YOKOHAMA -- Tests found 195 becquerels of strontium-90 per kilogram in sediment in Yokohama, the first time the radioactive isotope has been detected outside the 100-kilometer radius of the Fukushima No. 1 nuclear power plant.
YOKOHAMA -- Tests found 195 becquerels of strontium-90 per kilogram in sediment in Yokohama, the first time the radioactive isotope has been detected outside the 100-kilometer radius of the Fukushima No. 1 nuclear power plant.
The sediment, taken from the rooftop gutter of an apartment building in Kohoku Ward in July, was analyzed by a private-run laboratory. The Yokohama city government is conducting its own analysis.
Science ministry surveys found strontium-90, which has a half-life of about 30 years, in Fukushima Prefecture and in southern Miyagi Prefecture, but never beyond 100 km of the crippled Fukushima plant. Yokohama is 250 km from the plant.
The concentration of strontium-90 in the Yokohama sample is larger than the 77 becquerels per kilogram detected in soil in Fukushima city between April and May.
However, a simple comparison is not appropriate because the conditions in Yokohama at the time were favorable for the accumulation of radioactive substances.
"We decided on a reanalysis because the concentration was so high," a city government official said. "We will decide on our response measures after obtaining the results."
Strontium has similar chemical properties as calcium. It is soluble in water and tends to accumulate in human bones.
A resident of the five-story apartment house, built seven years ago, collected the sediment and had it tested at the Isotope Research Institute, an analysis laboratory in Yokohama's Tsurumi Ward.
The laboratory also found a cesium concentration of 63,434 becquerels per kilogram in the same sediment sample. The Yokohama City Institute of Health, which reanalyzed the sample, detected 105,600 becquerels of cesium per kilogram, although the results were not released because the sample came from privately owned land.
The city government was initially not too concerned about checking for strontium content because it was believed to be too heavy to travel long distances from the Fukushima plant and such analyses were expensive.
In early October, however, the city collected sediment samples from the apartment house rooftop in question, a gutter near the apartment house and elsewhere, and took them to the Isotope Research Institute for analysis.
"I knew that strontium would travel over large distances," said Hiroaki Koide, assistant professor of nuclear engineering at the Kyoto University Research Reactor Institute. "It is no surprise that fallout strontium from the Fukushima No. 1 plant was detected in Yokohama. The concentration appears somewhat large, but I can understand that because the same sample produced a high cesium concentration. Condensation probably took place."