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YOKOSUKA NAVAL BASE, Japan — The trace amount of radioactivity found near the USS Honolulu last month was not discharged from the nuclear-powered submarine, a Navy investigation concluded Thursday.

The Navy released its review of the ship’s “systems, procedures and operations” to the government of Japan and city of Yokosuka, saying there was neither deliberate discharge nor any accidents that would have allowed the material to escape. The review was done by a U.S. Navy expert outside the Honolulu’s chain of command who inspected the submarine, its documents and its commanding officer’s report, according to a Navy news release.

The investigation comes after Japan’s Ministry of Education, Culture, Sports, Science and Technology reported finding a small amount of cobalt 58 and cobalt 60 around the Honolulu’s stern as it left Yokosuka on Sept. 14. The amount was not considered dangerous to people or the environment, they said.

The ministry tests the waters every day that a U.S. nuclear-powered vessel is in port and again as it leaves. Since 1964, U.S. nuclear-powered warships have visited naval ports in Yokosuka, Sasebo and White Beach more than 1,200 times, according to the Navy’s release.

“The results of monitoring in these ports conducted by the government of Japan and the U.S. government, respectively, demonstrate that the operation of U.S. nuclear-powered warships has not had any adverse effect on human health, marine life or the environment,” the release stated.

The ministry’s findings largely agree with the Navy’s, Fumihiko Matsukawa, a ministry nuclear safety division official, said Thursday.

Although the “possibility” that the material came from the submarine cannot be denied, it was “unlikely that it originated from a power reactor or cooling system accident or trouble,” the ministry stated in a news release. It also ruled out fallout from 1950-1980 nuclear tests in the atmosphere and, due to where the radioactivity was found, dismissed that it came from local businesses that use these cobalts.

After testing all of the samples and examining every possibility, the cause remains “undetermined,” Matsukawa said, and the case likely will be closed.

“There will not be anything that will reveal new data,” he said.

Yokosuka civic groups have lodged concerns about the safety of nuclear-powered ships in response to the Navy’s plans to replace the conventionally-powered USS Kitty Hawk aircraft carrier with the nuclear-powered USS George Washington aircraft carrier in 2008.

Yokosuka city Mayor Ryoichi Kabaya moderated his opposition to the nuclear-powered carrier after visiting the San Diego area, where nuclear-powered Navy ships have been based for years without incident.

After the radiation was found, Kabaya asked the Navy to share the results of its investigation. According to Masashi Suzuki, assistant manager of the city’s military base division, the Navy contacted the city officials Thursday morning with the results.

“I appreciate the prompt response,” the mayor was quoted as saying in a written statement Thursday.

The USS Honolulu is a 21-year-old submarine scheduled for decommissioning in Washington state this month.

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