Past Sources of I-131 in the Environment | Current Potential Sources of I-131 Exposure

Past Sources of I-131 in the Environment

During the Cold War, national security policies prevented government authorities from disclosing the risks and health hazards associated with living near or working at weapon production facilities. These facilities released harmful levels of radiation into the environment.

Many people in the United States, especially those living near or working at weapon production facilities, such as the Hanford Nuclear Reservation, were unknowingly exposed to multiple sources of I-131, including fallout. The Nevada Test Site (NTS), which was used to test nuclear weapons, produced considerable amounts of fallout, which exposed most of the American population. The existing national security policies kept that information from reaching the American public.

• The main sources of I-131 in the environment have come from nuclear power plant releases and from the production and testing of nuclear weapons.
• The highest levels of combined I-131 releases occurred between the early 1940s and mid-1960s.

In the United States, past releases of I-131 have occurred at fuel reprocessing plants and some weapon production facilities of the Department of Energy (DOE). Since 1944, when the first production atomic reactor came into service, large amounts of I-131 have been periodically released into the atmosphere. I-131 was released to the atmosphere as a gas during nuclear weapons production (1945–1980s), aboveground nuclear tests (1951–1962), medical isotope production, medical administrations to patients, and unintentional releases. Multiple releases over time could have maintained constant or repetitive high levels of radioactivity, particularly around weapon production facilities. The highest levels of combined I-131 releases occurred from the early 1940s through the mid-1960s.

The annual dose of background radiation received by an average person in the United States comes from the following sources: radon gas, 55%; internal radiation, 11%; cosmic rays, 8%; terrestrial radiation, 8%; and manmade products, 18%. Less than 1% of the radiation from manmade products comes from nuclear power plant releases and fallout. Typically, little of this dose is from I-131 because of the short half-life of the element: it decays (loses its level of radioactivity) rapidly and rarely exists at any meaningful level in the environment. However, this changes if a major nuclear release occurs. When a nuclear bomb detonates or nuclear power plant fuel melts and causes an explosion, the volatile I-131 produced is forced up to various elevations (potentially exceeding 10 kilometers [6.2 miles]) by the intense heat, and is subsequently swept by the winds. I-131 can be deposited on the ground as dry deposition (I-131 adsorbs to particulates in the air and drops to the ground) or as wet deposition (I-131 dissolves in atmosphere moisture, some of which becomes rainwater and falls to the ground). The initial quantity released determines the significance of the fallout. Persons living in the direction in which the wind blows are referred to as “downwinders.” Many of the persons living downwind from the Hanford Nuclear Reactor could have received multiple exposures over time.

Total releases of I-131 worldwide equal 24,000,000,000 curies. The curie, or Ci, is the measurement for the rate of radioactive decay.

Worldwide, major significant I-131 releases occurred at the following locations.

The peak years for the releases at the Hanford Nuclear Reservation were 1944–1947 (92%), with minimal releases after 1947, except for two peaks in December 1949 (the Green Run) and May 1951 (filters removed). The largest I-131 releases from the Oak Ridge National Laboratory occurred between 1952 and 1956. An April 29, 1954, accident released 105 to 500 Ci over 2½ hours, accounting for about 6.5% of the total release for 1954. The Nevada Test Site had 90 nuclear tests that released almost 99% of the total I-131 released into the atmosphere from 1952 through 1957. The vast majority of I-131 releases at the Savannah River Site occurred between the mid-1950's and the mid-1960's. The Windscale release in the United Kingdom in 1957 was caused by a fire in the graphite moderator of an air-cooled plutonium production reactor. The Three Mile Island release in Harrisburg, Pennsylvania, in 1979, released 15–21 Ci of I-131 into the atmosphere. During and after the explosion and fire at the Chernobyl nuclear plant, large amounts of radioactive materials were released over a 10-day period, with 25% of the total amount released in the first day. These materials were subsequently spread over parts of Europe and the rest of the world by wind.