The Nuclear Regulatory Commission was told yesterday that two recent incidents where nuclear power plants were cooled down too rapidly have forced an investigation of the resulting thermal shock to see how much of a threat it poses to nuclear safety.

Thermal shock occurs in this instance when the temperature of the cooling water covering the hot uranium core drops so much and so quickly that it puts a sudden stress on the steel walls of the reactor vessel.

This stress could crack the reactor vessel if it has been bombarded for years by neutron radiation from the fissioning uranium fuel, which has caused it to lose its tensile strength and become brittle.

A small crack would result in a leak of radioactive steam into the concrete containment room, which would require shutting the plant. A large crack might rupture the vessel, which would cause the water to flood out of the reactor, uncover the core and possibly cause a meltdown.

"We think this is serious enough that we will be requiring some corrective action this summer," Stephen Hanauer, director of the NRC's division of safety technology, said at a briefing for the five NRC commissioners.

Of the five incidents of overcooling experienced by nuclear power plants in the United States in the last 20 years, Hanauer said in an interview, the two most recent incidents stand out.

One occurred in 1980 at Florida's Crystal River plant, 70 miles north of Tampa, where a loss-of-coolant accident caused the plant's operators to flood the reactor core with too much cooling water. Nobody knows how far the water temperature fell, because the instrument reading water temperature failed during the accident.

The other happened in 1978 at California's Rancho Seco plant, outside Sacramento, where a short circuit in the control panel wiring blacked out the instruments for more than an hour.

Not knowing what the reactor temperature was, the plant's operators overcooled it from a little over 600 degrees to 280 degrees Fahrenheit.

Neither developed a crack because the steel was still new.

Hanauer said that two nuclear plants in Germany and one in Finland have been reconfigured in recent years because of concern over thermal shock. The steel walls of the vessel in Finland grew so brittle from neutron radiation that the uranium fuel rods nearest the walls had to be removed, an action that reduced the number of neutrons striking the walls and lowered the plant's power output by as much as 10 percent.

Another option is to lower the enriched uranium content of the fuel rods nearest the reactor vessel walls, then raise the enriched uranium content of the rods farthest away from the walls.

This would reduce the number of neutrons bombarding the walls while not reducing power output.

A third solution is to shut down many of the nation's nuclear power plants one by one to anneal the vessel walls with intense heat, which would increase their resistance to the neutron bombardment.