Levels of strontium in soil near the Fukushima No. 1 nuclear power plant are up to six times the highest concentrations deposited in Japan by pre-1980 atmospheric nuclear tests, according to the science ministry.
Official maps of soil contamination by strontium and plutonium were published for the first time on Sept. 30 and reveal that, while concentrations are dwarfed by the radioactive cesium leaked from the stricken plant since March 11, significant quantities are present in some locations.
The survey team collected soil samples within a 100-km radius of the nuclear plant between June 6 and July 8.
The concentrations of different nuclides of strontium and plutonium per square meter were analyzed at 100 locations, including each of the 59 municipalities within an 80-km radius of the plant. Forty-one additional locations within the 20-km no-entry zone were studied.
Strontium-90, which has a half-life of about 30 years, was most concentrated at a location in Futaba town within the zone. The 5,700 becquerels per square meter detected there was six times the maximum of 950 becquerels per square meter found during fiscal 1999-2008 nationwide surveys by the science ministry and attributed to nuclear tests.
Strontium-90 concentrations exceeding 950 becquerels per square meter were detected at eight locations. Seven of them were either within 20 km of the plant or beyond that radius to the northwest.
The maximum concentration of plutonium-238 was 4 becquerels per square meter, while the maximum combined concentration of plutonium-239 and plutonium-240 was 15 becquerels per square meter. At all locations, the plutonium concentrations were below the maximum levels detected before the accident.
At six locations lying either within a 30-km radius of the nuclear plant or beyond that radius to the northwest, however, the proportion of total plutonium consisting of plutonium-238 was much higher than in residual levels left by the nuclear tests, clearly indicating that the contamination was caused by the Fukushima disaster, the ministry said. Plutonium from the latest accident had previously only been detected on the grounds of the Fukushima No. 1 nuclear plant.
On average, the strontium concentrations were less than 1 percent of the concentrations of radioactive cesium.
"Both the plutonium and strontium concentrations are very small compared to those of cesium. Future assessment of the impact of radiation exposure and the design of decontamination measures should be focused on cesium," a ministry official said.
The current, temporary safety standard for cesium in food includes a 10-percent increment that accounts for strontium.
"The ratio (of less than 1 percent) given by the latest study is low. You will probably not have to worry about strontium as long as cesium remains below the threshold level," said Shigeo Uchida, director of the Research, Development and Support Center at the National Institute of Radiological Sciences.
Wherever there is radioactive cesium, strontium is believed to exist in a low but fixed ratio to the amount of cesium. Strontium is easily soluble in water and resembles calcium in its chemical properties. It can accumulate in human bones.
Plutonium-239, which is used in the manufacturing of atomic bombs and nuclear fuel rods, has a half-life of about 24,000 years. It is not thought to be easily absorbed by digestive organs and tends to be excreted when taken into the body with food.
(This article was written by Hiroshi Ishizuka and Hisae Sato.)
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