The National Institute of Radiological Sciences (NIRS) knew about it since May last year, but decided to keep quiet. Why? It's yet another case of the researchers waiting until their data collection is published in a peer-review scientific magazine (the paper is linked at the bottom of the post).
For the NIRS researchers, their data has just been published in the UK's Scientific Reports (electronic version), reports Kyodo News (3/8/2012):
The National Institute of Radiological Sciences (NIRS, in Chiba City, Chiba) published the result of their measurement of plutonium-241 at three locations in Fukushima Prefecture 20 to 32 kilometers northwest and south of Fukushima I Nuclear Power Plant in the electronic version of the UK's science magazine "Scientific Reports" issued on March 8, 2012.
The level [of plutonium-241] will not affect human health. Plutonium-241 has relatively short half life of 14 years compared to other isotopes of plutonium. It decays into americium-241, which is easily absorbed through soil into legumes. The NIRS says "To avoid internal radiation exposure, it is necessary to survey the spread of plutonium-241 inside the 20 kilometer zone around the plant."
[Plutonium-241] was detected from the samples they collected in April and May last year - the dead leaves from the forests in Iitate-mura and Namie-machi, and the soil at J-village, which has been used as the staging area for the work after the Fukushima I Nuke Plant accident. Other isotopes of plutonium, plutonium-239 (half life 24,000 years) and plutonium-240 (half life 6,600 years), were also detected. From the ratio of isotopes, the NIRS researchers concluded that they were from the Fukushima nuclear accident.
The density of plutonium-241 [detected this time] is higher than that being detected in Japan after the atmospheric nuclear tests in the past. However, plutonium-241 has a short half life, and the density is lower than that during the 1960s when the radioactive fallout from the nuclear tests fell on Japan.
Plutonium is a radionuclide that hardly exists in nature. It is created when uranium in the reactor fuel absorbs neutrons.
Hmmm. If you want to avoid internal radiation exposure by ingesting food that may contain americium-241, a decay product of plutonium-241, don't you want to survey the area outside the 20 kilometer zone? After all, outside the 20 kilometer zone, Fukushima farmers will be tilling the land, ready to grow crops of all kinds again this year.
Half life of americium-241 is 432.7 years. As it decays, americium-241 emits alpha and gamma rays.
Asahi Shinbun has the numbers for plutonium-241:
Dead leaves in Namie-machi (26 kilometers NW of the plant): 34.8 becquerels/kg
Dead leaves in Iitate-mura (32 kilometers NW of the plant): 20.2 becquerels/kg
Soil at J-Village (20 kilometer south of the plant: 4.52 becquerels/kg
Farmers in Fukushima, are you still going to grow stuff on your land?
Researchers at the NIRS, did you think of at least informally telling the local authorities or the farmers about your findings? Or did you just sit and wait until your paper was published by a peer-review magazine?
(I know I'm wasting my breath.)
Scientific Reports carries their full paper online.