- Edited By Chris Reed -
One of the most expensive aspects of operating a nuclear power station is taking it offline for regular maintenance or refueling.
The cost of maintenance generally pales behind the income lost from not supplying power to the grid. So for most operators, shortening outages due to maintenance and repair downtime is a priority.
At Browns Ferry, a boiling water reactor generating plant near Decatur, Alabama, historic closures and upgrades have fostered a strict policy of safety-first, speed second, so ways to achieve shorter outages while maintaining safety are always welcome.
One of the more critical aspects of reactor work at Browns Ferry, and every other plant, is opening and resealing the vessel head flange, which consists of many large studs and nuts. These are tensioned to a very high load by applying hydraulic stud tensioners.
Tensioning these accurately is a huge undertaking, requiring technicians to operate in high radiation dose areas. The studs are tensioned and nuts are seated to elongate the stud; the correct tension signifying the reactor flange is properly seated and sealed.
Developments in systems to tension these studs have moved on considerably since Browns Ferry installed its four-way stud tensioning system in 1994.
This Biach Quick-Disconnect-Helical (QD-H) design has four stud tensioners mounted on a circular carousel supported by the reactor head flange lifting lugs. At each pass four studs can be tensioned, each at 90 degrees to each other, offering optimum positioning to reduce stresses. The units are then re-positioned several times, each time working on four studs at optimum spacing, until the vessel is completely resealed.
Further trim passes might be required if the final elongation readings do not meet the required specification.
Completing this operation quickly means reduced critical path cost and lower radiation dose to the crew members. Typically, this system is operated by one or two men per stud tensioner and a central pumping system operator, plus plant radiation technicians and supervisors.
The system is linked by heavy hydraulic lines running from each tensioner to a distribution manifold mounted on the central carousel and from there to the control unit, on the refuel floor, where space is at a premium.
So, Browns Ferry refueling floor manager John Blenkinsopp was keen to try the latest Biach tensioner from Hydratight.
“I don't want personnel to get injured, and I would never ask my team to do a job that I haven't done, or would be willing to do,” said John. “This system offered both time and personnel savings.”
The Biach SCT (Self Contained Tensioner) was proposed in 2006 and has been available commercially for a couple of years. But John was the first operator to use it on a large vessel — in this case, a 21ft reactor secured by 92 six-inch studs.
The SCT device greatly reduces tensioning time by putting the hydraulic and control systems into the tensioner unit, linking the four units together, then running just one power cable to each stud tensioner.
Each unit can control the others, to maintain the accuracy of the group — and only one man is needed to handle and reposition them, since each also includes a hoist controller. The control panel on each tensioner allows a supervisor to monitor all four tensioners and ensure work is going smoothly.
In the case of Browns Ferry, the SCT units were retro-fitted to the existing Biach QD-H tensioners, replacing the top plate of the existing tensioners with a complete SCT sub-assembly. The central pumping unit was replaced by on-tensioner pumps and controls and all of the hydraulic lines removed from the equation.
“The SCT unit saves time during installation of the carousel on the reactor head,” said John.
“We used to have to fly the carousel in, then run all the hydraulic hoses and hook them up. That would take us an hour on its own. Now all we do is run a 480V cable to each tensioner— which takes 10 minutes.”
The new system saves time and reduces the potential for injury during the tensioning operation.
“With hoist controls mounted to each tensioner and communication screens giving the status of each tensioner, safety and time management are greatly improved,” said John.
“Teamwork is crucial and anything that reduces the possibility of error and injury is welcomed.”
Outage performance for stud tensioning at Browns Ferry previously fell into the 2hr 30min-4hr 30min range, even with the industry-leading QD-H units.
But after training and experience of the new SCT system, Browns Ferry reduced tensioning to 1hr 25mins in the Fall 2010 outage.
“Industry outages used to take 60-90 days and stud tensioning would take two to three shifts,” John explained.
“In plants that don’t have quick-disconnect tensioners, the unit has to be physically screwed down on to the stud and that takes a long, long time.
“We installed the Biach QD tensioners in 1994 and had great success with them; so I’d recommend them to anyone. With the SCT units on-board, performance is improved even more.”
The reduction in tensioning time alone saves Browns Ferry in the region of $50,000 an hour per outage, across three scheduled outages every two years.
“Our team has worked with a member of Hydratight’s Biach team on every outage so far, to develop different methods to improve procedures,” said John.
“Safety is our most important point, obviously,” said John. “We focus on safety and quality, so essentially the schedule takes care of itself.
“Scheduling issues usually arise when mistakes have to be corrected. The SCT units allow us to do the job right the first time — so there is no loss of time, and we save money.”
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