A U.S. expert on atomic reactors said Thursday nuclear-powered vessels, including the aircraft carrier that will be deployed to the Yokosuka base in Kanagawa Prefecture, pose a radiation risk if their reactors are breached in an attack.

Gordon Thompson, head of a U.S. think tank, told reporters in Tokyo the probability of an accident caused by internal sabotage or terrorists "cannot be quantified."

As an example, he cited the boat-bomb attack on the conventionally powered destroyer USS Cole in Yemen in October 2000.

Thompson, executive director of the Institute for Resource and Security Studies, is visiting Japan at the invitation of a Japanese civic group to announce the findings of a report on the radiation risk posed by the planned deployment of the aircraft carrier USS George Washington to Yokosuka.

Thompson said a reactor in a nuclear-powered carrier may have a "comparatively high potential for a destructive hydrogen explosion" if its core sustains damage.

Yokosuka Mayor Ryoichi Kabaya announced June 14 that the city would inevitably be forced to host the George Washington to replace the USS Kitty Hawk, a diesel-powered aircraft carrier scheduled to be decommissioned in 2008.

"My judgment is that the probabilities of an accident in a nuclear-powered aircraft carrier reactor and a commercial reactor are rather similar," he said. "The probability of sabotage or attack cannot be quantified, so each person must reach their own judgment."

Masahiko Goto, a lawyer and representative of the civic group, said the group expects the state, prefecture and the city to further examine the safety of the planned deployment based on the report.

Goto criticized the mayor for accepting the deployment in line with a U.S. Navy fact sheet that "advertised the safety" of U.S. nuclear-powered warships.