A Note from Larry Smith at Vermont Yankee
February 24, 2010
Vermont Yankee engineers and technicians continue their investigation into the source of tritium in the plant’s groundwater.
Progress continues in our effort to excavate and expose the Advanced Off Gas concrete pipe tunnel for inspection. Last night, another 18 inches of dirt was removed. Excavation is proceeding today as weather conditions permit. Excavation has revealed a portion of the concrete encasement of the Off Gas OG-100 drain line, which comes off the AOG pipe tunnel. Preliminary visual inspection appears to show a defect in the concrete surrounding the pipe, where the line makes a 90 degree turn into the AOG building. This area will be studied closely as excavation continues and when the integrity test, described below, is conducted later in the week.
Soil samples continue to be taken during the excavation process. To date, soil samples taken are consistent with prior site characterization studies and do not show signs of process piping leakage. One sample taken last night did detect a trace amount of cesium 137, but the level is consistent to what would be found in soil due to atmospheric testing decades ago. As previously reported, radiation levels in the excavation pit increased slightly due to nearing underground process lines which contain radioactive fluids. A section of the dig site is being maintained as a radiological controlled area and all radiological controls for workers entering this area of the excavation are in place. An engineered catch basin to capture potential leakage encountered at the unearthed AOG tunnel has been designed and will be installed. A temporary shelter was placed over the excavation area last night to help ward off snow and allow work to continue. The final enclosure above the excavation area will be completed after final parts of the shoring are moved into place and excavation completed.
Plans are complete to test for leakage paths to the ground from the Advanced Off Gas concrete pipe tunnel using pure de-mineralized water. This will help determine if and where the tunnel may be leaking water into the ground. This test will be performed after excavation exposes the tunnel and the catch basin is installed allowing technicians to observe any leakage. The pure de-mineralized water used during the test will be collected and processed through plant systems.
Tritium concentration monitoring is ongoing. The installation of the two new deep monitoring wells is complete and they are in the process of being developed (GZ-13D and GZ-14D). Digging for shallow well GZ-19 is complete and drilling for the deep well GZ-19D is in progress. Due to interferences, the drilling of GZ-16 has been delayed until a better location can be determined. A new shallow monitoring well GZ-17, to be located just west of the excavation site in the maintenance shop, will commence tomorrow. On Friday, down hole geo-physics testing of six monitoring wells will begin. The purpose of this testing is to determine the structure of the bedrock. Geo-physics testing takes several days to complete. The information from the new wells and geo-physics testing will help engineers and hydrology experts to better understand how groundwater flows beneath the site.
As reported earlier this week, as a prudent precaution, the Construction Office Building (COB) drinking water well has been taken out of service. The well pump will be removed tomorrow. The layout and installation of the pipe line which will connect the main well to the COB water tank is in progress, with completion expected in a few days. The COB well will be included as one of the deep wells that will be part of geo-physics testing. After the geo-physics testing, a determination will be made if the well can be maintained as a monitoring well or if it will be closed and filled.
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.
