Construction of two 1200MWe Westinghouse PWR reactors at Watts Bar in southern Tennessee began in 1973, and carried on until 1985, when unit 1 was basically finished. That year, the Nuclear Regulatory Commission raised many questions that needed to be answered before unit 1 could be licensed (and it took another 11 years until the plant started commercial operation). Plant owner and operator Tennessee Valley Authority (TVA) created a plan to address these issues in 1985, but ceased construction on unit 2 at that time, having finished major structures, including pressure vessel, steam generators, pressurizer, turbines, condenser, generator and switchyard transformers, and having installed systems such as the reactor coolant piping.
In a 2007 letter to the NRC, TVA explained why it wanted to restart the project. It said that proceeding with the completion and operation of WBN Unit 2 is the best decision for TVA and the Tennessee Valley in terms of power supply, power price, generation mix, return on investment, use of existing assets, and avoidance of environmental impacts (by building new fossil-fuel plants, or siting new nuclear power stations elsewhere).
Although Watts Bar 2 is the first US nuclear construction project in almost 20 years, it is really closer to a restart than a new-build. In fact, TVA’s experience restarting Browns Ferry 1 in Alabama was a major influence. “With a rising demand for more baseload generation, TVA restarted Browns Ferry Nuclear unit 1 in June 2007, and the success of that recovery project helped us to understand what it would take to complete Watts Bar 2,” Ashok Bhatnagar, senior vice president of TVA’s nuclear generation development & construction, said recently.
Construction began on the three-reactor Browns Ferry BWR in 1967. The three units came online in the mid-1970s, but were shut down in 1985. Ten years later, units 2 and 3 were restarted. But it was not until 2007, after spending $1.9 billion, that unit 1 was restarted. TVA also operates three PWRs at its Sequoyah plant in Tennessee.
The resumed Watts Bar 2 construction project officially began in August 2007; a project office was set up in October and contracts were given out. Bechtel, which provided engineering, start-up and technical services to the Browns Ferry 1 restart project, won the engineering, procurement and construction project management contract.
Westinghouse won a $200m contract to upgrade and replace ‘most’ instrumentation and control systems, supply new reactor coolant pumps, steam generator services, crane replacement and services, probabilistic risk assessment, nuclear steam supply system plant design engineering services, drive rods, licensing services and safety analysis. Westinghouse is also installing its in-core information surveillance and engineering system to measure core power distribution.
A notable exception to the upgrade programme is the legacy Eagle 21 reactor protection system, which will not be changed, for at least two reasons. First, it is the same system on Watts Bar 1 (and other power stations), says TVA spokesman Terry Johnson. The similarity of units 1 and 2 is a key part of the development strategy that simplifies training and safety. “Using the same system on both units is part of the plan to have both units functionally the same from the operations and maintenance perspectives,” he said. By the same token, by retaining the same legacy systems as unit 1, TVA can use the same licence application and processes that eventually succeeded with unit 1 (after more than a decade of wrangling). This plan avoids many regulatory pitfalls.
Siemens won the contract to carry out turbine and generator work, and got to work early on to disassemble the turbines, take out the turbine rotors for refurbishment, and pull out the generator rotor for refurbishment.
Also in late 2007 began the years-long job of pulling out and replacing 27,000 copper-nickel alloy condenser tubes, whose material properties are now obsolete, with stainless steel alloy. As of March 2009, the job is halfway done. Workers are inspecting and replacing packing in penetrations – where pipes or cables go through concrete walls – and their seals.
Engineering walkdowns – where someone walks through the plant, following pipes with blueprints to make sure that what is supposed to have been installed actually is – also started fairly early in the process. Some valves, pumps and electrical components are missing. Over the years, staff at Watts Bar 1 have occasionally harvested components from Watts Bar 2, rather than waiting a week or more for a new part to arrive from a manufacturer.
The components that remain, which might not have been used in years, all need to be examined. Johnson, who toured the site recently, reported seeing two machinists next to a valve that they had opened up, making lots of micrometer measurements. They needed to make sure that the valve was in good condition and could be relied on to do its job. In a December 2008 presentation to the NRC, TVA estimated that the project would need to refurbish 80 pumps, 90 motors and about 5000 valves. Contractors are replacing some components, such as the main coolant pump motors, simply to be sure of their integrity.
Johnson says that the plant will be safe to operate, and is not obsolete, despite its age. Pipes are still pipes, pumps are still pumps, and motors are still motors, even though developments in materials and mechanical engineering have improved these essential components. So TVA will upgrade many of these with more modern equivalents. “Even existing plants which have been operating for years, we can replace the internals, such as a pump, to make them more efficient,” Johnson says. For example, controls to monitor flow rates in a pipe used to be exclusively analogue; now they may be replaced with more efficient digital controls.
He adds: “We are not going to make unit 2 a much more modern plant; we want to have licensed operators from unit 1 working on unit 2. Functionally they will operate in the same way, but where in unit 1 an analogue control speeds up a motor, on unit 2 a digital control does that.”
A team of 400 engineers based in nearby Knoxville is drawing up construction plans for Watts Bar 2. Once they are checked and verified to meet specifications and quality standards, they will be turned over to construction crews.
When construction stopped in 1985, the TVA estimated that the plant was 80-90% complete. After early walkdowns, engineers revised that figure to 60%. Completing the remaining 40% is not like adding a rear extension to a house, where an entire structure is built from scratch in one area. Instead, pieces need to be fitted in around existing structures.
“When you walk through the plant, and compare the operating unit to unit 2, in some cases there is a valve missing, there may be some components missing, but the person walking through the plant wouldn’t notice a lot different,” the spokesman says. “On the unfinished unit, the insulation has not been done – it’s one of the last things we do – and the equipment isn’t painted, and neither is the floor. It’s not finished, but you don’t get the sense that we are only just over half-way done.”
Site construction staff will peak at 2000-2300 in March 2010, by which time engineers in Knoxville will have had more time to develop and approve plans to pass on to construction teams. Bechtel says that the site will employ 250 permanent staff.
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| Watts Bar 2 resumed construction schedule |
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July 2007- TVA board decides to resume Watts Bar 2 construction |
