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Hot Spots, Hanford releases to the Air

topic posted Wed, November 26, 2003 - 6:29 PM by  Richard
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Hot Spots:
Weather and Hanford's Radiation Releases to the Air

The Hanford Nuclear Site in southeast Washington state released many radioactive substances to the air for more than 40 years. This fact sheet discusses how weather affected where the radioactive materials went and how "hot spots" might have formed. "Hot spot" is a term used to describe an area where the concentration of contaminants is greater than that in the surrounding area. Those contaminants can be either radioactive materials or toxic chemicals. As used here, the term "hot spot" refers only to radioactive materials from Hanford.

This fact sheet discusses how hot spots might have affected the amount of radiation a person received from Hanford. It explains how the effects of weather can create situations where, for at least periods of time, people living farther from Hanford could have received higher exposures than people who lived closer to Hanford.

This publication is intended to serve as an introduction to the subject of weather and hot spots. The Network also offers a publication on the history of Hanford's releases: The Release of Radioactive Materials from Hanford: 1944-1972. For those who wish to do further reading, a bibliography of books and articles concerning the topics discussed in this fact sheet is available.

Hanford began operations in 1944. Since then, scientists have measured the amount of radioactive contamination in the environment. Unfortunately, this information is not detailed enough to identify where Hanford's radiation went.

The Hanford Environmental Dose Reconstruction Project was established to estimate where Hanford's radioactive materials went and how much radiation people may have absorbed. The Dose Reconstruction Project selected an area in Washington, Idaho, and Oregon.

Computer models simulate the likely spread of Hanford's radiation. The models use mathematical calculations and historical weather information. Additional models use this and other information to estimate the radiation exposures people could have received. This is known as a "dose estimate."

The Hanford Environmental Dose Reconstruction (HEDR) Project was established to estimate what radiation dose people living near Hanford some time between 1944 and 1992 might have receivied from releases of radioactive materials. The Technical Steering Panel, which directed the study, completed its role in 1995. The federal Centers for Disease Control and Prevention (CDC) is now working with the HEDR Task Completion Working Group to continue public participation and to assure completion of the remaining HEDR activities.

Before describing how well the computer models handle weather and hot spots, a basic explanation of the behavior of pollutant materials in air is needed.

RADIOACTIVE POLLUTION IN THE AIR

When the radioactive pollution from Hanford's plutonium separations plants reached the top of the 200-foot smoke stacks, it began to be affected by the weather. The stability of the surrounding air, the wind speed and direction, and whether it was raining or snowing all influenced where and how much of Hanford's radiation fell back to earth.
Stability of the Atmosphere

Under different conditions, the atmosphere can be stable or unstable. Each influences how pollution spreads after it leaves a plant's smoke stack. Under stable conditions, airborne pollution diffuses relatively slowly. In the air near the ground, this tends to occur more often in the winter, especially during the night.

Under unstable conditions, pollution diffuses relatively rapidly. In the air near the ground, this tends to occur more often in the summer, especially during the day.

At Hanford, most of the emissions occurred at night when the air was cooler at ground-level than at the top of the stack. Hanford officials ordered night-time releases to make it less likely that the radioactive pollution would fall back to earth near the operating plants, which could have caused serious exposure to workers.1 While night-time releases tended to protect workers, the wind carried more radiation beyond the Hanford site to public areas than if the plutonium operations had been done during the day.
Wind Speed and Direction

The primary factors in determining where Hanford's radiation releases to the air went are the speed and direction of the wind. The wind's importance is symbolized by the fact that people who were or may have been exposed to radiation are referred to as "downwinders."

The direction of the wind determines where the pollution is carried. Based on what is currently known, the wind generally blew southeastward from the Hanford stacks to the Tri-Cities, toward Walla Walla, and then turned northeast toward Spokane and Northern Idaho. Places outside of this pattern, such as Yakima, did not receive as much of Hanford's airborne radiation. (See map of areas exposed.)

HOW HOT SPOTS FORM

Four main factors lead to the creation of hot spots: precipitation, wind patterns, stagnation, and impaction.

Now that we have a general idea where the wind blew Hanford's radiation, how might the radioactive material have concentrated into hot spots? Common sense suggests that as one moves farther from a pollution source there is a decrease in the amount and concentration of that pollution. This is due to some of the material falling back to earth and the remainder becoming more mixed with the surrounding air. In general, Northern Idaho received less fallout from Hanford than areas around Eltopia or Ritzville in Washington.

However, this was not always true. Because of variations in the wind patterns and the earth's surface (hills, valleys, etc.), the spread of radiation, even in the downwind direction, was not uniform. Recall that the term "hot spot" refers to an area that has more contamination than does the surrounding area. People who lived in a hot spot area or got their milk or other food from such an area could have been exposed to higher levels of radiation than others who were not affected by a hot spot. Four main factors lead to the creation of hot spots: precipitation, wind patterns, stagnation and impaction. Each is described below.
Precipitation

Snow and rain can wash radioactive substances from the air and deposit the contamination on vegetation and the ground. When the wind carried radioactive material from Hanford over an area where it was raining or snowing, that area could have received a greater concentration than the areas around it. The surrounding areas might not have experienced any precipitation or were not in the path of the wind-blown radioactive plume.

To illustrate this, let's use September 9, 1945, when Hanford released 1,658 curies of iodine-131. Assume that winds could have blown some of the iodine over the town of Ritzville (a distance of about 70 miles from Hanford, as the crow flies). Assume, also, that there was a rain shower over Ritzville at the same time and it washed much of the iodine from the air. This could have created a hot spot at Ritzville on that day.

Or, if it rained on Ritzville but the winds had carried Hanford's radiation toward Walla Walla that day, there would have been little or no additional contamination at Ritzville.
Wind Patterns

In the case of releases of pollution lasting many minutes or hours, the wind direction could change during this time. In some instances, the change in wind could send the pollution plume in two or more directions. Later, the winds could change speed and direction again and cause the plumes to combine over a particular area and form a hot spot.

This combining of plumes could be part of a regular weather pattern. If this were the case, then a particular area could have become a routine hot spot. A scientist in charge of developing computer models for calculating doses for the Hanford Environmental Dose Reconstruction Project believes that the Spokane area was possibly a hot spot of this type.
Stagnation

When the wind carrying radioactive material becomes calm, the radioactive plume may remain over a particular area for a longer time. The lack of wind would allow more radiation to deposit in one particular area, possibly creating a hot spot.

According to Dose Reconstruction Project scientists, this factor may have led to a hot spot over Walla Walla soon after the large Green Run release of iodine-131 in early December 1949.2 The winds carrying the iodine-131 died down over Walla Walla for several days before carrying the remaining radioactive material toward Spokane, Northern Idaho and Canada.
Impaction

Impaction occurs when the pollution plume meets the side of a hill or mountain. The contamination is deposited in greater amounts where the plume meets the side of a mountain than in other areas at the same elevation. It is well established in Hanford documents that this frequently occurred on Wahluke Slope (north of the Columbia River, across from the Hanford reactors) and on Rattlesnake Mountain (the southern border of the Hanford site).

HANFORD DOSE RECONSTRUCTION AND HOT SPOTS

It is nearly impossible to determine where hot spots actually occurred around Hanford over 40 years ago. The Hanford Environmental Dose Reconstruction Project's computer programs (used to calculate the spread of Hanford's releases to the air) cannot reliably predict when or where a hot spot may have occurred. Part of the reason for this is the limited precipitation data. In addition, the air modeling part of the computer programs does not directly take into account the effects of hills or mountains (impaction).

There is only enough historical information to reconstruct the general pattern of contamination resulting from Hanford operations. It is impossible to be more specific given the time that has passed and the large geographical area that was exposed.

Besides iodine-131, Hanford released other substances into the air, including plutonium and ruthenium particles.

Estimating whether or not there were hot spots might not really change the dose estimates for individuals from exposure to iodine-131. The main reason is that iodine-131 has a relatively short half-life of only eight days. A half-life is the amount of time it takes for a radioactive substance to decay by releasing radioactive particles or waves, and to lose one-half of its radioactivity. Another reason is that the Dose Reconstruction Project is adding an individual's dose over a full year or a lifetime.

It is important to remember that where people's milk and fresh leafy vegetables came from may be more important in estimating doses of iodine-131 than the exact location in which a person lived and worked.
Non-Iodine Air Releases

Besides iodine-131, Hanford released other substances into the air, including plutonium and ruthenium particles. Historical documents from the 1940s and 1950s revealed that government officials considered both particle problems to be serious. John Till, Chair of the Technical Steering Panel which directed the Dose Reconstruction Project, and Charles Miller of the Centers for Disease Control and Prevention (CDC) have recommended an in-depth study of this problem. In a report to the Technical Steering Panel, they stated: "The active particle problem at Hanford will require a very thorough and careful analysis." Because the current computer models do not handle particles, a special effort is underway to estimate doses from the plutonium and ruthenium particles.

The factors leading to the formation of hot spots are the same for these particles as they are for iodine. However, hot spots of plutonium or ruthenium particles could be of greater concern than hot spots of iodine-131. This is because they have much longer half-lives and thus decay more slowly than iodine-131 (the half-life for ruthenium-106 is 372 days; for plutonium-239, the half life is 24,000 years). This causes greater concern for exposure to humans because ruthenium and especially plutonium are in the environment for a long time.
Effects on Dose

Generally, the closer to Hanford a person lived in a downwind direction, the greater the exposure to airborne releases. However, those people living in the primary downwind direction or in a recurring hot spot may have received more exposure than those in other areas.

Where you lived is only one factor in determining your exposure to Hanford's radiation. It is also important to think of the time periods you lived in a particular place, what foods you consumed and where they came from, and whether there were other unique characteristics about your lifestyle that could have affected your exposure to radiation.

1. The practice began in May 1945 after two serious instances of worker contamination. Operation of Hanford Engineer Works, S Department, 1946 (known as the DuPont History). It is uncertain how long officials continued the the practice of night-time releases.

2. The Green Run was the largest single release of iodine-131 from Hanford (about 8,000 curies). It was part of a secret program by the intelligence agencies of the United States to develop ways of detecting radiation from Russia's nuclear weapons program. It is called "green" because the nuclear fuel dissolved during the experiment cooled for only 16 days after leaving the reactor. The usual practice at the time was 90-100 days. The length of the cooling allowed iodine-131 and other radioactive materials to decay. Certain key details of the Green Run are still classified.
Selected Sources:

* LIPNWD-2033 HEDR Vol.2, Iodine-131 Releases from the Hanford Site, 1944 through 1947, Volume 2 - Data, March 1993, p. 7.51.
* Meeting transcript for Van Ramsdell's presentation on hot spots to the Technical Steering Panel, Richland, WA, October 8, 1993, pp. 273, 274, 276, and 280-282.
* Dickeman, R.L. HW-42185, Meeting with Advisory Committee on Reactor Safeguards, March 1, 1956. Dated March 22, 1956, p.8.
* Memo from John E. Till and C. W. Miller to TSP. "Active Particle Problem at Hanford." Undated, p. 1.
posted by:
Richard
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