The Yomiuri Shimbun
Small amounts of three plutonium isotopes were detected in soil sampled at the Fukushima No. 1 nuclear power plant last week, but the levels do not pose a health risk, plant operator Tokyo Electric Power Co. has announced.
Plutonium-238, 239 and 240 were detected in soil samples taken at five locations inside the plant between the afternoon of March 21 and the morning of March 22 morning, TEPCO said Monday night.
"This is quite serious since it indicates fuel rods have melted to some degree," Chief Cabinet Secretary Yukio Edano said at a Tuesday morning press conference. Edano stressed the need to increase monitoring in the surrounding area.
Plutonium has been detected before after atmospheric nuclear weapons tests, but TEPCO said the plutonium measured this time was likely from the crippled reactors, judging from its characteristics. Only small amounts of substances used as nuclear fuel exist in nature.
The Japan Atomic Energy Agency conducted the analysis of the soil samples from March 23. It usually takes about a week to complete plutonium-detecting tests.
Levels of plutonium-238--a radioactive nuclide generated in nuclear reactors--in samples from a multipurpose field and a solid waste warehouse were 0.54 becquerels and 0.18 becquerels per kilogram of soil, respectively. The former amount is about 3.6 times the level normally detected domestically.
TEPCO Vice President Sakae Muto said in a press conference in Tokyo that plutonium-238 detected came from the recent crisis at the plant. Plutonium-238 made up a higher percentage of the samples than the other isotopes.
The plant's No. 3 reactor uses MOX fuel, a mix of uranium and plutonium. However, since plutonium is generated even when only regular uranium fuel is used, it is unknown which reactor was the source of the plutonium that was detected, Muto said.
Other radioactive substances that have been detected in and around the Fukushima No. 1 plant, such as iodine-131, are believed to have been generated in the reactors. TEPCO officials suspect plutonium was released together with these substances.
"I think the plutonium came from damaged nuclear fuel rods in the reactors, and escaped when water leaked out," said nuclear chemist Michiaki Furukawa, professor emeritus at Nagoya University.
"But it won't affect humans seriously since the amounts detected were very small. Also, since plutonium doesn't vaporize easily, it can't be carried far away by the wind," He said. "It's dangerous when it gets into the lungs but it hasn't been dispersed outside the plant."
TEPCO said it would continue taking soil samples to check for radioactive substances.
Plutonium is produced when uranium atoms are exposed to neutrons. It can be used as nuclear fuel similar to uranium. Plutonium's weight changes depending on the number of neutrons it has. There are several isotopes, including plutonium-238 and 239.
Plutonium, an alpha-ray emitter, has a very weak penetrating force. The risk of external exposure is therefore low. If plutonium is eaten, it is easily excreted, but if it is inhaled, it stays in the lungs and can cause cancer.
(Mar. 30, 2011)
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