Strontium-90

About 100 out of the 2,200 locations used for soil sampling were used for the analysis of strontium-89 and 90. The locations where strontium-89, having a half-life of 50.5 days, was detected is attributable to the accident, while locations where only strontium-90 was detected should be attributed to the result of weapons test fall-out made before the accident. In fact, locations without strontium-89 showed strontium-90 concentration lower than 950 Bq/m2, which is the level of weapons testing fallout. According to the contamination map created by Japan’s Ministry of Education, Culture, Sports Science and Technology (MEXT), locations where both strontium-89 and 90 were detected are distributed over an area of about 50 km radius from the NPP. In the area within about 50 km radius, the maximum concen­tration of strontium-90 was 5,700 Bq/m2, but it corresponds to only 0.12 mSv dose over 50 years, which is much lower than the effect of cesium-134 and 137. The ratio between detected strontium-89 to 90 was in the range from 1.9 to 4.0, which is in the possible range of the measurement error to be associated with the difficult detection of в-rays. On the other hand, the ratio of strontium-90 to cesium-137 was found to vary extensively from 0.00016 to 0.058 depending on the location, which suggests a non-uniform distribu­tion of strontium compared with cesium. The ratio of radioactivity of strontium-90 to cesium-137 in the nuclear fuels in the reactor core is in the range from 0.7 to 1.0, therefore, the low ratio of strontium-90 observed for the deposition indicates the lower volatility or mobility of strontium than cesium, being the most volatile and movable element in the fuel components.

Strontium-90 of 195 Bq/kg was found in the sediment on the roof of an apartment building in the city of Yokohama, south of Tokyo, and this news caused a controversy about the possibility of long-distance transfer of stron­tium from Fukushima. However, it is understood to be attributable to the result of past weapons testing fall-out, and its attribution to the accident was incorrect.

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