Lost H-Bomb Leaked Radioactivity but Is Harmless, U.S. Tells Japan

The United States has told Japan that a hydrogen bomb lost off a U.S. warship near Okinawa 24 years ago has leaked radioactive material but poses no threat to the environment, the Foreign Ministry said Monday.

Foreign Minister Sosuke Uno told a parliamentary committee that Japan accepts the U.S. evaluation but that an investigation is necessary to allay public fears.

The four-paragraph Defense Department report, sent to the Japanese Embassy in Washington on Friday, grew out of disclosures last week that an H-bomb settled to the Pacific Ocean floor about 70 miles from Okinawa after an A-4E Skyhawk carrying the weapon rolled off the deck of the aircraft carrier Ticonderoga in December, 1965.

The Japanese government said it first learned of the incident in a brief 1981 Pentagon report, which said only that the bomb was lost 500 miles from the Asian mainland in international waters.

According to Japanese officials, the Pentagon report acknowledged that the inert weapon experienced “structural failure” because of tremendous water pressure as it sank 16,200 feet to the ocean floor, resulting in leakage of unspecified nuclear material in the sea.

But the report said the radioactive material “would have settled very quickly to the floor of the ocean with other sedimentation; therefore, there is no environmental impact.”

The one-megaton bomb could not have detonated at the time of the accident “because the system was designed not to arm or receive an arming command under this accident circumstance,” it said.

The report did not say whether the vessel was carrying the bomb to a Japanese port in violation of an agreement between the countries.

On Monday, the Foreign Ministry appointed experts to evaluate the report and decide whether more studies are needed.