According to a report filed yesterday by the Vermont Department of Health, temporary modifications for the two hydrogen recombiner steam trap drain lines are in progress at the Vermont Yankee nuclear power plant in Vernon. The modification will eliminate the failed piping in the Advanced Off-Gas (AOG) pipe tunnel that has been draining directly into the tunnel and to the AOG pit sump through the floor drain, and will provide a direct path from the steam trap drain lines to the AOG drain tank. A more substantial enclosure over the AOG pipe tunnel excavation is also being constructed. This is necessary because the excavation will be open until the scheduled April and May 2010 refueling outage. During the outage, workers will more thoroughly clean out the AOG pipe tunnel to prevent the tunnel's floor drain from getting clogged with debris still left in the pipe tunnel. Vermont Yankee may then install a camera in the pipe tunnel for continuous surveillance until the outage, when more permanent fixes are planned. The Vermont Department of Health is tracking the progress made in mitigating the tritium leak at the plant. See maps below
Tritium Event Investigation AnalysisVermont Yankee engineers are returning to conducting a “root cause analysis” of the tritium leaks.This process is a formal means of evaluating the event to identify the possible causes, how the event compares to other nuclear power plant operating experiences, and whether the extent of conditions that led to the contamination of groundwater at the pipe tunnel are possible at other locations on site. There may be other pipes subject to the same type of corrosion that caused the two hydrogen recombiner steam trap drain lines to fail. There may be other floor drains that could get clogged with debris and cause contaminated water to collect in building or tunnel spaces. There may be more flaws in concrete joints that would allow contaminated water to leak into the groundwater. This root cause analysis should lead to actions that prevent such failures from recurring.
Remediation Plans for Contaminated Soil and GroundwaterVermont Yankee is now evaluating plans for remediation of contaminated soil and groundwater. Results from a detailed soil study in the excavation area should inform decisions about remediation.The soil study will for a second time use measurements of radioactivity in the soil near the leakage pathway to identify the extent and magnitude of soil contamination. The ability to shore up the walls of excavation deeper than it is already shored will also be a consideration in remediation plans. An extraction well may be drilled in the vicinity of well GZ-7 (see map of well locations) to pump out contaminated ground water. Vermont Yankee has to act quickly on this decision as the groundwater is moving, and there is a limited time frame for contaminated water to be available for extraction at a given location.
Geophysical Testing of Drinking Water WellsVermont Yankee has completed geophysical testing of the last of six drinking water wells on site. This testing is done to characterize the condition of the bedrock through which these deep wells are drilled, to better determine if the bedrock will prevent the more shallow groundwater from contaminating the deeper water drawn by the drinking water wells. Preliminary geophysical testing results from the first well tested, the Construction Office Building (COB) well, are expected on March 16. Vermont Department of Health and Agency of Natural Resources technical staff will be on site at Vermont Yankee, along with their counterparts from New Hampshire and Massachusetts, for that report.
Background
On January 7, the Vermont Department of Health was notified by Vermont Yankee Nuclear Power Station that samples taken from a ground water monitoring well on site (identified as GZ-3) contained tritium.
Tritium is a radioactive form of hydrogen. It is a by-product of the nuclear fission process in a nuclear reactor, and also occurs naturally in the environment in very low concentrations. Most tritium in the environment is in the form of tritiated water, which easily moves about in the atmosphere, bodies of water, soil and rock.
The finding of tritium in ground water signals that there has been an unintended underground release of radioactive material, and that other radioisotopes may have contaminated the environment.
Vermont Yankee officials are conducting an investigation to identify the source of the tritium, and the magnitude of contamination, with the Nuclear Regulatory Commission (NRC) in an oversight role.
The Vermont Department of Health has stepped up the frequency and number of water and environmental samples tested at the public health laboratory, and is doing independent analyses of health risk. With assistance from a number of other state agencies, a team of health and environmental experts will be on site as independent analysts.
Monitoring Well Test ResultsTesting by the Vermont Department of Health and Vermont Yankee of on-site and off-site drinking water well samples, as well as water taken from the Connecticut River, continue to show no tritium in excess of the lower limit of detection (
The Tritium Concentration Graphs (see below) show test rsults from Vermont Yankee over time. These graphs show generally decreasing tritium concentrations at well GZ-10, where a source of groundwater contamination was found and terminated on Feb. 14. The graphs of other tritium contaminated wells, especially GZ-7 and GZ-15 show a generally increasing trend. Taken together, the graphs show the plume of contaminated groundwater gradually moving from west to east, from GZ-10 to the Connecticut River.
With well GZ-21 now in service, there are now a total of nine contaminated wells. GZ-21 is very close to the presumed center line of the plume of tritium contamination, and between the Containment Access Building and the Advanced Off-Gas (AOG) building west of well GZ-15. In contrast, well GZ-10, which at one time resulted in tritium concentrations of more than 2.5 million picocuries per liter (pCi/l) now is down to 202,000 pCi/l. This well next to the original leakage pathway continues to shed its groundwater to the east.
The most recent well results reported by Vermont Yankee are listed below. All results are expressed in terms of picocuries per liter (pCi/l), or below the lower level of detection (
GZ-1:
Source: Vermont Department of Health. March 16, 2010