Excavation of pipes still on hold at Vermont Yankee

Posted
Saturday February 20, 2010

BRATTLEBORO -- Numerous obstacles have brought to a halt the excavation of a pipe tunnel at Vermont Yankee nuclear power plant, a tunnel that might be the source of tritiated water that has contaminated groundwater under the plant, said Bill Irwin, Vermont's radiological health officer.

On Wednesday, excavation of the pipe trench was halted when crews learned that some of the concrete forms of the foundation and structure of the advanced off gas pipe tunnel "stick out a little bit," said Irwin.

The pipe tunnel is about 10 feet wide, 20 feet long and 15 feet deep.

Instead of being able to excavate in a straight line around the pipe tunnel, he said, now they have to work their way around the concrete forms. In addition, some large boulders are getting in the way of installing shoring, which needs to be put in place before workers drop into the excavation pit.

"They may have some difficulty pulling those rocks out," said Irwin.

Because of the unexpected problems, the excavation "has been stuck for a few days," he said.

Irwin said the excavation crew and Yankee engineers are working on a new shoring plan they hope to begin implementing over the weekend.

Other plant structures and components are also slowing down the excavation, he said, including concrete ducts that service the power plant's emergency diesel generator.

Those ducts carry a pair of diesel fuel lines and a variety of electric cables.

The generator is safety-related equipment that is designed to provide electricity to run coolant through the reactor, the plant's control rod mechanism and its numerous valves in the event the plant's turbine goes off-line.

"Not only is it a complicated area," said Irwin, "it's kind of like an archaeological dig."

The excavation crew also has to be careful that it doesn't "seismically shock" or physically damage any of the pipes or ducts in the area, said Irwin, and the site is restricted by its proximity to the plant's turbine building and advanced off gas building.

"You can't just go in with an excavator or a backhoe," said Irwin.

The excavation needs to reach a point where the joints of the pipe tunnel and advanced off gas drain line concrete duct are exposed, he said. In addition, two pipes that serve the A and B recombiners also need to be exposed, said Irwin.

The off gas system is used to extract radioactive materials from steam circulating around the plant's condenser, which basically serves as a radiator for the plant. The radioactive materials are filtered out of the steam and disposed of in a hazardous waste site.

Recombiners take hydrogen gas produced during the hydrolysis of water in the reactor building and thermally combine it with oxygen to produce water, which is sent back into the system.

Whether the pipe tunnel is the source of the leak won't be known until it has been completely exposed for a visual inspection, said Irwin.

Even when the pipe tunnel is exposed, engineers will still have to rely on boroscopic inspections because there are no access hatches in the concrete structure, he said.

"In this particular case, it is not accessible to people at all," said Irwin. "To find leaks in liquid and radioactive steam systems you want to be able to walk those down and identify them. This pipe tunnel doesn't allow for that.

Cutting a hole in the pipe tunnel might actually be the quickest way to find the source of the leak, said Irwin, but that would interfere with operations of the plant.

Irwin said the long-term solution to the ongoing leak issue, whether the plant continues operation or not, is to site piping that carries radioactive materials above ground or in tunnels that have access hatches to accommodate visual inspections.

That way, he said, "you don't have to wait until you get contamination in the groundwater."

Pipe inspections also need to be incorporated into a routine maintenance program, said Irwin.

Currently, a plant operator only has to prove that its aging management plan is adequate to predict when and what pipes might need to be replaced, which Paul Blanch, a nuclear expert who testified before the Vermont Legislature on Thursday, called a "fix it when it breaks" program.

On Wednesday, Yankee engineers stopped a steam leak that was discovered in the off gas pipe tunnel. They also cleared standing water in the tunnel after they cleared a drain line.

But, said Irwin, "They do have indications in the pipe tunnel that there is continued leakage of liquid."

The boroscope revealed some water was dripping from pipe insulation and was flowing toward the drain where it is collected in a sump and removed, stated Larry Smith, Yankee's director of communications, in an e-mail to the media.

The pit sump is collecting about 100 gallons each day before being pumped to the radiation waste building, stated the Vermont Department of Health, in its daily Yankee update.

New monitoring wells have been drilled since the leak was discovered to help identify the source of the tritiated water, the size of the plume of contamination and where the contamination might be headed.

More wells are being drilled to characterize the separation of the layers of groundwater to determine if the contamination is uniform or higher or lower at different points, said Irwin. Those wells will also help to determine the flow of groundwater under the power plant and the extent of the plume at the southern boundary of the site.

Though tritium levels in the well closest to the pipe tunnel had been dropping, said Irwin, in the past three days they have held steady at about 2 million picocuries a liter.

"That could be related to having stopped the steam leak and unplugging the drain, but it could also be that it's just because of groundwater flow," he said.

Trace amounts of Cobalt-60 have been found in water in the pipe tunnel, said Irwin, but not in groundwater at this point. But, he said, when the leak is found, Cobalt-60 and other radioisotopes might be found nearby.

"As of today," said Irwin, "we continue to see no other radioisotopes in the monitoring wells and no contamination in the drinking water wells, both onsite and off.

Bob Audette can be reached at raudette@reformer.com, or at 802-254-2311, ext. 273.


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