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Monitoring Radiological Incidents

Since the mid-1900s, U.S. and foreign weapons tests, accidents, and other radiological events have released radioactive material into the environment. One of EPA's main roles during and after these events is to monitor the environment for radiation.

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The most common reason for the above-ground detonation of nuclear devices has been nuclear weapons testing. Other explosions were experiments in earth excavation ("cratering"), basic physics, and rocket engine research.

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How do nuclear devices affect the environment?

Exploding a nuclear device above ground can inject large volumes of radioactive material into the atmosphere. The explosion spreads the material from ground level up to very high elevations. Determining the exact type, amount, and fate of the fallout for any particular blast is difficult. It depends on the type of device, time and method of detonation, as well as regional and global weather patterns.

Nuclear Blast

Large particles injected into the atmosphere tend to fall close to the explosion site. Smaller particles and gases are carried higher, are likely to remain aloft, and can travel great distances on global air currents. They gradually return to earth as they settle out or are captured by falling rain drops.

Contaminants from large atmospheric explosions may remain for years or even decades in the stratosphere (the atmosphere above ground level air up to a height of fifty miles). These contaminants only gradually settle out as small deposits of fallout in the ground-level air (the troposphere) or onto the earth's surface.

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Learn more about the atmosphere from NASA

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What kind of contamination is in fallout?

Fallout typically contains hundreds of different radionuclides. Some of these stay in the environment for a long time because they have long half-lives. Some have very short half-lives and remain in the environment for only a few minutes or a few years. Some produce high levels of radiation. Both long-lived and highly radioactive materials are potential risks to human health and the environment.

Some of the more important radionuclides detected by EPA's RadNet include:

RadNet monitoring stations also detect and routinely measure the radiation these radionuclides produce:

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Related Terms
half-life | radionuclides | risk

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How have people been exposed? What were the effects?

Exposure of people and other living things occurs by various routes or pathways. External or direct exposure comes from small amounts of fallout on the ground. Internal exposure occurs when radioactive particles are inhaled or when they are ingested (swallowed) following uptake by crops and livestock. Radionuclides that emit alpha and beta particles are less of an external exposure threat because they don't travel very far in the atmosphere. Alpha particles can be stopped by the dead cells on the skin's surface. Gamma rays travel much farther in the atmosphere and can penetrate the body.

Inhaled or ingested radionuclides continue to emit radiation directly to living tissue, increasing the risk of negative health effects. The most well-known effect is cancer, caused by damage to DNA in the cells. The health risks from fallout have been described in many studies.

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When did the tests occur? Does RadNet's data identify the contaminants?

Annual yield of above-ground nuclear detonations
by Country and Year

The U.S. conducted the first above-ground nuclear weapon test in southeastern New Mexico on July 16, 1945. Between 1945 and 1963, hundreds of above-ground blasts took place around the world. The number and size (yield) of blasts increased, particularly in the late 1950s and early 1960s. Following the signing of the Limited Test Ban Treaty of 1963 by the U.S., U.S.S.R., and Great Britain, most above-ground blasts ceased. (Some above-ground weapons testing by other countries continued until 1980.) The graph at right, illustrates this.

The atmospheric radiation measured by ambient monitoring systems in place at the time rose and fell with the size and number of blasts. An example is shown by the graph below of strontium-90 concentrations in milk. Concentrations peaked in 1963, following the intensive nuclear weapons tests of 1961-1962, then decreased as tests decreased. Since testing stopped,the remaining strontium-90 as decreased through radioactive decay and can no longer be detected.

A 2000 report by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) described how blast yields corresponded with global deposits of radionuclides such as strontium-90, cesium-137, and tritium (H-3).

Correlation between blast yield and SR-90 concentrations in milk.
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Photo: Chernobyl Nuclear Power Station

Chernobyl Nuclear Power Station in April of 1986, with red glow of Unit 4 visible near the center.

On Saturday, April 26, 1986, reactor number four at the Former Soviet Union's Chernobyl nuclear power station, exploded and burned. The accident, which occurred during unauthorized testing, emitted large quantities of radioactive material. The heat from the fire was so intense that the glowing reactor could be seen even from space, as shown in the satellite photo at right.

The U.S. detected slightly elevated radioactivity levels, but they were well-below levels requiring protective actions.

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How did EPA respond?

In the days following the accident, the Soviets released little data on the severity of the accident. Almost no data were available on on the extent of radioactive fallout in Europe and the rest of the world. In response to Americans' concern about potential health effects in the United States, the White House assigned the responsibility for leading the U.S. response to EPA. The Agency immediately took several steps:

  • monitored radioactivity levels in the United States
  • established a group to provide advice on preventing contamination of the food supply and protecting public health
  • established an information center to gather and distribute facts and data about the accident
  • arranged daily press conferences to keep the public up-to-date and to give EPA an opportunity to answer the public's concerns.

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How did EPA monitor the plume from Chernobyl as it crossed the U.S.?

One source of monitoring data was daily samples from EPA's Environmental Radiation Ambient Monitoring System, a forerunner of RadNet. The system first detected radiation from the accident at ground level on the West Coast one week after the accident. Radioactivity levels were somewhat higher than usual. However, they were well below levels that would have required any action to protect public health. The animation below shows the path and the timeframe for the passage of the Chernobyl plume across the United States.

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How did EPA protect U.S. citizens in Europe?

EPA sent experts to Europe to monitor and assess levels of radioactivity around U.S. embassies. For sometime after the accident, EPA scientists measured radioactivity in the Black Sea and Kiev Reservoir in cooperation with the former Soviet Union.

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Chernobyl Disaster: An Inside Tour

Chernobyl: An Inside Tour (Slide Show)
Ten years after the 1986 explosion of Unit 4 of the Chernobyl Nuclear Power Plant near Pripyat, Ukraine, EPA staff member Gregg Dempsey was given a rare tour inside the Sarcophagus that surrounds Unit 4. His slide show from that visit offers a unique historical snapshot of the conditions and challenges inside the plant. It provides views of the interior of Unit 4 not seen elsewhere in Chernobyl photo collections. In addition, it offers the personal perspectives of workers who were present at the time of the accident. One of these workers returns to his flat in the nearby city of Pripyat for the first time since leaving it 10 years earlier.

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On March 11, 2011, a 9.0-magnitude earthquake struck northern Japan. The epicenter of the powerful earthquake was under the Pacific Ocean, approximately 80 miles east of Sendai, where the Fukushima Daiichi nuclear power plant is located. The plant’s automatic earthquake detectors successfully inserted all the control rods into the three reactors that were operating at the time. However, 46 minutes later, a massive tsunami inundated the Fukushima power plant, causing widespread destruction and knocking out the reactors' emergency cooling systems. The reactors overheated, damaging the nuclear fuel and producing chemical explosions which breached the reactor buildings and allowed radioactive elements to escape into the environment

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How did EPA respond?

EPA’s RadNet system was operating normally on the day of the earthquake and detected nothing unusual in the first week after the Fukushima accident. During this time, EPA deployed additional portable monitors in Alaska, Hawaii, Idaho and U.S. Pacific Territories. They were set up and tested before the first radionuclides from Fukushima were expected to arrive on the winds from Japan. The RadNet system went on an emergency schedule, with expanded and accelerated sampling and analysis of air, precipitation, drinking water and milk. RadNet added laboratory analyses to test for additional radionuclides that are created in a nuclear power plant.

On March 18, 2011, a RadNet air monitor in Hawaii detected tiny levels of a iodine-131, a radioactive isotope that would be expected from the Fukushima nuclear incident. During the rest of March and April, RadNet detected tiny amounts of iodine-131 and other isotopes expected from a nuclear incident in samples collected throughout the country. All of the radionuclides detected in the U.S. from Japan were far below levels of public health concern. No protective actions were ever needed in the U.S. or its Pacific Territories. After a thorough review of all the sampling and monitoring results showed declining levels of radiation from Japan, RadNet returned to a routine sampling schedule on May 3, 2011.

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Sharing Data with the Public

To keep the public informed, EPA launched a Japan 2011 website which displayed near real-time radiation monitoring results from RadNet. EPA posted all laboratory results as soon as they were available. Agency staff used a new system on the site where users could view the results in a spreadsheet and download it to their own computers.

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We benefit from using radioactive materials for medical diagnosis and treatment, electric power generation, industrial processes, and research. However, these uses do have some risks. Careful planning and design can minimize risks from accidents but not completely prevent them. Below are two incidents that were detected by RadNet

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Tokaimura, Japan, September 30, 1999

Three workers at the Japan Nuclear Fuel Conversion Company transferred several times the allowable limit f enriched uranium into a precipitation tank, bypassing criticality controls. The transfer caused an uncontrolled, self-sustained nuclear reaction. Though the accident released radioactive noble gases and gaseous radioiodine, most of these substances were confined to the building.

EPA fully activated the nationwide RadNet system, monitoring radioactivity in air and pasteurized milk. No increase in radioactivity above typical background levels was measured in any of the samples analyzed, so protective actions were not needed.

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Atmospheric Nuclear Tests, People’s Republic of China, 1976

The People’s Republic of China conducted two test detonations of nuclear weapons on September 26 and November 17, 1976. Both detonations were conducted above ground, injecting radioactive material into the atmosphere.

EPA fully activated ERAMS, now RadNet, to monitor radioactivity in air, precipitation, and pasteurized milk. EPA’s monitoring system identified low, but measurable, quantities of radioactive material throughout the United States from the September 26 test. No additional material from the November 17 test was detected. As a result of the findings, state agencies in Connecticut and Massachusetts ordered farmers to switch dairy herds to stored feed only, minimizing the potential impact on milk supplies. EPA continued to monitor radioactivity levels until they returned to normal background levels in November 1976.

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