YOU HAVE COME TO THE RIGHT PLACE to display atmospheric dose reconstruction estimates resulting from Hanford site radionuclide

YOU HAVE COME TO THE RIGHT PLACE to display atmospheric dose reconstruction estimates resulting from Hanford site radionuclide emissions for the highest release years of 1945-1951. These estimates come from the Hanford Environmental Dose Reconstruction (HEDR) study conducted for the U.S. Department of Energy, at the Pacific Northwest National Laboratories (PNNL) operated by Battelle. The study was completed in 1994.

Related research includes the Hanford Thyroid Disease Study (HTDS) being conducted by the Fred Hutchinson Cancer Research Center and sponsored by the Centers for Disease Control and Prevention. The primary purpose of this epidemiological study is to determine whether thyroid disease is increased among persons exposed to the releases of radioactive iodine from the Hanford Site between 1944 and 1957. The initial study results, made public on January 28, 1999, do not show a link between the estimated thyroid dose from iodine-131 and the amount of thyroid disease in the study population. While the study did find thyroid diseases among HTDS participants, those who had higher estimated radiation doses appeared to be no more likely to have thyroid diseases than were those who had lower doses.

GO TO THIS WEB SITE for detailed information on this and other Hanford-related Studies: http://www.doh.wa.gov/hanford/resources/studies.html

The purpose of HEDR was to estimate radiation doses since the first year of Hanford operations, that people could have received as a result of radionuclide emissions from the Hanford Site. Exposures to radioactive materials released to the atmosphere resulted primarily from the consumption of food containing radioactivity, inhalation of contaminated air, or direct exposure to radioactivity in soil or air. Initial study efforts indicated that atmospheric doses resulting from exposures to surface, irrigation and drinking waters were minor in comparison to other exposure routes, therefore, they were not studied in detail.

The greatest contributors to dose from the atmospheric pathway were the chemical separation plants that began operations on December 26, 1944 (because there were only 6 days of separation plant operations in 1944, the data from those days have been attributed to 1945). Major separation plant activities ceased in November 1972. The PUREX (plutonium/uranium extraction) plant did operate for a limited time in the late 1980s.

Once the last chemical separation plant ceased operations, radioactive releases to the atmosphere decreased dramatically. Consequently, detailed studies began for estimating doses in the years 1944-1972. Because of the lower radioactive releases after 1972, the doses published in the Hanford annual environmental report were used to complete the dose history for the period of 1973-1992.

Dose estimates prior to 1973 were calculated for representative individuals in a 75,000 square-mile area surrounding the Hanford Site, including eastern Washington, northeastern Oregon, and western Idaho. Each estimate was the median (middle value) of a distribution sorted by increasing size of its values. Each distribution, consisting of 100 values, was determined using a "Monte Carlo" statistical sampling technique.

Doses from iodine-131 and five other radionuclides account for 99% of the potential dose from the atmospheric pathways. The other radionuclides are strontium-90, ruthenium-103, ruthenium-106, cerium-144, and plutonium-239. These doses are estimated in terms of effective dose equivalent (EDE), a measure of dose that attributes weighting factors to body organs and tissues, to produce an estimate of total risk from exposure to multiple radionuclides. EDE is reported in units of "rem" ("roentgen equivalent man").

The doses for locations near the Hanford site are larger than those farther from the site and correspond to the I-131 deposition at each location. A principal exposure pathway for many individuals was milk from cows that ate vegetation contaminated by iodine-131 released into the air. The doses from the atmospheric pathway are dominated by exposure to iodine-131 in the years 1945-1951. Although by 1965, iodine-131 was no longer the dominant radionuclide, its cumulative effect on dose for the period 1945-1972 contributed 99% of the cumulative EDE potentially received by an adult living in Richland, WA. In general, the magnitude of the doses is directly proportional to the amount of iodine-131 released during the year. Releases and dose estimates were the largest in 1945, accounting for about 75% of the cumulative EDE during 1944-1972. A representative adult maximally exposed in the years 1945-1992, at the highest impact offsite location, Ringold, WA (an area adjacent to the east-central border of the Hanford site), would have received approximately 1.2 rem.

Because atmospheric radiation doses for the maximum release years of 1945-1951 were dominated by exposure to iodine-131, for which the thyroid is the critical organ; dose to the human thyroid is the best indicator of radiation exposure. This dose is measured in terms of "rad" (a unit of measure for the amount of "radiation absorbed dose"). Age-dependent dose conversion factors were used to convert iodine-131 intake to absorbed dose.

The cumulative thyroid dose for a maximum representative adult at Ringold, WA, for the period of 1945-1972, is approximately 39 rad. For a maximum representative child at the same location for the same period, the median of the absorbed cumulative dose to the thyroid is approximately 230 rad. The range of the distribution is from the 5th percentile of 60 rad, to the 95^th percentile of 840 rad.

Thyroid dose estimates for twelve groups of representative individuals, distinguished by sex, age and milk source, are provided for 1102 locations (sectors) within the HEDR study area. The sources of other consumed foods were assumed to be backyard gardens or small farms.

On average, an estimated 55% of the iodine-131 releases were deposited within the 75,000 square-mile study area. Some 10% decayed within the study area during atmospheric transport. The remaining 35% either decayed or were deposited outside of the study area.

Estimates of radionuclide releases, atmospheric transport and environmental accumulation all led to the estimation of radiation doses for representative individuals in the study area.

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HEDR STUDY CONCLUSIONS:

Reliable and useful doses and their uncertainties have been reconstructed for potential exposures to representative individuals from historical releases of radioactive materials from the Hanford site to the atmosphere.

The most important contributor to dose was iodine-131 released from the

chemical separation plants during 1945. The most important means of exposure from the atmospheric pathway was the consumption of fresh milk.

The highest estimated dose was from the consumption of milk produced by cows fed on fresh pasture, near Ringold, WA. The commercial distribution of milk had an effect on doses. An important impact resulted in lower doses in what otherwise would have been a high impact location (e.g. Richland, WA). For most representative individuals at any location, 90% of the estimates are within a factor of 15 (i.e. the 95th percentile is about 15 times higher than the 5th percentile).

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