Vermont Yankee provides update on tritium

Last Friday, October 8, Vermont Yankee announced that a water sample from rock fissures beneath the plant tested positive for tritium at an amount of 1,040 picocuries per liter. A statement issued by a representative of the Vernon nuclear power plant said that this amount of tritium is significantly below the US Nuclear Regulatory Commission’s required level to report tritium findings (which is 30,000 picocuries per liter) and significantly below the US Environmental Protection Agency’s standard for permissible levels of tritium in drinking water (20,000 picocuries per liter). The rock fissures from which the sample was taken fed a former industrial drinking water well that previously served the plant’s Construction Office Building, known as the COB well. This well was retired earlier this year (March) as a conservative precaution shortly after the tritium leak from the plant was discovered.
This water sample test result does not indicate any new leak of tritium from Vermont Yankee.
Rather, this test result confirms the migration of tritiated water that was previously released from the leak in the advanced off gas (AOG) system pipe tunnel that was sealed and repaired earlier this year.
Further, this test result does not indicate any threat to public health or safety because the sample was not taken from any active drinking water well, nor has tritium been detected in or near any current drinking water well.
Vermont Yankee is implementing an industry-leading program to protect groundwater at the site. This program focuses on inspecting, monitoring and selectively replacing plant piping and other components to prevent leaks. Also, additional monitoring wells are being installed to promptly detect any leaks should they occur.
As noted, the Environmental Protection Agency’s limit for tritium in drinking water is 20,000 picocuries. A picocurie is one trillionth of a curie. To place these figures in context, there are at least 6 curies (or 6 trillion picocuries) of tritium in an average self-illuminated red “EXIT” sign at your local movie theater or school gym.
Vermont Yankee believes this test result is consistent with previous findings, showing that there is no threat of contamination to drinking water sources AND the tritium remediation plan has been comprehensive and effective.
Vermont Yankee’s tritium remediation project has continuously monitored drinking water wells on the VY site and at a number of locations in the area around the site and no tritium has been detected in any drinking water wells. The off site locations range from the Vernon elementary school, which is located just across the street from the plant to the water system of the town of Hinsdale, NH, located across the Connecticut River.
As Vermont Yankee continues to finalize the remediation of tritium that the company first discovered and self-reported in January 2010, the company re-emphasizes today that it is committed to maintaining increased sampling frequencies for tritium and increased communications. Both of these initiatives were part of a March 25 public commitment that the company made as part of its industry-leading groundwater protection initiative.
Vermont Yankee has shared the results of the recent water sample test with the U.S. Nuclear Regulatory Commission and the Vermont Department Health and will continue to cooperate with both organizations. Further, Vermont Yankee has continued to extract groundwater – a process that began several months ago -- as part of its efforts to remediate the previous tritium leak from the AOG pipe tunnel.
The shallow groundwater zone has been the main focus of Vermont Yankee’s groundwater efforts to date. This zone, which extends to 70 feet below the surface, includes soils of many different compositions. This is the level where shallow groundwater moves. The general flow at the Vermont Yankee site is west to east, away from drinking water supplies.
This zone has been continuously monitored and tested since the first discovery of tritium in January of this year.
The depth to bedrock varies locally across the region, but in this area of the Vermont Yankee site, the depth to bedrock is approximately 70 feet. Groundwater flows through the bedrock via cracks and fissures. This water also flows west to east. When a deep drinking water well is installed, it bisects these cracks, which allow water to supply the well casing and eventually be pumped to the end user.
A total of 25 monitoring wells have been installed and seven more are currently being installed. These wells are used to track the direction and volume of the groundwater under the site and to measure the tritium content.
As a result of the data gathered from these monitoring wells, a number of extraction wells have been drilled at the points of significant concentration. More than 267,000 gallons of tritiated water have been extracted from the groundwater and pumped into holding tanks on the site. As the higher tritium concentrations move eastward, consistent with the general hydrology of the site, new wells are drilled in the path of the flow and contaminated water is pumped out.
There currently are two extraction wells in operation. Vermont Yankee is assessing the flow rates and tritium content of water being extracted and present in the monitoring wells to determine whether further mitigation is warranted
The deep bedrock layer is the site of the current testing that has produced the relatively low-level sample of tritiated water that is the subject of recent media stories.
By using the pre-existing COB well, Vermont Yankee technicians have been able to collect and test two water samples from fissures in the bedrock adjacent to the well. These samples were taken at 200 and 300 feet below the surface by a process called “Packer testing.”
Packer testing consists of lowering a 20-foot long hollow steel tube into the well. The tube has bladders (packers) at the top and bottom, which can be inflated to seal a 20-foot section of the well. Existing water in the well is pumped out, and samples of water contained in fissures of the adjacent bedrock are then collected in the tube. The sample is then brought to the surface and samples are taken to be tested for tritium.
In the case of these recent measurements at Vermont Yankee, the test at the 200-foot level required 48 hours of continuous extraction to collect the volume of water required for a representative sample. This site had been identified by a preliminary test in March as the site of a low-yielding fracture in the rock. It was this sample that contained a concentration of 1,040 picocuries of tritium per liter of water.
The test performed at the 300-foot level encountered free-flowing water from the adjacent fissures. The sample was easily collected and no tritium was found.
It also should be noted that other isotopes such as Strontium-90 and Cesium-137 have never been detected in groundwater samples taken at Vermont Yankee. This includes water samples taken from monitoring well GZ-10 which was installed in early February in the immediate vicinity of the advanced off gas (AOG) system pipe tunnel where the tritium leak occurred.
Strontium-90 and Cesium-137 and other isotopes were identified in limited quantities in soils directly adjacent to the leak for the AOG pipe tunnel. Soil in the area of the leak has been removed and is being stored in shipping containers on-site waiting for disposal at a licensed off-site waste facility. Empirical data gathered to date supports the conclusion that Strontium-90 and Cesium-137 and other radionuclides, unlike tritium, which is very similar in molecular structure to water, are not transported across the site by groundwater.
Source: Entergy Nuclear Vermont Yankee. Vernon, VT/October 11, 2010