Despite an easing of the Three Mile Island nuclear crisis, state, county and local officials today continued to advance plans for a possible, precautionary evacuation of up to 636,000 central Pennsylvania residents.

Even as word spread through the state capital complex that the size of the gas bubble in the nuclear plant's reactor had shrunk dramatically, officials at the state Emergency Management Agency were busy stocking radiation measurement equipment and reexaming planned evacuation routes.

"No plan of this type, even in its final form, is perfect," said Charles Crowe, an emergency planner. "We have to keep working on it," he said.

Part of the work involves setting up about 200 evacuee shelters in church, school and hospital buildings outside the evacuation area. Dave Butcher, a spokesman for the Red Cross, which would be responsible for operating the shelters, said most have already been stocked with foodstuff and bedding.

"Most of the shelters could be operational within two to three hours of an evacuation notice. I'd say we're ready if we're needed," Butcher said.

An evacuation order must come from Pennsylvania Gov. Richard Thornburgh. But, yesterday, emergency management officials did not seem upset about the uncertainty over whether Thornburgh would issue such an order. The key factor was that the governor gave no directive to scale down exacuation readiness, they said.

"All our evacuation planning is finished. We're ready to go," said Lancaster County emergency management director Paul Leese. He said the country will maintain its state of "advanced readiness" until told otherwise.

The advanced state of readiness seems to cover everything from schools to jails. Some Examples:

Most schools in the Harrisburg area remained closed today, even through Thornburgh recommended Sunday night that only schools within five miles of the Three Mile Island plant shut down.

Red Cross officials said today that about 40 percent of the estimated 133,672 persons living within a 10-mile radius of the plant have left the area voluntarily. (There was no estimate of how many, if any, of those persons have returned to the area.)

A spokeman for the Hospital Association of Pennsylvania said the six general hospitals within a 10-mile radius of the plan are releasing ambulatory patients and restricting admissions to emergency cases. The hospitals now have about 50 percent - about 1,100 patients - of normal occupancy, the spokesman said.

All of the hospitals within the 10-mile radius are prepared to implement evacuation plans, according to the hospital officials. County officials have also drawn up evacuation plans for remaining hospitals within a 20-mile radius, the officials said.

One state penitentiary, the state correctional institution at Camp Hill, is within the 20-mile radius. It has 1,160 inmates, about 200 of whom are small-time offenders who would be released in an evacuation. State corrections officials, who last week said they were having trouble finding transportation for the remaining prisoners, said today the problems have been solved.