It will be least a year before workers can reenter the containment building at the Three Mile Island nuclear power plant to inspect the damage done to the uranium core by the March 28 accident there, federal officials have been told.

The estimate of the one-year period was made to the Nuclear Regulatory Commission and the Presidential Commission on the Accident at Three Mile Island by executives of General Public Utilities Corp., which owns Metropolitan Edison Co., the operator of the Three Mile Island plant near Harrisburg, Pa.

Reopening the containment building at Three Mile Island in one year doesn't mean the cleanup of the contamination inside could begin then. All it means, sources at the NRC said, is that an inspection could be made to assess how much radiation there is and where it is located inside the containment.

"When we assess radiation damage," one source said, "then we can assess the damage that's been done to the core inside the reactor vessel."

Assessing the damage has a lot to do with estimates of cleanup costs, which now range from $140 million to $250 million or more. General Public Utilities has told the NRC it will take at least two to three years to clean the containment and remove the heavily damaged core and reactor vessel.

Based on what it already knows, the NRC believes that 30 percent of the metal that held the 36,000 uranium fuel rods in place inside the reactor has been eaten away by heat and oxidation. As many as one-third of the uranium oxide fuel pellets that were inside the rods have been displaced; some have been dislodged and are on the floor of the reactor vessel.

"We don't believe there's any danger in their reheating," one NRC source said. "We don't think there are enough of them on the floor to start any kind of chain reaction."

The water being circulated through the core to cool it is treated with boron salts to absorb whatever neutrons are coming off the uranium fuel bundles so that a chain reaction doesn't restart. Boron serves as a sponge to soak up excess neutrons inside a reactor vessel, to slow a chain reaction or stop it.

At least one of the fuel assembles holding 220 rods was so damaged in the accident that cooling water can't circulate freely around it. This has created a hot spot in the core of 320 degrees that will probably get no cooler in the near future.

Outside the reactor vessel in the concrete containment are so many radioactive fission products that came out as gas and dissolved in waste water that there are from 20,000 to 30,000 "rads" of measurable radiation inside.

Sources said radioactive debris is everywhere in the containment. It's on the walls, in the dome, on pumps, valves and pipes. Most of it is radioactive iodine, which has a short half-life of eight days. Some is cesium-137, whose 29-year half-life means it will be dangerously radioactive for at least half long.

Temperatures of about 3,6000 degrees Fahrenheit were believed to have been reached inside the core when it was uncovered by cooling water for 50 minutes.During that time, steam provided the only coolant to the core and it was then that the zirconium cladding around the fuel rods oxidized and broke open, releasing the fission products that had built up inside the rods.

It is not known how much long-lived fission material is in the containment. Some is on the walls, but much may be dissolved in the estimated 425,000 gallons of waste water that lie five feet deep in the containment basement.

The estimate of the one-year wait to open up the containment for a look inside is based at least partly on conjecture that the inside radiation will have dissipated enough to allow robot devices inside for an inspection. There is no guarantee that the containment will be cool enough to permit that.