Federal officials raised yesterday the possibility that violations of federal regulations at the Three Mile Island nuclear power plant were serious enough to cost Metropolitan Edison Co. its license to operate any atomic facility.

"When we get to the bottom of all this loss of license is a distinct possibility," a Nuclear Regulatory Commission source told the Washington Post.

The discussion of the company's operating license came as, separately, NRC officials in Pennsylvania said that several months ago they talked to Babcock & Wilcox, the plant's manufacturer, about a pressure gauge that apparently misled an operator in the control room about the level of water in the reactor at the time of the accident that began March 28.

Sources at the NRC now say the operators at Three Mile Island located near Harrisburg, Pa., "blocked off" all three auxiliary feedwater pumps for maintenance one hour before the chain of events began that badly damaged the reactor core and released radioactivity to the air and water. This is a violation of NRC rules and one that could force the NRC to take away Metropolitan Edison's nuclear operating license.

The operators periodically, had been locking the "block valves" for two weeks before the accident so technicians could run the pumps and check the seals without fear that pressure might back up through the valves and damage the pumps. Earlier reports said the pumps were shut off completely for the two weeks.

NRC sources said the switches in the Three Mile Island control room that can be turned on manually to open and close the block valves were "tagged," meaning operators in the control room could see that the block valves were closed and locking out the auxiliary pumps.

In fact, long after the chain of events began that caused the accident, the operators must have noticed that the auxiliary pumps were locked out because they three the switches that opened the block valves and started up the pumps. By that time, the fuel in the reactor core had been badly damaged and the worst of the accident had taken place.

The logs at Three Mile Island show that 30 seconds after the main coolant pumps failed the automatic controls that switch on the auxiliary pumps came on and started the pumps. Cooling water began to flow through the auxiliary pipes toward the reactor, then abruptly stopped flowing when it ran against the closed block valves.

"It was outrageous," one NRC source said. "Our rules state that if you lock out an auxiliary feedwater system for maintenance of any kind, you're supposed to hit the down button in a helluva hurry and shut down the reactor."

The source said that "it boggles the mind" that a nuclear plant superintendent allowed the reactor to operate while locking out an auxiliary cooling system, the second line of defense against a loss-of-coolant accident that could damage the reactor core or even bring on a meltdown of it.

"The rules allow two backup pumps to be down at any one time and then only for a short time," the source said. "If you have all three down, you're supposed to shut the plant down."

Meanwhile, in Middletown, Pa., where Three Mile Island is located, Harold C. Denton, Director of the NRC office of reactor regulation, said that NRC staffers had questioned several months ago the builders (Babcock & Wilcox) of Three Mile Island about the reading or misreading of a pressure gauge in the control room. The pressure gauge tells the operator what the coolant pressure is, which in turn should tell him what the water level is in the reactor.

Three Mile Island logs show that the operator read the pressure gauge as saying that the coolant pressure was high enough to support a water level that covered the core. In fact, the water level was falling and would soon fall to where it uncovered the core and damaged it, creating radiation, some of which ended up in the environment.

After getting the water level wrong, the operator shut down the emergency core cooling system that was also supplying water to the core. When he did that, the water level dropped dramatically and the fuel suffered its first damage.

Denton was asked if the question the NRC raised with Babcock & Wilcox over the reading of the pressure gauge were printed and passed on to Metropolitan Edison for its instruction.

"You have to understand that we operate on a risk aversion basis and that no matter what happens in any plant you can find a thick stack of documents where we've been attempting to improve that area," Denton said. "I'm familiar with these documents but I haven't gone back and studied them to see whether or not in hindsight we should have picked something up and done it differently.

"But that would be part of our overall investigation," Denton said. "I think there were people in the commission who were concerned about some of these things early on."

Denton also said that five years ago questions were raised about the specifications in the Babcock & Wilcox design that call for an automatic isolation of the concrete containment structure when the pressure inside rises to 4 pounds per square inch.

Scientists advising the NRC have questioned this design, pointing out that other nuclear power plant designs isolate the containment from the reactor when the emergency core cooling system is turned on and before the pressure is allowed to rise.

By the time the containment at Three Mile Island had been isolated and cut off from the reactor, hydrogen and radioactive gases like xenon, krypton and iodine had leaked into the containment. Not only did this badly contaminate the containment, it also produced at least one small explosion of hydrogen inside the containment that threatened to break through concrete.

Pennsylvania Gov. Richard Thornburgh, in a report to the State last night, said that conditions near the plant were safer now than at any time since the accident. But the governor did not suspend his evacuation advisory.

Thornburg's advisory that pregnant women and children under school age residing within five miles of the plant should stay away from home remains in effect.

Thornburgh said developments have convinced him that he was right to limit the evacuation advisory to the most vulnerable segments of the population. "We've survived . . . without the psychic and physical damage a mass evacuation might have caused," he said.

The governor asked Pennsylvanians to "stay calm in the face of the storm," so that residents can prevail in "this struggle against forces over which we have no control."

On the scene at Three Mile Island, technicians continued to go about the task of drawing off the hydrogen gas still left in the reactor that is preventing it from reaching what the NRC calls a "cold shutdown" condition.

The hydrogen dissolved in the reactor coolant is brought out in the waste water through pipes to an auxiliary building outside the reactor. The hydrogen and radioactive gas in the water is then pumped back into the containment building, where devices call "recombiners" remove the hydrogen and force it to recombine with oxygen to form water.

When the NRC believes that all the hydrogen gas in the reactor has been removed, it will start to bring the reactor pressure down so the fuel rods in the core can be cooled down to a completely safe level.

Depressurizing the reactor while hydrogen remains in the core means the gas can reform into a bubble that poses a potential explosive hazard. It could also serve to block cooling water being pumped in and again overheat the core which is badly damaged. CAPTION: Picture, Jody Crater sports timely T-shirt in Middletown, Pa., near A-accident site. AP