While federal officials expressed evident surprise and relief today at the overnight shrinkage of the dangerous hydrogen bubble that had blocked the cool-down of the Three Mile Island nuclear power plant reactor,experts from the firm that operates the plant insisted they were expecting the drop in the bubble's size all along.

"The whole thing has been a planned process since it began last Friday," said John Hilbish, a nuclear engineer from the Metropolitan Edison Co., operator of the plant.

"Based on the game plan," said Hilbish, "this is exactly what we expected to happen."

What did happen was explained in step-by-step fashion today by Hilbish and other nuclear experts who have gathered here to shut down the malfunctioning reactor. According to their accounts, the process went this way:

On Friday, utility nuclear experts began "degasifying" the then-1,000-cubic-foot bubble by spraying the circulating liquid in the reactor's primary cooling system into a portion of the plant called a "pressurizer."

The tall, cylindrical pressurizer is designed to allow the operators of the plant to raise or lower pressure in the primary cooling system of the reactor. The device is located outside the reactor itself but within the surrounding containment building.

By spraying the coolant into the pressurizer, hydrogen was removed from the liquid. The coolant was then recirculated into the reactor where it absorbed additional hydrogen from the trapped bubble. As the hydrogen went into the coolant, the size of the bubble decreased.

According to Thomas Elsasser, a nuclear engineer for the Nuclear Regulatory Commission, the bubble gave up hydrogen at different rates as varying degrees of pressure were applied to the primary cooling system.

"They operated in a band between 900 pounds per square inch and about 1,100 pounds per square inch (psi) of pressure on the system," said Elsasser. "The variation of pressure was the key. They just kept applying different levels until the process worked."

One problem, he explained, was that below 900 psi, hydrogen absorbed in the coolants might have been released prematurely into the vacuum pump circulating the coolant through the primary system. If that happened, he said, the pump could suffer vapor binding and fail.

Officials say that when the pump was shut down last Wednesday by a plant operator, the radioactive core of the reactor became partially exposed and damaged, triggering the entire nuclear crisis.

What occurred during the degasification of the bubble, nuclear experts said, was a process similar to squeezing a sponge. The bubble was squeezed by the pressure on the primary system and hydrogen was forced out into the coolant where it was carred back to the pressurizer.

Back in the pressurizer the entire process began again with the coolant sprayed out and the hydrogen released. From there the radioactive hydrogen was vented from the pressurizer into the surrounding containment building.

Federal officals said one of two "recombiners"-machines that convert hydrogen and oxygen from a gaseous state to water-was started up late this afternoon to keep down the level of potentially explosive and mildly radioactive hydrogen in the containment and prevent it from reaching the outside atmosphere.

Utility officials speculated that the initial reduction in the size of the bubble may have been slowed because the hydrogen already saturated in the coolant had to be removed before the new absorption process could begin.

At his news conference here today Harold Denton, head of the NRC team here, indicated that he was initially skeptical about the amount of reported shrinkage in the bubble. Earlier, Denton had expressed doubts over whether the degasification system could effectively decrease the size of the trapped bubble.

But Denton and Roger Mattson, an NRC division chief, said that if the measurements were correct "the bubble is for all practical purposes gone."

Mattson said that experts would continue to monitor the coolant system closely to prevent hydrogen from leaking back into the bubble.