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YOKOSUKA NAVAL BASE, Japan — Trace amounts of radioactivity turned up in Yokosuka Harbor this month after a visit from a U.S. nuclear-powered submarine, the Japanese government reported Wednesday.

Testing of water and soil continue, according to Japan’s Ministry of Education, Culture, Sports, Science and Technology, which did not directly attribute the traces to the submarine.

In a statement, the ministry said the radiation posed “no threat” to humans or the environment. The ministry tests the waters for radioactivity when nuclear-powered ships arrive and daily while they are in port. The ministry also follows ships out to sea, taking samples from their wake.

A small amount of the radioactive isotopes cobalt 58 and cobalt 60 showed up around the stern of the USS Honolulu as it left Yokosuka on Sept. 14 after a weeklong visit, said Fumihiko Matsukawa, the ministry’s nuclear safety division official.

“It is about one 100,000th of the radioactivity people take into their body a year,” Matsukawa said. “It was such a small amount that it was barely detected.”

While cobalt occurs naturally in the water and in the earth, these two radioactive isotopes typically are associated with medical use and nuclear power plant operations, according to a U.S. Department of Health and Human Services Web site.

Matsukawa did not point to the submarine as the definite cause, saying the ministry will re-examine the water sample and test a soil sample from the same day.

The ministry has surveyed U.S. nuclear-propelled warships visiting Yokosuka, Sasebo and Okinawa ever since a nuclear ship first visited in 1964. The ministry has found radiation before but has not attributed it directly to the ships, Matsukawa said.

The U.S. Navy plans to forward-deploy its first nuclear-powered ship to Japan in 2008, when the USS George Washington replaces the USS Kitty Hawk as the fleet’s aircraft carrier.

The Navy does not expect this finding to delay preparations for the George Washington, said Cmdr. David Waterman, spokesman, U.S. Naval Forces Japan.

“This does not have a foreseeable impact on our plans,” he said, adding that the Navy currently is investigating the matter.

The City of Yokosuka also asked for an investigation and asked the Navy to disclose the results “including causal association with the nuclear-powered submarine Honolulu,” Yokosuka Mayor Ryoichi Kabaya was quoted as saying in a written statement.

Masahiko Goto, a Yokosuka attorney and activist opposing bringing a nuclear-powered carrier to Yokosuka, said the incident could have far-reaching implications. Contending that the Navy’s assertion of no nuclear accidents on ships is wrong, Goto said he hopes that Navy will disclose information, including the submarine’s logbook, toward identifying the cause.

“Yokosuka city and the national government should reconsider [the] nuclear carrier deployment issue because it is dangerous,” Goto said.

The USS Honolulu is a 21-year -old submarine permanently based in Pearl Harbor, Hawaii. Scheduled for decommissioning in October, it is on its final run.

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