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Historical Radiological Event Monitoring

Since the 1940s, U.S. and foreign nuclear weapons tests, reactor accidents and other radiological events have released radioactive material into the environment. One of EPA's roles during and after these events is to monitor the environment for radiation. 


On March 11, 2011, a 9.0-magnitude earthquake struck off the coast of 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, less than an hour 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 hydrogen explosions that breached the reactor buildings and allowed radioactive elements to escape into the environment. Read about EPA's radiological monitoring related to Fukushima.


On Saturday, April 26, 1986, a nuclear accident at 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. Read about EPA's radiological monitoring related to Chernobyl.

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Tokaimura Accident

On September 30, 1999, three workers at the Japan Nuclear Fuel Conversion Company transferred several times the allowable limit of 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 used the Environmental Radiation Ambient Monitoring System (ERAMS), now RadNet, to monitor the radioactivity in air, precipitation and pasteurized milk. As expected, no increase in radioactivity above typical background levels was measured in any of the samples analyzed, so protective actions were not needed.

Nuclear Testing

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 used the Environmental Radiation Ambient Monitoring System (ERAMS), now RadNet, to monitor the 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.

ERAMS monitored each atmospheric nuclear test, as the system is continuously in operation. EPA also responded to the last recorded atmospheric test on October 16, 1980 (which occurred in China) with increased milk collection.

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