The first step was to form a Cobalt Reduction High Intensity Team (HIT) with representatives from the Radiation Protection, Chemistry, Engineering, Maintenance, Modifications, Planning, and Procurement groups. The next order of business was an evaluation of each group’s radioactive cobalt (Co-60), reduction plans.
Clearly, one of the best methods for reducing the source term for radioactive cobalt is to reduce cobalt-bearing materials. The second-best approach is to control and mitigate the activity from plant out-of-core surfaces. Browns Ferry utilized both approaches for the Unit 1 restart program.
During August 2003, TVA asked the Electric Power Research Institute (EPRI) to perform an independent assessment of the BFNP Unit 1 restart plan. The goal of this evaluation was to determine measures that would assist TVA in making the restart world-class for cobalt dose rate control. The goal was to use all of the tools, technologies, and guidance that EPRI had compiled to date to reduce Unit 1’s radiation source term for radioactive cobalt.
EPRI’s initial assessment identified several areas that required further investigation, but unfortunately, much of the design work for many plant systems had already been completed and approved by the NRC. At that point, any new design changes had the potential to delay the projected plant restart date of May 2007. However, adequate time and resources were available for TVA to implement a number of the EPRI suggestions.
One important EPRI recommendation was that, at a minimum, all primary system piping should be electropolished (EP) and pre-oxidized to minimize radionuclide deposition on piping surfaces. For forged piping, the interior surface is smooth, and electropolishing alone provides a limited reduction in true surface area. Thus, it is imperative that pre-oxidation follow electropolishing. If the schedule permitted, it was recommended that the reactor recirculation pump bowls and large system valves be electropolished and pre-oxidized. In this latter case, if pre-oxidation was not possible, at least the electropolishing should be done, as some benefit is realized for these cast stainless steel surfaces. Unlike forged piping, the cast surfaces of pumps and valves are very rough, and electropolishing provides a reduction in the true surface area.
The best surface pretreatment for new components is the EPRI stabilized chromium process (SCrP). However, that recommendation encompassed a large percentage of the plant primary system components and was not possible considering the tight project schedule. Instead, SCrP was used on carefully selected valves, piping, pump casings, impellers, valve bodies, and process radiation monitor housings (see EPRI 1003373, Stabilized Chromium Process Final Report, August 2002).
After performing a chemical decontamination to reduce worker exposure, the majority of the major piping systems in Unit 1 were removed for replacement, and at one point it was even possible to walk into the drywell in street clothes.
Email
Comments
Print
Reuse
