For Immediate Release: March 12, 2007
Contact: Communication Office
BURLINGTON - An independent study from Oak Ridge Associate Universities (ORAU) released today resolves a discrepancy between how the State of Vermont and Vermont Yankee measure site boundary radiation dose at the Vermont Yankee Nuclear Power Station in Vernon.
The report concludes that the measurement methodology used by the Vermont Department of Health since 1973 was less accurate than current methods used by Vermont Yankee. Actual doses to the public over this time span, the report concludes, have been over-estimated using the Department of Health’s network of dosimeters.
ORAU reviewed more than 35 years of Vermont Yankee site boundary dose measurements and reports on compliance to Vermont Department of Health regulatory limits. The report recommended significant improvements in the methods used by both the Health Department and Vermont Yankee to measure and interpret site boundary doses.
The Vermont Department of Health, which monitors the plant 24 hours a day using 70 thermoluminescent dosimeters (TLDs), will now incorporate “main steam line” radiation measurements as part of its overall assessment of radiation dose at the site boundary. TLDs will continue to be used, but as part of a wider array of dose determination methods.
The previous measurement methodology used by the State of Vermont was less accurate, the report concluded, because it did not account for background radiation adequately — this led to inaccurate estimates of radiation dose from Vermont Yankee. Previous assessments, the report states, also did not accurately account for differences between what people might be exposed to and what people actually absorb as dose.
“The report concludes that the past 35 years of site boundary exposure measurements by the Health Department overestimated the actual radiation dose, “ said Health Commissioner Sharon Moffatt, RN, MSN. “We wanted to ensure that the dose measurement was accurate. Using the measurement methodology supported by Oak Ridge Associated Universities, the measurements of site boundary doses have been compliant with regulatory limits.”
At no time has the dosage from the Vermont Yankee plant posed a measurable risk to public health.
The U.S. Nuclear Regulatory Commission limits radiation doses for the general public to 100 millirem per year. The limit set by the U.S. Environmental Protection Agency is 25 millirem per year from radioactivity in water, air and soil. The Vermont Department of Health limits are more restrictive – 20 millirem per year.
If site boundary doses exceed 20 millirem a year, Vermont Yankee is required to investigate the cause, initiate a program to reduce levels, provide a report to the Health Department within 14 days and implement corrective action. Corrective action may include reducing the power level, applying controls to shield radiation, or extending the site boundary.
In February 2006, the State of Vermont more than doubled the number of dosimeters in preparation for a 120 percent power upgrade at the facility that began in March 2006. This helped the Department better assess radiation doses around the entire perimeter of the plant.
Site boundary exposure measurements by the Health Department in 2006 showed that the 20 millirem per year regulatory limit may have been exceeded as a result of the power uprate. Using its previous methodology, the radiation exposure at the site boundary ranged from 20.3 to 24.0 millirem for 2006. However, to account for measurement error of up to 25 percent, measurements of up to 25 millirem are considered within the regulatory limit. Using the ORAU recommended exposure to dose relationship, actual public doses would be estimated to be well less than 20 millirem in 2006.
ORAU, formed in 1946 to advance scientific research and education in nuclear science, started work on the report in December 2005, and the final report was completed on March 1, 2007.
The full report is available at: http://healthvermont.gov/enviro/rad/vt_yankee.aspx