The process of cooling the crippled Three Mile Island nuclear power plant began yesterday even as the Nuclear Regulatory Commission said that radioactivity inside the plant's containment structure would remain at lethal levels for months to come.

The cooling down began at 10 a.m. yesterday morning and by 1 p.m. the temperature of the water around the reactor core had fallen from 280 degrees to 262 degrees. It continued to fall almost 10 degrees an hour through the rest of the day.

By late afternoon, the temperature was down to 230 degrees where it was held while the NRC studies several proposals for taking the reactor all the way down to a "cold shutdown."

"We'll take another four to five days to cool down," said Victor Stello, chief of the NRC's division of operating reactors. "We want to get down to below 200 degrees because that's where the big pumps kick in to get the water down to where it's not much more than room temperature."

Efforts to cool the reactor have been under way since March 28 when the plant, near Harrisburg, Pa., overheated in an accident.

Meanwhile, analysis of the waste water in the containment building surrounding the reactor confirmed NRC's worst fears that the water is contaminated with so much radioactivity it will be months before a cleanup can even be attempted.

More than 8 percent of the radioactive iodine produced as a fission product in the reactor has escaped from the reactor vessel and is in the containment water and on the walls and ceiling of the 157-foot-tall building. Three percent of the radioactive cesuim produced in the reactor is also in the containment building.

"That means we had millions of curiers of radiation in that containment structure the first day of the accident," one NRC source said. "And while the iodine is losing its radioactivity, it's still hotter than hell inside that building and will stay that way for a long, long time."

Iodine-131 has a half-life of eight days, which means that more than half its radioactivity is already gone. But so much radioactive iodine left the recotr and got into the containment that it will be months before its radioactivity is dissipated.

The iodine came out as a gas when temperatures of the damaged fuel rods climbed from their normal 600 degrees to 2,000 degrees. When the gas poured into the containment and raised the pressure there, spray pumps automatically came on an poured sodium hydroxide into the containment to change the iodine to liquid sodium iodide which "plated out" on the containment walls.

While the iodine was responsible for most of the radioactivity in the containment, the cesium-137 also in the structure presents a far more enduring radioactive hazard. Its half-life is 28 years and even though its radioactivity night be measured in thousands of curies instead of millions, most of it could still be there 25 years from now.

"The cesium makes the cleanup a very, very difficult job," an NRC source said. "And that stuff is all over everything, on the walls, the ceiling, the pumps, the steam generators, the pressurizer, the quench tank, the flood tank; you name it, there's cesium on it."

Besides the sludge-like iodine and cesium, there are from 150,000 to 200,000 gallons of radioactive water in the containment. The water level is almost four feet deep in the building, which has an inside diameter of 130 feet and a wall thickness of four feet.

The levels of iodine and cesium contamination tell the NRC that as much as half the rector core was damaged by temperatures that reached 3,000 degrees. The NRC has said it believes that the cooling water in the reactor sank so low that almost half the length of the fuel rods lay uncovered by water for at least eight hours.

The NRC believes there were three separate periods of uncovering, the last one doing the most damage because it apparently lasted unchecked for six hours. The "uncovering" overheated the fuel rods to where they cracked and crumbled and vented radioactivity into the containment.