The earliest meltdown on his list occurred at a reactor being tested at the federal government's Santa Susana Field Laboratory, near Los Angeles, in 1957. Also on the list are Enrico Fermi Unit 1, near Detroit, in 1966; Three Mile Island, in 1979; and Chernobyl, in Ukraine, in 1986.

Given that in the history of nuclear energy, 582 reactors have operated for a total of 14,400 years (counting each year of operation by one reactor as a reactor-year), a core-damage accident has happened once every 1,309 years of operation. With 439 reactors now operating worldwide, the rate would yield an accident an average of once every three calendar years.

One variable is that some of the accidents were at types of reactors that are no longer commercially operated.

In contrast, the Nuclear Regulatory Commission has set a goal of no more than one accident per 10,000 years of collective operation. A corollary goal was that no more than one in 10 of those should result in significant off-site releases of radioactive material.

The commission's position is that all American reactors operating today meet that goal, but the conclusion is reached by calculating the probability of various failures and not by actual experience.

Mr. Cochran is scheduled to testify about his calculations before the Senate Environment and Public Works Committee on Tuesday. The Natural Resources Defense Council has called for a moratorium on new reactor licensing in the United States and on extensions for some plants.