John Comey, the stocky, 33-year-old spokesman for the State Emergency Management Agency, knows "The Plan" by heart.

He's been reciting it since Wednesday, when a series of mechanical breakdowns in the nearby Three Mile Island nuclear power plant triggered the nation's most serious nuclear power accident to date.

The plan is what to do if it becomes necessary to evacuate the 636,073 people living within a 20-mile radius of the facility.

The plan sounds simple, but beneath that simplicity flows a sea of problems that could swell into a tidal wave of confusion during crucial moments.

Basically, it involves:

The state Department of Environmental Resources would notify the state Emergency Management Agency in the event of a radiation threat.

The Emergency Management Agency would consult immediately with the governor's office on whether to evacuate.

The evacuation order, if necessary, would be issued by the governor through the Emergency Management Agency. The first counties to be so notified would be those considered in immediate danger-York, Dauphin and Lancaster.

The Emergency Management Agency would notify local emergency agencies through the use of police, fire and rescue unit communications.

State and county officials would broadcast the evacuation order over commercial radio and television stations at the discretion of country officials. Air raid sirens might also be used to alert the public.

The Emergency Management Agency might also use the National Oceanic and Atmospheric Adminstration (NOAA) radio network as an additional tool to notify the public.

Through the use of the broadcast media and local emergency officials working neighborhood by neighborhood, the public would be instructed to move to designated shelters-called mass care centers. The primary means of transportation would be private automobile. Buses also would be used to move people who couldn't get out by car.

Comey said the mass care centers are basically public institutional buildings such as schools and state hospitals. He said tthey would be stocked with food and bedding, and would be staffed by a variety of human resource personnel, such as nurses and ministers. State officials say evacuation sites have been designated, but they have taken no steps to move supplies to them. They said this could be done quickly.

Evacuation preparations are being coordinated by the state civil defense chief, Oran Henderson, who was acquitted on charges of covering up the Mylai massacre in 1968.

The evacuation of some residents already has begun. On Friday, Gov. Richard Thornburgh urged pregnant women and pre-school children within five miles of the atomic plant to leave the area.

Today, a fleet of ambulances was used to move almost 300 patients of nursing homes in Middletown to a sanitarium near Chambersburg, about 60 miles away. Nursing home officials said the move was necessary because said the move was necessary because many of the staffers, alarmed by the crisis, had left the area.

Also in the area are eight hospitals with a total of 3,637 beds, and 22 nursing homes with a total of 2,463 beds. Not all the beds are filled.

According to the evacuation plan, the 1,200 inmates to other penitentiaries.

In all, there are three prisions, one state and two country lockups, within the maximum 20-mile evacuation area.