Since a magnitude 9.0 earthquake rocked Japan and set loose a massive tsunami March 11, the Tokyo Electric Power Company (TEPCO) has been scrambling to avert a nuclear disaster at its hardest hit plant. The Fukushima Daiichi
nuclear power station, home to six nuclear reactors, has witnessed
explosions at three reactors and a fire in a spent-fuel pool at a fourth. At two reactors, units No. 2 and 3, the vessels containing the nuclear material are suspected to be compromised.
A handful of plant workers remain on the site, implementing
emergency cooling measures at the stricken, overheating reactors. Radiation levels have fluctuated wildly during the crisis, and the extent to which the workers' health has been endangered may not become apparent for years. But so far, the releases of radiation have been limited compared to the 1986 Chernobyl disaster, an explosive event that caused dozens of cases of fatal radiation poisoning among plant workers and that has been implicated in
thousands of thyroid cancer diagnoses in the years that followed. (Nuclear fission of uranium fuel produces radioactive iodine, which gathers in the thyroid gland.) As
many nuclear experts have noted, the Fukushima Daiichi reactors are better designed than the Chernobyl reactor that failed.
Below are some facts and figures about the radiation hazard of the Fukushima Daiichi plant and how it compares to other nuclear accidents in history. Many of the figures are measured in millisieverts, an international unit of radiation dosage. One sievert is equal to 100 rems, so one millisievert is 0.1 rem.
Radiation dose at the boundary of the Fukushima Daiichi plant at 2:30 P.M. Japan time on March 16: 1.9 millisieverts (mSv) per hour
Peak radiation dose measured inside Fukushima Daiichi nuclear power station on March 15: 400 mSv per hour
Maximum allowable exposure for U.S. radiation workers: 50 mSv per year
Average exposure of U.S. residents from natural and man-made radiation sources: 6.2 mSv per year
Estimated total exposure at the boundary of the Three Mile Island site during the 1979 accident there: 1 mSv or less
Average total radiation dose to the 114,500 individuals evacuated during the 1986 Chernobyl disaster: 31 mSv
Half-life of iodine 131, a dangerous radioactive isotope released in nuclear accidents: 8 days
Half-life of cesium 137, another major radionuclide released in nuclear accidents: 30 years
Decay products of iodine 131 and cesium 137: gamma rays and beta particles (electrons or positrons)
Amount of nuclear fuel in the Chernobyl 4 reactor that exploded in 1986: 190 metric tons
Estimated nuclear fuel and fission by-products released into the atmosphere during Chernobyl disaster: 25 to 57 metric tons
Approximate amount of nuclear fuel in each crippled Fukushima Daiichi reactor: 70 to 100 metric tons
Sources: Japan Atomic Industrial Forum, International Atomic Energy Agency, U.S. Nuclear Regulatory Commission, National Council on Radiation Protection and Measurements, U.S. Environmental Protection Agency, United Nations Scientific Committee on the Effects of Atomic Radiation, National Institute of Standards and Technology, Nuclear Energy Institute
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