CONCORD, N.H. – A speaker at an international watchdog agency warned Monday that few of the world’s nuclear plants could withstand an attack like that on the World Trade Center last week. But a spokesman for Seabrook’s nuclear plant said the plant could handle such an attack.

“We have the utmost confidence that Seabrook Station would have been able to contain an attack like that,” Alan Griffith said. “Our containment structure is incredibly strong.”

He said the plant was designed to repel a direct hit from a bomber plane. The architects of most nuclear plants designed them to be safe from ground attacks, David Kyd, an International Atomic Energy Agency spokesman, said in Vienna, Austria, on Monday.

Most plants were designed to withstand only accidental, glancing impacts from the smaller aircraft widely used in the 1970s, he said.

“If you postulate the risk of a jumbo jet full of fuel, it is clear that their design was not conceived to withstand such an impact,” Kyd said.

Delegates from 132 nations opened the agency’s annual atomic energy conference with calls for tighter security and calls to make sure nuclear materials are kept out of terrorists’ hands.

Japan’s minister for economy, trade and industry, noted that his country’s nuclear plants were built to withstand earthquakes – not “hits from above by missiles or aircraft.”

The Seabrook plant was designed in the 1970s and licensed for commercial use in 1990.

The 200-foot-tall reactor dome includes two concrete barriers. The outer one is 11/2 feet thick and the inner one is 41/2 feet thick. The two are separated by a 5-foot airspace. Inside the dome, the reactor is protected by steel walls between 9 inches and a foot thick.

Security has been increased at Seabrook, as at other nuclear plants, after the terrorist attacks last week. Griffith said he was unsure how long the extra security measures would be in place.

Maine Yankee, the closed nuclear plant in Wiscasset, Maine, has increased security, spokesman Eric Howes said. Maine Yankee’s responsibility is to safeguard spent fuel stored at the site, Howes said.

Vermont’s nuclear engineer said he does not know whether the Vermont Yankee nuclear power plant could withstand a terrorist attack similar to those on the World Trade Center.

But state nuclear engineer William Sherman, as well as Vermont Yankee officials, have said the 2-foot-thick, steel-reinforced concrete shell over the nuclear reactor in Vernon could withstand a significant attack. But now, he said he has his doubts.

“I think the question is a good one,” Sherman said. “I don’t know the answer.” Sherman said nuclear power plants are far stronger structures than the World Trade Center or the Pentagon.