|By Brian Schimmoller, Contributing Editor|
Bob Williams, TVA’s Watts Bar Fukushima project manager, was in a tight spot in August 2012 when the U.S. Nuclear Regulatory Commission issued its orders concerning nuclear plant modifications in response to the Fukushima Daiichi accident. Construction of TVA’s new nuclear plant, Watts Bar Unit 2, was well underway when Fukushima happened. The NRC order required operating plants to be in compliance within two operating cycles, but required construction permit plants like Watts Bar 2 to be in compliance before receiving an operating license. So while the existing Watts Bar Unit 1 would need to be in compliance by October 2015, Unit 2 had to get there by early Fall 2014.
“The original plan was to do Unit 2 first because of the schedule requirement,” said Williams. “However, as the mitigating strategies were developed and designs were initiated, we discovered that approximately 70 percent of the connection points were common (shared) between the two units. With a small amount of additional work in the Unit 1 refueling outage, we could bring both units into compliance at once. This solved an operational issue with operators in a common control room having to operate common equipment to two different sets of procedures. It also allowed us to touch procedures, instructions, drawings, training once instead of twice.”
Because of the compressed schedule imposed by the order, Watts Bar had to begin design and procurement activities before any of the regulatory and industry guidance around flexible coping (“FLEX”) was finalized. TVA had to make some conservative assumptions as to what would be required. “Basically, because of the timing and the unknown requirements, our design is more robust than the minimum requirements,” said Williams. “We also have less reliance on the Regional Response Centers than many utilities will have.”
The FLEX equipment at Watts Bar is truly robust. A new Flexible Equipment Storage Building (FESB) has been constructed to house the FLEX equipment, which includes: three Triton diesel pumps, three diesel Dominator pumps, two 3 MW diesel generators, two tow vehicles, a compact track loader for debris removal, two diesel intermediate-pressure pumps, two diesel transfer pumps, hose trailers, portable light stands, small portable diesel generators, and disaster kits. The FESB building has been constructed above the probable maximum flood level and is designed to remain functional at two times the site safe shutdown earthquake. For coping with spring and summer storms, the FESB is designed to withstand a design basis 360 mph tornado (rotational plus translational speed). Williams noted that the monster tornado that swept through Tuscaloosa, Alabama, in 2012 had total speeds in the high 200-mph range.
To ensure rapid emergency response, 225 KVA diesel generator connections are provided to the vital battery chargers, and the 480V shutdown boards; and 3-MW diesel generator connections are provided for the 6900V shutdown boards. “The site was originally designed for a fifth diesel generator, which enabled us to use some existing switchgear for our FLEX strategy,” said Williams. “Prior to the modifications, we only had four paths by which to get power to the plant; now we have nine.”
Although water (too much of it) initiated many of the troubles at Fukushima, it was the lack of water (and an inability to get it where it was needed) that ultimately contributed to the subsequent core meltdowns. Water should not be a problem at Watts Bar. The new auxiliary feedwater supply tank has a capacity of 500,000 gallons, and meets the same design basis criteria and seismic basis as the FESB. The tank will provide 15 hours of clean water to both units, or about 30 hours to one unit. Maintenance procedures are in place to periodically test the isolation valve, and as a passive system, the tank can supply water on either loss of power or control air.
TVA also has installed connections to the refueling water storage tank, the boric acid tank, the demineralized water storage tank, the primary water storage tank, and the tritium water storage tank if additional sources of clean or borated water are needed. And although they’d obviously rather not use it, TVA has access to a nearby water source, Watts Bar Lake.
Because it was designed ahead of the industry guidance related to Fukushima, the FLEX equipment at Watts Bar duplicates some of the equipment provided by the Regional Response Centers. Equipment from the Regional Response Centers can then be used as replacement equipment for a failed piece of equipment or to prevent multiple moves of site equipment as events proceed.
TVA is developing preventive maintenance procedures for all of the FLEX equipment. “We are contracting with the equipment manufacturers to conduct maintenance on some of the larger pumps and diesel generators,” said Williams. “FLEX equipment similar to existing plant equipment such as breakers and valves are being added to existing maintenance procedures.”
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