It’s difficult to make any sense of the data being reported from various quarters regarding dose rates and contamination levels at varying distances from the Fukushima Dai-Ichi nuclear plant.
The International Atomic Energy Agency (IAEA) could do a public service by establishing a consistent reporting framework so the public can assess whether radionuclide release rates are changing, and in what direction. However, its daily updates are only adding to the confusion.
Today, the daily update appears to show a significant upward trend in reported contamination levels. However, because it is not clear exactly where the measurements are being taken, one cannot do an apples-to-apples comparison.
For instance, in its March 27 update the IAEA reports that the highest level of daily deposition of iodine-131 was measured on March 26 at 7,500 becquerels per square meter (Bq/sq. m) in Yamagata prefecture; for cesium-137, it was reported as 1,200 Bq/sq. m. (Actually, the statement does not make clear if these are daily rates or total cumulative deposition levels, although from the context it appears to be the former.)
Compare these to the peak daily deposition rates reported in the March 26 update for measurements made on March 25. These were 480 Bq/sq. m for I-131 in Ibaraki and 110 Bq/sq. m for Cs-137 in Yamagata. But can these be compared? It would appear that the peak rate of Cs-137 daily deposition in Yamagata has increased by a factor of ten. But without knowing how comprehensive these surveys are, we don’t know if these represent actual increases or sampling discrepancies.
In either case, the higher level is cause for concern, although even at a daily deposition rate of 1,200 Bq/sq. m it would take more than a year before Cs-137 levels rose to the threshold of 555,000 Bq/sq. m that triggered strict land-use controls after the Chernobyl accident. But IAEA measurements of total beta-gamma contamination suggest much higher deposition rates may have occurred in the first few days after the accident in Japan (see below).
Other measurements reported by the IAEA also show disturbing upward trends. For instance, on March 27 the IAEA reported dose rates on March 26 measured in a region from 30 to 41 kilometers from Fukushima as ranging from 0.9 to 17 microSievert per hour, and total beta-gamma contamination levels ranging from 0.03 to 3.1 megabecquerels per square meter (MBq/sq. m). On March 26, it reported measurements in a region from 23 to 97 kilometers from the site on March 25 ranging from 0.73 to 8.8 microSievert per hour, and total beta-gamma contamination ranging from 0.02 to 0.4 MBq/sq m.
Thus even though the closest measurement point on March 26 was further away from the site, the peak dose rate—which is now about 100 times background—was twice as high as on March 26 and the peak beta-gamma rate was eight times higher. But we do not know if the measurements were taken in the same directions on both days. All we do know is that peak measurements continue to be disturbingly high.
It would also be helpful if the IAEA provided some insight to the relationship between measured beta-gamma levels and the much lower reported I-131 and Cs-137 deposition rates. Many short-lived isotopes have already decayed away. However, the reported beta-gamma rates were significantly lower than 3.1 MBq/sq. m even on March 21. This indicates continued high levels of deposition that cannot be explained by the reported deposition rates of I-131 and Cs-137.
The IAEA data would be much more useful if it also provided enough detail to allow apples-to-apples comparisons from one day to the next.
UPDATE: March 27, 8 PM
On March 24, the IAEA reported beta-gamma measurements of 3.8 to 4.9 MBq/sq m. in a northwesterly direction from the Fukushima site, which is in the direction of the highly contaminated plume that DOE identified a few days earlier. The much lower readings of 0.4 MBq/sq. m or below reported on March 26 were taken in a southerly or southwesterly direction. So it is likely that the high reading of 3.1 MBq/sq m. reported on March 27 was taken from the northwesterly direction again, although IAEA did not say so.