The volume of radioactive substances in the air spewed from the damaged Fukushima No. 1 nuclear power plant fell significantly in the month following the disaster.
This finding emerges from observations made by the High Energy Accelerator Research Organization (KEK).
The observational data, which is based on the results of precise measurements performed in collaboration with the National Institute for Environmental Studies (NIES), shows that as of June the concentration had fallen to one hundred-thousandth of the amount measured immediately after the crisis triggered by the March 11 Great East Japan Earthquake.
Graph 1, which is available on the KEK homepage, uses a logarithmic scale for the vertical axis, with each increment being 10 times greater than the one below. The uppermost notch, 1.00E-04, represents a concentration per cubic centimeter measured in becquerels equal to 1 x 10 to the negative fourth power, while the lowermost notch of becquerels is 10 to the negative tenth power.
Try thinking of a becquerel as a unit expressing the amount of radioactive substances. The drop in the total volume of radioactive substances (the black line)--from 10 to the negative fourth power to the negative ninth power--shows that it has dropped to a one hundred-thousandth of its initial volume.
Certain elements with short half-lives disappeared quickly: technetium-99m (half-life of 6.4 hours), tellurium-132 (3.2 days) and tellurium-129m (33 days).
Iodine-131, with a half-life of eight days, was no longer detected by the end of May, even though a large amount was released. Only cesium remains.
Another notable aspect of this graph is that although all the elements are trending downward, there are many sharp fluctuations in the process.
A closer examination of measurements taken later in different areas shows that, at times, the amount of radioactive substances in the air at a given location sometimes increased abruptly by a multiple of 100.
According to a team of researchers from Ibaraki University, the University of Tokyo and elsewhere, this has to do with dryness in the air and wind direction. When dust carrying radioactive cesium soars up into the air, it is carried by the wind.
This is called "resuspension."
Soil in Japan has properties that allow it to easily absorb cesium; and once it does, it is difficult to separate. That explains the lack of cesium in large amounts in underground water. Instead, its location changes when soil is moved.
Wind and rain can also affect places where radioactive substances adhere to vegetation.
It is difficult to predict changes in radiation levels caused by those meteorological phenomena. Although scientists and researchers employ high-performance supercomputers to analyze vast amounts of observational data and produce weather forecasts, they are not always correct. In light of this, the difficulty of predicting changes in radiation levels should come as no surprise.
A group led by Yuko Hatano, associate professor of environmental risk assessment at the University of Tsukuba's Graduate School of Systems and Information Engineering, produced equations to make long-term air pollution predictions based on experience gained at the time of the Chernobyl disaster in 1986.
Factors the group took into consideration included fluctuations caused by resuspension as well as reductions in the amount of radioactive substances adhering to vegetation. The group also considered reductions based on the established half-life of each substance.
Having sorted out the predictive equations, the group then drew a graph that almost mirrored findings covering more than a decade from the Chernobyl disaster.
However, drawing the graph required the group to match three parameters with actual measurements. This presented a problem as measurements that can determine these parameters for the Fukushima disaster have not been collected.
This is where graduate student Kensuke Ota and his co-workers came to the rescue. The group devised the fastest and slowest case scenarios for the rate of dissipation of radioactive materials by referencing data from Chernobyl. The group used these constants to draw a graph showing long-term predictions of atmospheric concentrations of radioactive substances starting at 50 days after the March 11 earthquake. This is Graph 2.
Since this is also a logarithmic graph, the rate of decline would appear much steeper if drawn on a linear scale. We can see that after three years, say 1,000 days, the concentration will at best fall to one five-hundredth and, at worst, to one one-hundredth.
This prediction is of the amount of radioactive substances floating in the air. In my previous article, I presented estimates predicting that if--under hypothetical and unrealistic circumstances--you were to stand outdoors 24 hours a day, then your total amount of annual radiation exposure would drop to three-fourths after the first year and to one-half after three years. However, the figures in this article include all the effects of radiation from substances absorbed by soil and clinging to vegetation. In other words, it is the total radiation exposure on the outside of your body.
Radioactive substances floating in the air can be inhaled. According to this long-term prediction, such internal radiation exposure appears to fall rapidly.
Eating contaminated food can also cause internal exposure. When the United States and the Soviet Union conducted atmospheric nuclear tests in 1961 and 1962, all Japanese people received internal exposure as a result. According to data from the National Institute of Radiological Sciences (NIRS) based on regular measurements of radioactive cesium in adult males taken with whole body counters, body levels were highest in October 1964 at 730 becquerels. This dropped to under 100 becquerels by the end of the 1960s. It rose again by 23 becquerels due to the Chernobyl accident, but the effect dropped to half that amount some 15 months later.
Since 1990, the amount of cesium has been so low that whole body counters cannot detect it.
In some countries in Europe, levels were higher at the time of the Chernobyl accident than during atmospheric nuclear testing. According to data from the U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 850 becquerels were recorded in Poland during the testing, but this jumped to 1,700 after Chernobyl. Readings also rose in Germany from 782 to 1,500 becquerels, while Austria experienced a drastic increase from 337 to 2,800 becquerels. These levels started to drop a year after the accident.
Forecasting is an inaccurate science, but we can make rough forecasts based on prior experience.
Is the government doing enough to formulate those forecasts? I believe that we should try not to be too optimistic or pessimistic. Instead, we should make rational judgments based on the data available. But I'm not so sure this is going to happen.
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