![]() Three months after the accident (after fission ceased) I-131 had virtually disappeared as a problem. Iodine is readily taken up by the body and accumulates in the thyroid gland. Iodine-131 decays to inert and stable xenon-131. Regarding releases to air and water leakage from Fukushima, the main radionuclide from among the many kinds of fission products in the fuel was volatile iodine-131, which has a half-life of 8 days. (It is the reason that access to a BWR turbine hall is restricted during actual operation.) There is also often some leakage from fuel elements of fission products, including noble gases and iodine-131. N-16 has a half-life of only 7 seconds but produces high-energy gamma radiation during decay. Radioactivity in the cooling water flowing through the core is mainly the activation product nitrogen-16, formed by neutron capture from oxygen. The International Commission on Radiological Protection (ICRP) allows up to 500 mSv for workers in emergency rescue operations. At the end of October this was reduced to 100 mSv for new workers. On 17 March, NISA set 250 mSv as the maximum allowable dose for Fukushima recovery workers, under health physics controls. An instant dose of 100-250 mSv can slightly increase the risk of later developing cancer, but if this dose is spread over time there is less risk of any effect. A short-term dose of 1000 mSv (1 Sv) is about the threshold of acute radiation syndrome (sickness). It is the chief hazard for the plant workers, who wear film badges so that the dose can be monitored. This is only a hazard for those on the plant site, and the level diminishes with distance from the radioactive source. ![]() ![]() Exposure to ionizing radiation can also be by direct radiation from the plants and fuels themselves, though not released to the environment. Whether this is in the air or settled on the ground, it may expose people to ionizing radiation, and the effect of this is measured in Sieverts, or more typically milliSieverts (mSv). Radioactive releases are measured by the amount of (radio)activity in the material, and quoted in Becquerels.
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