Speaking on ABC's "This Week," Joseph Cirincione of the Ploughshares Fund said: "This is already one of the worst nuclear accidents in history if it stops right now. And we're dealing with multiple meltdown possibilities.
"There were actually incidents at other nuclear facilities in Japan that would have been significant incidents by themselves, but they're caught in the wake of these major crises at these nuclear reactors that possibly will melt down in the next couple of days," he added.
"Nuclear reactors are built to withstand crises, and even multiple crises," Cirincione said. "But it's very hard to build a facility that can withstand this."
The United States is sending radiation experts not only to help the Japanese, but to monitor the possible risk of radiation spreading to Pacific Islands, Hawaii or even Alaska and the West Coast in the event of meltdowns, ABC News said.