Cooper Nuclear Station’s construction of a replica of a quarter of the plant’s drywell area helped to save an estimated 600 man hours of outage work and reduce radiation dose from a predicted 44 REM to 31 REM during work to replace four fan coil units.
“The fan coil replacement – an infrequently conducted evolution – was a tremendous success for Cooper,” said Entergy Nuclear’s Brian O’Grady, Vice President and CNO of Cooper. “We came in under our dose goal and on schedule, both of which are directly related to the use of our drywell mockup.”
Construction of the drywell replica
Cooper, an 810-megawatt boiling water reactor, is owned by the Nebraska Public Power District and managed by Entergy Nuclear. The plant came online in July 1974. In 2010 the station received a license renewal from the US Nuclear Regulatory Commission to operate for another 20 years. Cooper is located 80 miles from Omaha, Nebraska in the central United States.
Chris Pelchat, Cooper project manager, said the work included contracting local metal fabricators, carpenters and other specialists to build the $250,000 (roughly €200,000) replica, which was housed inside a closed school at a nearby town. The replica took three months to build.
The replica supported a project to replace the plant’s four belt-driven fan coil units with direct-drive units, an enhancement related to Cooper’s change from a 12-month fuel cycle to an 18-month cycle.
The drywell replica was built and an adjoining training facility with classrooms and a workstation for the radiation simulation tracking was constructed. A wireless system was set up to analyse movements in the radiation area, with location tags worn with the dose rate metres. The location tags gave workers information on the best path while performing work in the replica to lower the dose. Real-time video added to the practice work and the video of the actual drywell work was available to anyone with access to the plant’s internal website.
A radiation survey of the actual drywell in the station, taken during the last refuelling outage, supplied the data for the mockup radiation simulation. Imagery also was taken during the last outage by a 360-degree laser measuring of all the elevations of the drywell.
Training and teamwork
Groups of plant employees, supplemental workers and vendors spent three weeks at the replica, practicing ways to limit dose. The coursework was broken into two-hour rotations of three teams, who would then evaluate their training through video playback and discussion.
“We went through the training with the workers and went from basic to more complex work," said Pelchat. “For example, the drywell mockup is pretty well lit for the initial training. But then as training progresses, they have to hang the temporary lighting, be dressed out and perform all the tasks that they would have to do in the actual evolution.”
“The team members have come up with some incredible ideas – how can we get around this, how can we make this better – it’s a questioning attitude that makes this whole project even better,” said Pelchat.
Pelchat said ideas solicited from the workers resulted in an easier way to move equipment and tools to and from the drywell mockup, without the hazards and raised dose from rigging.
A metal box was constructed to roll on rails of ball bearings to and from the entrance of the drywell mockup, reducing dose and saving time. “That gave us a lot more flexibility to move something, to get it out of the way,” said Pelchat.
The mockup was painstakingly designed from the original plant blueprints, with details down to small upward kinks in the metal-grating floor that workers would step on while performing the evolution.
The replica fan motor assemblies and coil assemblies are the precise weights of the actual plant machinery. The training also included disassembling pipes in order for welders to reassemble them.
“Just sharing an identical replica is beneficial,” said Pelchat. “When the workers come down here they can see the challenges other teams or personnel face. It really makes the teams think about how to work well together.”
Pelchat said while building the mockup, nearby residents would come by the closed school to see what was going on. He said the interest prompted a community outreach day sponsored by Cooper. More than 200 people attended.
“This gave us a chance to tell the people how we are working to assure their safety and be transparent about the changes we’re making to the plant,” said Pelchat. “There was a lot of community interest and a great chance for us to tell our story.”
“I am definitely happy with the results,” said Pelchat. “The vendors on the project were all new to nuclear prior to this. The drywell mockup gave them the opportunity to develop proper radiation worker behaviours better than any classroom training. The project’s aggregate dose, duration and overall success can be attributed to their practice and lessons learned while rehearsing the mockup.”