Bursts of radioactive iodine have been escaping from the damaged Three Mile Island nuclear plant since Saturday, hampering efforts to cool the plant down.

"The iodine release is tying up the time of people who should be concentrating on the cooldown," one source at the Nuclear Regulatory Commission said today. "We want to get our arms around this iodine thing and fix it before we do anything else."

The periodic escapes of iodine reached a peak Sunday, ehen 94 picocuries of iodine-131 were measured on the east bank of the Susquehanna River six-tenths of a mile from Three Mile Island. The allowable limit of radioiodine away form the site of a nuclear plant is 100 picocuries.

On Three Mile Island itself, as much as 380,000 picocuries were measured inside a vent stack on top of the auxiliary building alongside the stricken nuclear reactor. This peak measurement was taken from an iodine filter cartridge that had been exposed to radioiodine escaping from the plant between 8 a.m. and 6 p.m. Sunday.

The NRC's Vis Stello, director of the division of operating reactors, said that the offsite release of 94 picocuries of iodine-131 posed no threat to the public health.

"An adult exposed to 100 picocuries would hae to inhale it 24 hours a day for a full year," Stello said, "before the dose reached a half a rem to te thyroid. A child's thyroid is smaller."

Nevertheless, the NRC wants to pinpoint the cause of the radioiodine release and stop it because inhaled and ingested iodine settles in the thyroid where large doses can damage the gland. A dose of 150 rems would be enough to damage the thyroid of a child and a dose of 400 rems would be fatal to anybody.

Stello said the iodine releases coincided with the changing by technicians Friday night and Saturday morning of filters in the Three Mile Island auxiliary building that scrub out radioiodine. Stello also said the ventilation fan in the building may have tripped the wrong way, pushing the iodine out instead of keeping it in.

"The iddine releases go up and down so it's hard to pinpoint where they're coming from in the auxiliary building," Stello said. "We have some ideas but we don't know for sure yet."

Stello said it's most likely the iodine is being released from tanks in the auxiliary building where radioactive decay gases have pumped from the reactor. Radioactive iodine was the most abundant and one of the first radioactive fission products to escape from the badly damaged fuel core at Three Mile Island.

The iodine escaped from the reactor mostly as a gas but it also came out dissolved in waste water, where it would still be in waste water tanks in the auxiliary building. There is a slight chance there is some iodine on the floor of the building but Stello said it's unlikely because the floor has been sprayed with sodium hydroxide to turn the iodine gas into liquid sodium iodide to keep it inside the building.

A little progress was made today in the cooling down operation with the hottest fuel bundles inside the damaged reactor falling four degress to 344 degress Fahrenheit. Still, four thermocouples inside the reactor core are reading temperatures of more than 300 degress from the most badly damaged parts of the core, which is hindering the cooldown.

The NRC wants to cool down the core to less than 200 degrees, so it can drop the pressure inside the reactor all the way down to zero where the water will be in no danger of boiling. The NRC now figures it will be at least five days before it can do that and possibly as long as several weeks.

Over the weekend, General Public Utilities Corp. revised downward its earlier estimate of replacement electricity costs for Three Mile Island from $1.1 million a day to $800,000.

"A previous estimate was based on replacement costs for a single day during which both Three Mile units would be operating at full capacity," said GPU chairman William G. Kuhns. "The current estimate of $800,000 a day more accurately reflects normal operating levels of the two units."

The damaged reactor and a second out-of-service reactor at Three Mile Island supplied electricity to three GPU utilities: Metropolitan Edison Co., Jersey Central Power and Light, and Pennsylvania Electric Co. The replacement costs are being shared equally by Jersey Central Power and Light and Metropolitan Edison, with Pennsylvania Electric picking up a little less than half of what the other two are paying.

All three utilities are buying electricity from other companies in Pennsylvania, New Jersey, Maryland and Ohio. The costs to each of the three are based on how far it has to be transmitted. CAPTION: Picture, John MacMillan, vice president of Babacock & Wilcox Co.'s nuclear power generation division, testifying on March 28 accident at Three Mile Island plant. By Margaret Thomas-The Washington Post