Trace amounts of cobalt-60 found
BRATTLEBORO -- Trace amounts of cobalt-60 were found in standing water in the advanced off-gas piping tunnel Thursday at Vermont Yankee nuclear power plant, said John Dreyfuss, Yankee's director of nuclear safety assurance, during a conference call with the Nuclear Regulatory Commission, the Vermont Department of Health and the Vermont Department of Public Service.
But Dreyfuss told the agencies that finding trace amounts of cobalt-60 in such a location is not surprising, just as was finding it in standing water in a pipe trench in the plant's radioactive waste building on Jan. 22.
In that instance, cobalt-60, at 13,000 picocuries per liter, and zinc-65, at 2,460, were found, along with tritium, in the pipe trench. Drinking water limits for Cobalt-60 are 100 picocuries. For Zinc-65, the drinking water limit is 300.
At the time, tritium levels were at 1.6 million picocuries, with a drinking water limit of 20,000.
The water in the off-gas pipe trench drains into a sump pit, which then drains into a tank.
In Thursday's update from the DOH, it stated the off-gas pit sump appears to have pumped 100 gallons of water into the radioactive waste building over the last 24 hours.
"This is a possible indicator of leakage within the pipe tunnel, which drains to this sump," stated the DOH. "This is the drain that was recently unclogged, and may have collected water from nuclear steam leaking into the tunnel."
During the conference call, Dreyfuss told the agencies that it is not believed the trace amounts of cobalt-60 are making their way to the groundwater, stated Neil Sheehan, spokesman for the NRC, in an e-mail to the Reformer.
Cobalt-60 has a shorter half-life -- 5.27 years -- than tritium, stated Sheehan.
"However, it is a gamma emitter, rather than a weak beta emitter like tritium," he said.
On the other hand, cobalt-60 is not as transportable as tritium, which behaves like water. If cobalt-60 were to get in the ground, it would likely be filtered out by the soil.
Those exposed to a gamma emitter such as cobalt-60 are at significant risk, according to the Environmental Protection Agency.
The magnitude of the health risk depends on the quantity of cobalt-60 involved and on exposure conditions such as length of exposure, distance from the source and whether it was ingested or inhaled.
"Again, Entergy is saying trace amounts are involved," stated Sheehan.
The water in the off-gas pipe tunnel has been removed, stated Larry Smith, Vermont Yankee's director of communications, in an e-mail to the media. Additional boroscopic inspections of the tunnel will be conducted today, he wrote.
"Thus far, the well monitoring data is not conclusive as to whether the AOG tunnel is the source of tritium in groundwater at the site and the investigation of this and other potential sources is ongoing," wrote Smith.
Excavation of the site was put on hold Wednesday because of concerns for worker safety and for nuclear safety requirements.
"Workers are finalizing plans to excavate from 12 to 15 feet below grade to check the tunnel for leakage," wrote Smith.
Engineers and vendors are updating their plans for the design of shoring and bracing of the site, he stated.
The excavation site has been expanded to include a monitoring well that had levels of 1.99 million picocuries for the past two days, wrote Smith.
An enclosure building is being built over the excavation site to isolate the area from the environment.
Two deep wells have been dug to help better understand groundwater flow beneath Yankee, stated Smith.
Other wells have been drilled since the contamination was discovered to help define the extent of the plume and identify the source of the leak.
"While this investigation continues, it is important to note that there has been no tritium levels found in any samples taken from drinking water wells or the river," stated Smith.
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