A Note from Rob Williams at Vermont Yankee
February 12, 2010
Vermont Yankee engineers and technicians working diligently to identify the source of tritium in the plant’s groundwater continue to make progress.
A 13 X 7 foot area just to the west of the plant’s advanced offgas building is being excavated to uncover underground piping and structures that are deemed a possible source of the tritium. The monitoring well in that location has shown the highest concentration. One of the underground structures in that area is a concrete pipe tunnel that runs between the advanced offgas sytem and other buildings. Technicians using a boroscope have identified standing water in the tunnel. Further analysis will determine the source of the water and whether it is a possible source of the tritium.
The excavation work at this point has progressed to four feet and materials for shoring the sides at further depth are being installed over the weekend. The integrity of structures and components in the excavation area is supported by a comprehensive engineering analysis that will ensure safety at its final depth of 15 feet.
All excavation is being done using a relatively gentle process using pressurized water or air to loosen the fill material and a vacuum to remove it. All the hydro- and air-excavation equipment is operated from one truck.
Meanwhile, other monitoring wells are being installed. These wells will further characterize the tritium concentrations in the groundwater.
Of the 14 monitoring wells presently in service, six have tritium concentrations which continue to vary day to day. The highest concentration, as referred to above, is now at 2.23 million picocuries per liter — a slight reduction from previous readings. The second highest is to the northeast of the advanced offgas building and is today at 948,000 picocuries per liter — a slight increase from 937,000. The third highest is further east, but still within the plant’s security-protected area. It is now reading 86,400 — down from its most recent concentration of 92,800. The other wells that show tritium are also showing such variation at lower concentrations.
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.
For more details on the tritium investigation, the Vermont Department of Health has a thorough status report on the investigation at this web link: http://healthvermont.gov/enviro/rad/yankee/tritium.aspx
Also helpful is the Nuclear Regulatory Commission web page on tritium monitoring: http://www.nrc.gov/reactors/operating/ops-experience/grndwtr-contam-tritium.html
- Rob Williams
Entergy Vermont Yankee
Rwill23@entergy.com
