One of the more critical aspects of nuclear reactor maintenance work at Browns Ferry, a 3.4 GW boiling water reactor plant near Decatur, Alabama, USA, 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.
Browns Ferry refuelling floor manager John Blenkinsopp was keen to try the latest Biach tensioner from Hydratight. 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. “We used to have to fly the carousel in, then run all the hydraulic hoses and hook them up,” said Blenkinsopp. That would take us an hour on its own. Now all we do is run a 480V cable to each tensioner, which takes ten minutes.”
Outage performance for stud tensioning at Browns Ferry previously fell into the 150–270 minutes range, even with the industry-leading QD-H units. But after training and experience of the new system, Browns Ferry reduced tensioning to 85 minutes in the autumn 2010 outage.
“Industry outages used to take 60–90 days and stud tensioning would take two to three shifts,” he said. “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.” 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.
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