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Abstract
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The disaster which hit Japan on March 11 was unique in the atomic age: an earthquake, followed by a tsunami, followed by a meltdown. There have been other earthquakes, other tsunamis and other nuclear accidents, such as Chernobyl, but the calamitous sequence at Fukushima Daiichi had not happened before. It put a full stop to arguments both for and against nuclear energy; time had come to cope with the crisis, to monitor the site, and to gauge its effects on humans and the environment. A clear, rapid account was expected so that: “Civil society in all countries could assess the record of nuclear power and draw the conclusions.” It didn't quite play out that way.
Japanese authorities followed the playbook of other national disasters (the BP oil spill in the Gulf, Hurricane Katrina). They went through the stages of administrative grief: Shock and Disbelief followed by Denial and Anger, Bargaining and Guilt. They issued frequent and often contradictory accounts; but pretty soon the facts of the meltdown emerged. The logs of the International Atomic Energy Agency show that the world had been watching and counting.
On March 11, Japanese authorities claimed that the disaster had caused “no release of radiation from any of the nuclear power plants affected by today's earthquake and aftershocks.” And on March 15, they insisted that “there has so far been no release of radiation from any of the nuclear power plants.” But massive douches of seawater had failed to back up the crippled cooling systems in all functioning reactors. Hydrogen explosion followed hydrogen explosion, fires broke out to release I131 and Cesium137 into the air and seas. By March 22, it became clear what had really happened inside the plant: three of the six reactors were in advanced stages of meltdown. The zirconium alloy sheaths that surround the reactor's fuel rods had ruptured, and pellets of molten uranium dropped to the bottom of the reactors where they congealed. The core of Unit 4 landed in a cooling pond in open air after two hydrogen explosions macerated the building. All that water added to cool the cores had overheated and needed extrusion. The pot was boiling.
One month after the tsunami hit, the bad news was spelled out in becquerels and sieverts. The total radioactivity released was between 360,000 and 600,000 trillion becquerels, and at least 21 workers were exposed to over 100 millisieverts.
These units require explanation: A becquerel (Bq) is 1 disintegration of a radioactive substance per second and describes the total amount of radioactive energy released. A sievert, on the other hand, expresses the effect of radiation on humans. A sievert (Sv) is the energy (J) absorbed by an amount (kg) of human tissue exposed to various sources of radiation (w).Therefore, 1Sv=1J/kg.w and that “w” is the human factor. Humans are exposed to natural background radiation of 2.4 mSv per year; we tot up an extra 3 mSv during a mammogram or 5 to 30 mSv from a CT scan. In most countries the current maximum permissible dose to radiation workers is 20 mSv per year averaged over five years, with a maximum of 50 mSv in any one year.
Language: en |