The Navy said Friday it discovered the leak July 17 when a gallon of water spilled from a valve while the submarine was in dry dock for routine maintenance at Pearl Harbor in Hawaii. An investigation showed water may have been slowly leaking from the valve since March as the Los Angeles-class submarine traveled around the Pacific.
The total amount of radioactivity released into the environment from the USS Houston at each stop was less than one half a microcurie, U.S. Pacific Fleet spokesman Capt. Scott Gureck said.
Gureck said that was a negligible amount, equivalent to the radioactivity of a 50-pound bag of fertilizer.
Akihiro Yoshida, a city official in Sasebo where the USS Houston made a port call in late March, said that government monitoring showed no abnormal increase of radioactivity in the area's waters during the submarine's calls.
"Still, we are rather concerned," Yoshida said.
Many people in Japan, the only country to have suffered atomic bombings, are sensitive about the military use of nuclear technology and the presence of American forces. The U.S. atomic bombings of Hiroshima and Nagasaki in August 1945 killed at least 200,000 people.
News of the incident also comes just weeks ahead of the controversial arrival of the nuclear-powered aircraft carrier USS George Washington to be based in Yokosuka, just south of Tokyo.
The carrier's arrival originally was set for August under a Japan-U.S. security alliance, but was delayed until late September because of a fire aboard the vessel in May. The George Washington is relieving the soon-to-be decommissioned USS Kitty Hawk and will be the first U.S. nuclear-powered ship to be stationed permanently in Japan.
The George Washington's deployment already has triggered protests, and the fire escalated concerns many Japanese have about nuclear power.
Masahiko Goto, a lawyer representing a citizens group opposing the George Washington's deployment, sharply criticized the U.S. Navy.
"They had discovered the radiation leak weeks ago and did not inform the Japanese government immediately," Goto said in a statement.
"The U.S. Navy's handling of the accident and lack of transparency showed there is no way we can trust them," he said.
The Navy said it didn't publicize the leak itself because the radioactivity released was below a level that would warrant a public announcement.
The Foreign Ministry acknowledged that it was told of the leak by the U.S. Navy on Friday but waited a day to announce it because the amount was negligible. The delay stirred a flurry of criticism.
Chief Cabinet Secretary Nobutaka Machimura called the delay "not good," and Defense Minister Yoshimasa Hayashi said "We'd have liked to hear from the Foreign Ministry earlier."
The delay also embarrassed Foreign Minister Masahiko Komura, who acknowledged that he learned about the leak from local media reports Saturday morning and said, "We should have made the announcement sooner."
The USS Houston is based at Apra Harbor in the U.S. territory of Guam in the Western Pacific. It visited a U.S. naval base in Sasebo in late March, and then stopped in Guam from late May to mid-June. The submarine sat in Pearl Harbor for about three weeks before it was dry-docked in mid-July.