Scientists made a shocking finding in early November at the Fukushima No. 1 nuclear power plant's No. 2 reactor: They detected radioactive xenon, which indicated that nuclear fission was still occurring.
The good news is that it was not occurring in a continuous state of "criticality," or a sustainable chain reaction.
After analyzing temperature and pressure readings, among other data, the scientists concluded that not enough energy was being released for a "China syndrome" scenario.
Although there are no guarantees, it seems highly unlikely that the Fukushima plant will release large amounts of radioactive material again.
Then we would like to know how the amount of radiation in the released materials will change in the future.
A study involving universities and research institutes from across Japan, and led by the Fukushima prefectural government and the science ministry, has shown us where radioactive materials have fallen. We also have a pretty good idea of the types and amounts.
Iodine-131 constituted the bulk of the material. But it has mostly dissipated due to its half-life of only eight days.
The next largest amount is cesium.
Cesium-137 and cesium-134 were released in fairly equal amounts.
Most of the cesium-137 still remains since it has a half-life of 30 years. But cesium-134 has a half-life of only two years, meaning that half of it will decay by March 2013 if no decontamination efforts are made. Small amounts of strontium and plutonium were also detected in the study.
Yoko Fujikawa, an associate professor of nuclear engineering at Kyoto University, estimated the amount of each radioactive material released by the Fukushima nuclear power plant. The estimates show that the overwhelming majority of material was released by the No. 2 reactor.
The containment vessels inside the No. 1 and No. 3 reactors, whose buildings exploded, were probably not heavily damaged.
At the No. 2 reactor, an explosion occurred in the suppression chamber into which a large amount of radioactive gases inside the reactor enter through the piping. If the suppression chamber was badly damaged, then the natural result would be a massive release of radioactive materials.
According to the estimates, the overwhelming majority of the radioactive material released was iodine. The amount of released strontium was about one-thousandth of that of iodine-131 while the amounts of plutonium-239 and plutonium-240 were about one-100-millionth of the figure for iodine-131, respectively.
The amount of radioactive material released during atmospheric nuclear testing in the 1960s was far greater than the amount released in the Chernobyl disaster.
Nuclear testing produced 375 times as much iodine-131 and 11 times as much cesium-137. I am again reminded of my anger at the selfishness displayed by the superpowers of the time, the United States and the Soviet Union. Later, China would also release large amounts of radioactive material by conducting atmospheric nuclear tests.
However, although the total amount of radioactive materials produced by nuclear testing was huge, it must be said that the amount that fell on Japan was not so great.
Fujikawa also compared the amounts of radioactive fallout in Japan from the Fukushima disaster and from nuclear testing.
Although large amounts fell in Fukushima Prefecture, the accident there caused much more to drop on Ibaraki and Tokyo than did nuclear testing. The only saving grace is that iodine, which accounts for the majority of the fallout, has a short half-life.
In addition, Fujikawa calculated the air radiation dose from results of a soil survey conducted by Fukushima Prefecture at the end of March. After verifying that the radiation levels she calculated closely matched actual measurements, Fujikawa estimated the proportion of radioactivity in each location based on the material found there.
This showed that radiation levels varied widely according to location; that radioactive materials were distributed unevenly in each place; and that iodine accounted for roughly half of the radiation as of the end of March.
Average radiation levels in the 40 locations studied were 4.6 microsieverts per hour for iodine, 2.7 microsieverts per hour for cesium-137 and 7.7 microsieverts per hour for cesium-134.
Assuming that the soil's condition remains unchanged, it was calculated how much radiation a person would absorb by standing there all day, every day for a year. The figure was 40 millisieverts in the first year and 31 millisieverts in the second year. The estimates dropped to 25 millisieverts in the third and 20 millisieverts in the fourth.
Although it is unrealistic to assume a person would stand outside for such long periods, thinking about it this way allows us to grasp how the radiation levels are falling. In the second year they will be six-eighths of their starting level, five-eighths in the third year, and then four-eighths--or one half--in the fourth.
But in reality, radioactive material in the soil does not just sit there until it decays. Rain and wind, as well as decontamination work, can move it. The wind may blow it away, but it can also be brought in from somewhere else.
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Mariko Takahashi is a senior staff writer of The Asahi Shimbun.
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