The Night Hanford Released Radioactive Iodine on Purpose

On the night of December 2–3, 1949, operators at T Plant on the Hanford site in Washington State processed uranium fuel that had been out of the reactor for about 16 days. Normal practice allowed 90 days of cooling or more. The water scrubbers that ordinarily cleaned the plant’s off-gas were not run.

The release that followed, later known as the Green Run, sent about 7,800 Ci of iodine-131 and 20,000 Ci of xenon into the air, according to the classified test report the Atomic Energy Commission (AEC) issued in May 1950. The pre-test prediction for iodine had been roughly 4,000 Ci. The output came to about twice the plan, the winds shifted, and doses to people living off the site were not estimated at the time.

The Green Run is a documented case in which a measurement goal shaped a radioactive release. Its iodine settled on pasture and crops around Hanford, and the National Park Service says contaminated vegetation turned up from Kettle Falls, Washington, to Klamath Falls, Oregon. The offsite population received no advance warning, and the test became public only in 1986.

A signal too faint to follow

Between November 1948 and March 1949, the Air Force and the AEC ran aerial-monitoring tests over routine releases at Oak Ridge and Hanford. At Hanford, the off-gases could be detected for less than 2 miles. The Green Run used fresher fuel and no scrubbing, which produced a far stronger source.

The GAO describes its purpose as an atomic-energy intelligence experiment and a study of how the gases diffused. It took place about four months after the first Soviet nuclear test in August 1949. Staff of the Advisory Committee on Human Radiation Experiments (ACHRE) wrote that “the primary source documents that describe the purpose of the Green Run remain classified by the Air Force.”

The 1950 report itself was largely declassified in 1989, with some passages kept classified by the Air Force. The GAO found that those passages do not mention radiation-warfare or human-effects testing.

What an eight-day half-life does to green fuel

Iodine-131 has a half-life of 8.02 days, meaning half of any quantity decays in that time. Fuel cooled 16 days has passed through only two half-lives. Fuel cooled 100 days has passed through more than twelve.

That arithmetic is the reason for the cooling period. ACHRE staff wrote that the Green Run used three tons of fuel and that 100 days of cooling would have reduced its iodine activity about 1,000-fold and its xenon about 60,000-fold.

Iodine-131 emits beta particles and weak gamma rays. The Washington State Department of Health lists the exposure routes as milk, which peaks a few days after iodine settles on forage, along with other food, inhalation, and external exposure from ground deposits.

Where the wind went

The release was scheduled around the weather. General Electric’s Health Instruments Division set five conditions, among them a ground-level inversion and specific limits on the wind, according to the National Park Service.

The weather did not hold. Wind shifts brought the emissions down close to the ground over populated areas, and long-range tracking was not possible. An aircraft still detected the cloud more than 100 miles to the northeast.

On the ground, the highest vegetation reading was 28 µCi/kg, on site. Nearby communities measured between 0.1 and 4.3 µCi/kg. Animal thyroids reached up to 80 times the 4 µCi/kg limit.

Twice the forecast, or more

The 7,800 Ci figure was later challenged. In a June 1992 reanalysis in Health Physics, Robkin, working from the measured xenon-133, estimated that about 0.40 ± 0.12 PBq (11 ± 3 kCi) of iodine-131 had been released. He chaired the Source Term Subcommittee of the Hanford Environmental Dose Reconstruction (HEDR) project.

That is about 40 percent more iodine than the test report’s figure. For xenon, Robkin’s estimate of about 16,000 Ci was lower than the test report’s 20,000 Ci.

Curies released are not thyroid dose

A release in curies measures radioactive activity, the rate of decay in what left the stack. Deposition describes what landed on grass and leaves. Absorbed dose is the energy absorbed per unit mass of tissue, and it depends on what people ate, drank, and breathed, as well as external exposure from deposited material.

ACHRE staff wrote that individual doses from the Green Run were relatively low, largely because it happened in winter, when cows were not grazing on open pasture. They also noted that the milk pathway was unknown in 1949. Their November 1994 draft memo also cites a personal communication giving a maximum thyroid dose of 188 mrad.

Set against Hanford’s routine operations, the Green Run was not the largest source. In 1945, routine releases exceeded 45,000 Ci of iodine per month. By one estimate cited by the GAO, the Green Run accounted for about 1.1% of the iodine released from 1944 through 1949. The National Park Service gives a total of 739,000 Ci for 1944 through 1972.

The GAO noted that routine releases from 1945 to 1947 may have given infants doses above present limits through milk. It also reported that the Department of Energy counts the Green Run among the 14 most significant safety-related incidents in Hanford’s history.

Two memories of one decision

Who approved the weather is a matter of conflicting recollection. Of two AEC contractor officials, one said AEC contract officials judged conditions acceptable. The other said the AEC did not want to proceed and that the Air Force decided. Day-to-day authority over when fuel could be processed rested with GE’s Health Instruments Division.

That division’s view afterward is on record. A monthly report by H. M. Parker states that the division “would resist a proposed repetition of the test.”

Forty-five years after the night at T Plant, ACHRE staff set down the remaining problem in their draft memo as two questions: “Who proposed the experiment? Who approved it?”

About the sources

Most details of the 1949 events come from a November 1993 fact sheet by the U.S. General Accounting Office (GAO), which relays the May 1950 AEC test report, and from a November 9, 1994, draft staff memo of the Advisory Committee on Human Radiation Experiments (ACHRE). The release figures are as reported by the GAO from the 1950 report, and the H. M. Parker quotation is as reproduced in the ACHRE draft memo.