Radiation and the Japanese Nuclear Reactors
A fact sheet describing the events at the Fukushima Daiichi and Daini nuclear plants in Japan following the earthquake and tsunami nearby. Also a description of radiation dose limits and exposures to workers and the public in the United States, with a placeholder for radiation doses at the affected reactors in Japan.
An earthquake measuring 9.0 on the Richter scale struck off the northeastern coast of Japan on March 11, triggering a tsunami. Along with the loss of life and damage, Japan also faces the challenge of stabilizing nuclear power plants in the hardest-hit region.
After the earthquake, all the operating reactors at the Fukushima Daiichi and Fukushima Daini nuclear plants shut down automatically, as they are designed to do. However, due to the loss of offsite power and failure of the backup diesel generators, there were difficulties powering the waste heat cooling systems at the Fukushima Daiichi plant.
To reduce the resulting increase in containment pressure, Tokyo Electric Power Co., vented steam—containing small amounts of radioactive material—from the primary cooling circuit of reactors 1 and 3. The released vapor passed though filters designed to remove radioactive components such as iodine and cesium. Upon release, the slightly radioactive vapor dispersed into the atmosphere.
Residents have been evacuated from a 12.5-mile radius around the Daiichi plant and about two miles around the Daini plant. The precautionary measures taken to evacuate residents near the sites are intended to prevent or mitigate any radiation dose from radiation releases that might occur as the situation develops.
A buildup of hydrogen gas in the secondary containment structure at Daiichi Unit 1 led to an explosion at that reactor (see reactor diagram). However, the integrity of the primary containment structure was not compromised and there were no large leaks of radiation from the reactor core.
The U.S. Nuclear Regulatory Commission’s (NRC) annual limit for worker exposure to radiation is 5,000 millirems (mrems). The average U.S. nuclear power plant worker receives 120 mrems annually. A typical X-ray provides 10 mrems per film.
The NRC’s public radiation dose limit is 100 mrems annually. The average U.S. public exposure from the commercial nuclear fuel cycle, including nuclear power plant operations, is less than 1 mrem per year.
The average American receives more than 600 millirems of radiation exposure annually—about half from naturally-occurring sources and the rest from medical applications, such as CT scans and X-rays. Although there is scientific evidence for health risks following high-dose radiation exposures, risks of health effects are either too small to be observed or are nonexistent at levels below 5,000–10,000 mrem. 
A small amount of radiation was released in the 1979 Three Mile Island accident in Pennsylvania, but studies found that it did not have an impact on health or the environment. About half of the reactor fuel at TMI Unit 2 melted during the early stages of that accident.
 Position Statement of the Health Physics Society, “Radiation Risk in Perspective,” U.S. Health Physics Society, July 2010.