福島核電事故 3個反應爐「爐心熔穿」
日本讀賣新聞報導，日本政府向國際原子能總署（IAEA）所提出的報告書今天曝光，報告中坦承福島第一核電廠內的1號機至3號機，可能發生爐心熔穿現象，這比當初預期的核心溶熔還嚴重，是最嚴重的核災事故狀況。
報導指出，福島第一核電廠的第1到3號機反應爐壓力槽很有可能從底部熔解，核燃料露出掉至儲存容器內，發生爐心熔穿。
此外，日本福島核電站在地震後一周內洩漏出的輻射量，比當初公布的要多一倍。日本當局在今天舉行首次調查會議前表示，當時的輻射量高達77萬兆貝克，最初估計僅37萬兆貝克。
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CNN 有一災難目擊記整理
紐約時報兩篇日本核災相關報導
Radiation Understated After Quake, Japan Says
By HIROKO TABUCHI
Published: June 6, 2011
TOKYO — Japan said Monday that radioactive emissions from the stricken Fukushima Daiichi nuclear power plant in the early days of the March 11 earthquake and tsunami disaster might have been more than twice as large as a previous estimate, suggesting the accident was more grave than the government had publicly acknowledged.
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It is unclear whether a more accurate reading of emissions levels would have promoted a swifter or wider evacuation from around the plant. Still, the lag in reporting the true extent of the emissions added to what some critics have called a litany of confusing and contradictory data and analysis from the Japanese authorities, putting officials on the defensive about whether they delayed, or even blocked, the release of information to the public.
Last month the government acknowledged that three of the plant’s reactors had probably suffered fuel meltdowns, after having denied that possibility.
On Monday, Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency, said that the reactor pressure vessel at one of the plant’s reactors appeared to have been compromised as early as five hours after the quake.
The agency also said it now estimated that the radioactive release from the plant totaled 770,000 terabecquerels in the first week after March 11. The agency had previously estimated 370,000 terabecquerels released in the first month.
A terabecquerel is a trillion becquerels, a commonly used measure of the radiation emitted by a radioactive material.
The agency suggested that the higher emissions estimate was equivalent to only about 10 percent of the radioactive materials released in 1986 by the explosion and fire at Chernobyl, still widely considered the world’s worst nuclear plant disaster, in the former Soviet Union. But the 770,000 terabecquerels figure in fact comes to about 40 percent of the official Soviet estimate of emissions from Chernobyl.
Most experts say that the true emissions from Chernobyl were 1.5 to 2.5 times as high as the Soviet Union acknowledged. Assuming that true emissions from Chernobyl were twice the official figure, the Fukushima nuclear accident has released 20 percent as much as Chernobyl, according to Japan’s new estimate.
Japanese officials have stressed other differences between Fukushima and Chernobyl. At Chernobyl, a burning graphite reactor pushed radioactive particles high into the atmosphere and downwind across Europe. At Fukushima, the leak mostly produced radioactive liquid runoff into the ocean and low-altitude radioactive particles that have dispersed into the ocean, which suggests that the crisis could pose fewer health risks.
Japan’s assessment has been based largely on computer models showing heavy emissions of radioactive iodine and cesium from March 14 to 16. Officials have said that the emissions peaked during those days, and have dropped sharply since.
Even at the time of the first estimate in April, Japan’s Nuclear Safety Commission, an independent government panel, had called the reading too low. Its own estimate was 630,000 terabecquerels in the first month, or about 34 percent of the official Soviet estimate and 17 percent of the unofficial higher estimate.
The commission relied on a computer model that uses radiation measurements taken at various distances from a nuclear accident. The model produces an estimate of the radioactive material escaping from the source.
But the Nuclear and Industrial Safety Agency based its number on estimates of the damage to the reactors’ radioactive cores. Its latest reading more accurately reflects the radioactive material spewed after hydrogen explosions at Reactors 2 and 3, the agency said.
Officials cautioned that there was a wide margin of error involved in both calculations.
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Radiation’s Unknowns Weigh on Japan
By MATTHEW L. WALD
Published: June 6, 2011
As officials in Japan agonize over what constitutes a safe radiation dose for people who live near the Fukushima Daiichi nuclear reactors, the state of the science has been a daunting problem. Studies on the effects of exposure are based mostly on large doses delivered quickly by atomic bombs, while radiation from the Fukushima disaster would more likely result in small doses delivered over many years.
Kyodo, via Reuters
Workers removing topsoil from a playground in Fukushima Prefecture in Japan to reduce children's exposure to radiation.
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So far the debate in Japan has centered on the risks to children. Government guidelines set after the disaster allowed schoolchildren in Fukushima Prefecture to be exposed to 20 times the radiation dose previously permitted. The new level is equal to the international standard for adult workers at nuclear power plants.
After a huge outcry from parents, the government promised that it would lower the permissible level and that it would pay to remove contaminated topsoil from school grounds.
But the debate is not limited to children; the authorities have to weigh the risks of allowing thousands of people, including the elderly, to be exposed to levels that remain far above natural background radiation.
The general assumption is that when people are exposed to small doses for decades, the incidence of cancer will rise over time. But that prediction is based on extrapolating from data on people who were exposed to acute brief doses when atomic bombs were dropped on Nagasaki and Hiroshima in 1945 — not on observing individuals exposed to small doses over decades.
Some researchers argue that all humans are regularly exposed to a low natural level of radiation, and that it is not harmful when below a certain threshold, although fetuses may be an exception. Another vocal minority argues that there is statistical evidence for higher cancer rates among people exposed to tiny incremental doses.
Still, the mainstream view is that extrapolating from the Hiroshima and Nagasaki data is more prudent.
“There’s a point beneath which you just don’t know, and a straight line is the simplest assumption,” said Dr. Richard R. Monson, an epidemiologist and chairman of the committee that wrote an influential report released in 2006 by the National Academy of Sciences on low-level radiation exposures.
His committee based its recommendations on a hypothesis known as the “linear, no threshold model.” Under this hypothesis, if a given dose will cause fatal cancers in a certain number of people in a population, then half that dose will cause fatal cancer in half as many people, and a millionth of that dose will cause fatal cancer in a millionth as many people.
Dr. Monson’s committee largely extrapolated from the health records of thousands of Japanese civilians exposed to a sudden burst of high-energy gamma radiation by atomic bombs. Over the next 65 years, most of those people died from cancers that may or may not have been caused by radiation, and others from causes common to old age.
Their death rate from cancer exceeded the one recorded for populations of Japanese not exposed to the radiation. But applying this data to the risks faced at Fukushima Daiichi is problematic, experts say, and could lead to overstating or understating the risk to people who live near the plant.
The most obvious difference is that the bomb survivors’ exposure in 1945 was nearly instantaneous. People in the Fukushima area are confronting regular levels of contamination in the range of 5 to 10 times what people are normally exposed to in natural background radiation.
What is more, some of the radiation to which people are being exposed around Fukushima is inside the body; it comes from radioactive materials that contaminated their food or water. At Hiroshima and Nagasaki, many of the victims experienced only a quick external irradiation.
Evan B. Douple, the associate chief of research for the Radiation Effects Research Foundation, a joint Japanese-American science institute that analyzes health data from the bomb survivors, said that a dose delivered slowly over time was less damaging than an equal dose delivered quickly.
“It is well known in radiation biology that radiation-induced damage from a given dose of radiation is less effective if it is protracted or fractionated,” he said. The reason, he said, is that the body’s repair mechanisms work during the extended period of exposure.
The 2006 report by the National Academy of Sciences estimated that the effect of a given amount of radiation is 1.5 times worse when the dose is given all at once than when it is extended. But there are no authoritative details on varying doses over time.
As if this were not complex enough, another school of thought suggests that the radiation effect on people exposed at Hiroshima and Nagasaki was worse than the official statistics show.
This theory holds that weaker individuals were killed off by the bombs and by the hardships suffered in those cities at the end of World War II. The people who survived past that period, on whom the estimates are based, are not representative because they were stronger than average.
So the deaths counted in the following decades occurred among a hardier-than-average population, critics say.
In the United States, most of the policies involving radiation exposure involve people who are exposed to low levels on the job, like nuclear plant workers. If the United States faced decisions like those now confronting Japanese officials, “there really isn’t any coherent policy,” said Robert Alvarez, a former senior staff member at the Energy Department who works as a consultant for groups worried about nuclear risks.
Kuniko Tanioka, a member of the Japanese Parliament who traveled to Washington to research how the United States government conducts independent inquiries after major technological disasters, said that advising the public after a nuclear accident poses grave challenges in both countries.
Ms. Tanioka suggested that the best course that Japan could take would be to distribute all the raw data it has on radiation exposure to the international community and allow outside interpretations.
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