(Last updated 5/18/11)
The unit of measurement of the amount of radiation received by a person (“radiation dose”) is a millirem. This is similar to the units of inch (for length), ounce (for weight) and degree (for temperature).
Natural Background Radiation
On average, Americans receive about 310 millirem per year of radiation dose from naturally occurring radiation in the environment (commonly known as “natural background radiation”). This includes radiation from space (cosmic radiation), and radioactive material in the earth, the air, and food and water.
- Radon in home and workplaces accounts for two-thirds of natural background radiation dose (about 200 millirem per year).
- A round-trip flight between New York and Los Angeles will result in 4 millirem due to being at a higher altitude and receiving more cosmic radiation.
- Flight crews receive 200-400 millirem per year.
Medical and Dental Radiation
Americans also receive radiation dose from medical and dental procedures in the form of X-rays, CT scans, nuclear medicine, etc. Although the average amount of radiation dose is calculated as more than 300 millirem per year, much larger amounts of radiation dose are received by people undergoing diagnosis or treatment of specific medical conditions.
- A typical chest X-ray is 2 millirem.
- An upper GI tract examination is 600 millirem.
- A CT scan of the chest or abdomen is 800 millirem.
Federal Radiation Safety Limits
Federal agencies such as the U.S. Nuclear Regulatory Commission (NRC) establish radiation dose limits to protect the health and safety of the public and nuclear workers. The limits are based on the current scientific understanding of potential risks from radiation exposure.
- The NRC radiation dose limit for protecting members of the public is 100 millirem per year.
- The NRC radiation dose limit for protecting nuclear workers is 5,000 millirem per year.
Radiation from a Nuclear Power Plant – Routine Operations
As a part of routine operations, small amounts of radioactive material are released from nuclear power plants into the environment in accordance with requirements that are specified in the operating licenses issued by the NRC. Radiation dose to members of the public from these releases into the air and water are routinely monitored by the plants, the states and the NRC and are documented in public reports submitted to the NRC. The average radiation dose to a person living near a nuclear power plant is much less than 1 millirem per year.
Radiation from a Nuclear Power Plant – Accident
Under accident conditions at a nuclear power plant, involving fuel damage or melting, radioactive fission products are released from the fuel elements into water and steam surrounding the fuel within the reactor vessel. Primary fission products include radioactive noble gases (krypton and xenon), iodine, cesium and, to a lesser extent, strontium. As pressure and temperature build up in the reactor, the safety relief valves release the steam into the suppression pool in the torus (BWR design). Among other functions, the suppression pool helps to cool and trap the majority of cesium, iodine and strontium in the suppression pool water. Periodic venting from the suppression pool may be conducted to manage temperature and pressure and protect the integrity of the primary containment.
Releases to the environment associated with venting consist primarily of radioactive noble gases, which, by their chemical and physical nature, disperse in the atmosphere and provide a source of radiation exposure in a passing plume or vapor cloud. The current conditions at the Fukushima Daiichi plant may result in a reduced effectiveness in trapping the radioactive cesium and iodine in the suppression pool. Therefore, the releases may contain higher levels of radioactive cesium and iodine, which provide sources of exposure from inhalation and ingestion.
Specific information on key radionuclides is as follows:
- Krypton-85 – radioactive noble gas; beta-gamma emitter; 11-year half-life.
- Xenon-133 – radioactive noble gas; beta-gamma radiation emitter; five-day half-life
- Cesium-137 – radioactive form of cesium; beta-gamma radiation emitter; 30-year half-life
- Iodine-131 – radioactive form of iodine that concentrates in the thyroid gland; beta-gamma radiation emitter; eight-day half-life
- Strontium-90 – radioactive form of strontium; beta radiation emitter; 29-year half-life.