Once again, the forces that are bound and determined to shut down the Vermont Yankee nuclear power plant and remove its 620 MW of reliable (90% capacity factor) electricity from the competitive supply of electricity in New England have found another hook. The incredibly detailed analysis that is being conducted of the soil next to the location where the leak from the Advanced Off-Gas (AOG) pipe tunnel was detected and fixed has revealed the presence of Cesium 137 (Cs-137) at a level of approximately 300-1200 picocuries per kilogram (0.00000000003 to 0.00000000012 curies per kilogram). In the same soil sample, the investigators have detected manganese-54, cobalt-60, zinc-65.
(See the March 31, 2010 update from the Vermont Department of Health on their page titled Investigation into Tritium Contamination at Vermont Yankee Nuclear Power Station)
Cs-137 is not a naturally occurring isotope; it is only produced when uranium is split in a fission reaction. It is not produced with every fission, breaking uranium nuclei with a neutron is a process that nearly always produces two different atomic nuclei of various sizes, but exactly which two is a matter of probabilities. There are more than one hundred different combinations where the number of protons between the two elements add up to 92 and the number of neutrons in the isotopes adds up to about 140 or 141 (if the nucleus that breaks apart is U-235). (If the original nucleus that fissions is Plutonium-239, which happens even in a uranium fueled reactor, the number of neutrons will add up to either 142 or 143.) The reason that the number of neutrons in the fission products is less than the number in the original source nucleus is that several neutrons get releases as individual particles in atomic fission.
Here is an illustration of the fission yield curve for various fissile materials.
Image Credit – Wikipedia (http://en.wikipedia.org/wiki/File:ThermalFissionYield.svg. This work is licensed under the Creative Commons Attribution 3.0 License.)
Fission products are not a “natural” part of our environment, but they are a part of our existing environment at minute quantities, mainly due to the left-overs from the atmospheric testing of nuclear weapons conducted during the 1940s-1960s. Between the US and the Former Soviet Union, hundreds of fission weapons were exploded, allowing fission products to be widely distributed among surface soils all over the world. Since Cs-137 has a 30 year half life, a bit more than 50% of whatever was released during that testing still exists and continues to decay.
It is unlikely that atmospheric testing is the source of the tiny quantities found near the leak at Vermont Yankee at a depth of 15 feet. The fission products were most likely deposited in the soil as a part of the same water that included the tritium that was the focus of months worth of discussion and speculation.
In a nuclear power plant, every effort is made to keep fission fragments inside the fuel elements through the use of corrosion resistant cladding as a first layer of containment, but cladding failures are not entirely unknown. There are a LOT of fuel rods in a nuclear fission core bundle and they occasionally fail.
According to some reports, Vermont Yankee has had a few fuel element failures in its 38 year operating history. It is possible that some of the corrosion products that form what nukes know as “crud” still contains a bit more Cs-137 than is typical for those normal bits of rust and that some of those bits ended up in the AOG piping to be carried into the soil along with the nearly, but not completely, pure H2O that leaked out into the soil.
(Remember, the new report of finding another isotope is NOT a report of a new leak from piping at the plant. It is the SAME leak that has been found and stopped. The Cs-137 is simply another component of the SAME fluid that has been found at levels several times above background but still in extremely tiny quantities – 0.00000000003 to 0.00000000012 curies per kilogram – and still in a very limited quantity of soil. That soil was found 15 feet below the surface of an industrial facility. It is not a public health risk.)
Breathless headline from WMUR New Hampshire – New Contamination Found At Vermont Yankee
Ruthland Herald – New radioactive isotope found at Vermont Yankee
Huffington Post – Second radioactive substance at Vermont Yankee
(The commercial media has to capture attention so they can sell ads, after all.)