Tools | Repair & maintenance
Preventing half-SCRAMs with insulated screw bit3 September 2012
No manufacturer produced a non-conductive bit for a torque screwdriver, until now. Staff at Entergy’s Pilgrim Nuclear Station developed the tool to prevent short-circuits in I&C cabinets.
All torque screwdrivers include a variable-torque range, and all have a torque-limiting clutch that disengages once the preset torque has been reached. This prevents over-tightening that can cause possible damage to components and the final product. While torquing terminal screws in an ATS (analogue trip system) cabinet, the metal screwdriver bit can inadvertently short-out the adjacent terminal screw, causing an unexpected half-SCRAM. A half-SCRAM event greatly challenges plant operations with respect to controlling the plant; and a full reactor SCRAM challenges every department from operations, chemistry, maintenance and more.
Entergy’s Pilgrim team was frustrated with not being able to find a non-conductive bit that could be used in ATS cabinets, especially since there was a history of half-SCRAMs from this function. Non-conductive material provides electrical insulation to prevent inadvertent short of screw terminals. Team members researched, tested and machined revisions until they were successful.
Attacking the half-scram issue
Beginning in October 2010, employees at the 685 MW Pilgrim BWR in Plymouth, Massachusetts began researching non-conductive materials. It was soon decided that ceramic would be the material of choice for this unique and demanding function.
During testing, silica-based ceramic was found to be too brittle and cracked easily under stress. A second version was made with zirconia ceramic. Zirconia ceramic is harder than steel and must be machined with a diamond cutter or grinder. After obtaining the material and a diamond cutter, the next iteration was ready for testing.
The zirconia ceramic bit performed well. It went through a series of tests to the maximum electrical termination screw torque nuclear requirements of 18 inch-pounds (2.04 Nm) without failure.
A brass hexagon-fitted bit is used. A Delrin rod—a plastic/nylon non-conductive polymer—is the connective material to hold brass and zirconia ceramic together. Glue would neither hold nor bond to the ceramic.
Delrin is an engineering thermoplastic used in precision parts that require high stiffness, low friction and excellent dimensional stability. It was discovered by Hermann Staudinger, a German chemist who received the 1953 Nobel Prize in Chemistry and first synthesized by DuPont research chemists around 1952.
The hex bit is machined with a deep groove. The zirconia ceramic tip is diamond-cutter machined with a deep tongue. The bit and tip are press-fitted into the Delrin rod, which was machined 0.05 of an inch smaller; it fills in around the inserted bit and tip when pressed; and the Delrin plastic is sufficiently strong to withstand nuclear torquing requirements.
Production, testing, input and revisions took about two months.
At Pilgrim, there was a stop-work order on the ATS cabinet until it was proven that the new zirconia ceramic torquing bit would work.
After testing, all of the remaining ATS screws at the Pilgrim station (more than 1500) were worked/torqued without any incident. About 2 percent (approximately 30) were found loose and out of specification. Now that all 1500 screws in the ATS cabinets are torqued to specs, they only need to be routinely checked.
The use of the insulted screwdriver bit eliminates inadvertent ATS cabinet short-outs that can cause half-SCRAM events.
Radiation savings could be up to 50 to 100 Rem of exposure that would occur during forced outage activities that would need to be performed by workers due to a reactor SCRAM. One reactor SCRAM would cause a loss of revenue of up to $1 million dollars for every day that the plant is not generating electricity.
Pilgrim Nuclear Power Station has had more than 25 unexpected half-SCRAMs in the past five years. About half of these (12) could have been prevented by the insulated screw bit. Additionally, there have been approximately 12 days of lost generation within the past five years due to related issues, equating to approximately $12 million in lost revenue.
The World Nuclear Association reports that unplanned automatic scrams result in thermal and hydraulic transients that affect plant systems. On average, each plant experiences 0.6 SCRAMs per year. This equates to approximately $600,000 loss of revenue per year.
The zirconia ceramic torquing screwdriver bit has also been used for lifting and landing leads for surveillance testing, a common nuclear function. This improves occupational and nuclear safety further.
“This is a great addition to our tool chest. The team worked together from the research and production of the bit at the Pilgrim shop to testing it in the plant. This is another way we are making operations safer and more productive here at Pilgrim,” said Bruno Giorgio, nuclear control technician, I&C Maintenance, and team lead for the project. Other members of the team were Amy Niederberger, senior engineer, Chris Hultstrom, lead nuclear control technician, and Bob Wheaton, lead nuclear control technician.
Pilgrim started using the new bit in December of 2010. All I&C technicians are trained on the use of the new screwdriver bit and it is presently being used on any terminations where there is a high risk of shorting terminations to each other or to ground.
This article was originally published in the August 2012 issue of Nuclear Engineering International