An article in yesterday's editions on nuclear reactor problems should have specified that any explosions following a core meltdown would be from steam and not from nuclear reactions.

The Nuclear Regulatory Commission is checking two "potentially serious" problems of plumbing and stress that could lead to meltdowns in any of the nation's operating nuclear power plants and a resulting disastrous explosion of radioactivity.

One involves pressurized-water reactors, which comprise about two-thirds of the nation's 74 nuclear plants. It is possible, according to Harold Denton, NRC's director of reactor regulation, that some have brittle walls that could crack if they received a sudden high-pressure burst of cold water when they are very hot. A jet of cold water might be injected during a malfunction or accident, crack the reactor vessel, and cause a meltdown.

The other problem involves the 23 boiling-water reactors. If there is a break or leak in one of the pipes that carry water out of the reactor after a sudden shutdown, it would be hard to stop the leak and prevent all the water from escaping the reactor, again causing a meltdown.

Scientists have long known that neutron radiation from the reactor core makes the vessel walls brittle over time, but recent research has found it occurring faster than expected. They have also known that cold water on hot metal causes thermal stress, and built reactors to mix incoming cold water with hot water to cushion the impact. But Denton told reactor owners last week that research had also found "quite severe thermal shocks in more or less normal operation" with relatively small accidents.

Ironically, new rules in effect since the accident at Three Mile Island two years ago could make things worse. The rules require operators to leave high-pressure water pumps running if they come on automatically because operators at TMI turned them off and the reactor core overheated as a result. But leaving them on, it turns out, increases pressure on the hot walls at the worst possible moment -- just as the cold water hits.

The boiling-water reactor problem involves several pipes ranging in diameter from 3/4-inch to 6 inches that carry about 600 gallons of water out of the reactor core to a storage tank when there is an automatic shutdown. The pipes also undergo "significant thermal shock" and apparently are used so little that they do not fall under the NRC's regulations for routine inspection, said Stuart D. Rubin, lead reactor systems engineer at the NRC.

If one or more should break, highly radioactive water would spill into the reactor building where it would threaten the plant's safety systems, including water pumps directly below, and the flow might continue until the core was uncovered, Rubin said.