The commission plans a meeting Friday with reactor owners to ask them to demonstrate why their plants should not be shut down until the problem can be fixed.

At the Nuclear Energy Institute, the trade association of reactor owners, Angelina Howard, a vice president, said in a statement that the settlement was "hugely significant."

To date, reactor owners have paid more than $10.5 billion.

In contrast, in 1999 the office found that the reactor owners were collectively 3 percent short.

The procedure for Shoreham was simpler, because that deal was made in the days when reactor owners were regulated utilities.

Now the reactor owners are suing for billions of dollars.

But the utility industry sees Mr. Pollard as its most technically adept opponent, one whose work has caused toil and tribulation for reactor owners.

This overview shows that many of these significant events occurred because reactor owners, and often the NRC, tolerated known safety problems.

According to commission documents, some reactor owners simply asserted that they could use such alternate means under the terms of their licenses.

But reactor owners are being urged to begin work now.