President Carter visited Three Mile Island nuclear power plant yesterday as officials there reported that though the reactor core was cooling, serious danger remained from the gas bubble that has impeded efforts to cool the reactor to the safe point.

At the end of his tour of the plant near Harrisburg, Pa., Carter promised "a thorough inquiry" into the causes of the nation's worst nuclear power accident and pledged that he will be "personally responsible for thoroughly informing the American people about this particular incident and the status of nuclear safety in the future."

Officials delayed evacuation moves from the affected area until further determination of the danger of a gas explosion, a meltdown of the nuclear core or leakage of radiation into the atmosphere. Current evacuation plans cover a 20-mile radius around the plant with a population of about 630,000.

The president assured residents of the surrounding pennsylvania communities that radiation levels near the plant now are "quite safe," but cautioned them that officials at the plant face critical decisions in the next few days on measures to dissipate the bubble and stabilize the reactor core.

This was an implicit reference to the possibility that Pennsylvania Gov. Richard Thornburgh may order an evacuation.

It remained unclear last night. When, or if, an evacuation would be ordered.

However, in a move that indicated that evacuation was not imminent, Thornburgh said last night he is asking that all state offices continue to conduct business as usual, beginning this morning. He said he recognizes that some families who left the area over the weekend because of the accident may have some problems getting back to the Harrisburg area. Because of that he said that personal and vacation leaves would be granted to state workers and that those leaves would be charged to all absentees. State employes who are pregnant or who have preschool children will be excused with no loss of vacation time, Thornburgh said.

The president, accompanied by his wife, Rosalynn, flew by helicopter early yesterday afternoon from the White House to Middletown, Pa., three miles north of the nuclear plant. Carter's journey, said White House press secretary Jody Powell, was meant as a display of the president's "personal concern" for the people in the area and to obtain "firsthand information" about the situation.

Mrs. Carter said she went along "because I was interested."

At the end of the two-hour tour around the area, the president stopped at the Middletown Town Hall where he issued a statement to waiting reporters.

"The primary and overriding concern for all of us is the health and the safety of the people of this entire area," he said.

Carter said he had been assured by officials at the plant, whom he called "highly quaified," that the reactor core "is indeed stable." But he warned that in the next few days "important decisions" would be made on how to deal with the gas bubble and that Thronburgh may "ask you and others in this area to take appropriate action to ensure your safety.

"If he does, I want to urge that these instructions be carried out calmly and exactly, as they have been in the past few days."

Powell, warning against "jumping to conclusions," said Carter's remarks were written in advance and shown to Thornburgh and were not meant to suggest an imminent evacuation order.

Consumer advocate Ralph Nader called for immediate evacuation of residents within a 30-mile radius of the plant, citing the possibility of a hydrogen explosion that could lead to a melting of the core and a large release of radioactive gases.

One good turn yesterday was the cooling of the damaged reactor fuel rods. The highest temperatures recorded yesterday inside the reactor were 495 degrees, down from the 700-degree temperatures measured earlier.

Four "bundles" of fuel rods were still hotter than 400 degrees, probably because they are the bundles that suffered the most damage. There are roughly 225 rods in each bundle, so that 900 of the 36,000 rods in the reactor are very badly damaged.

One reason the rods have begun to cool is that the radioactivity generated in them has begun to decay. Another reason is that the borated water flooded into the reactor on Saturday may have absorned enough of the neutrons still coming from the uranium fuel to slow down the generation of heat inside the fuel rods.

Borates (salts of boron) in water "soak up" excess neutrons in any fissioning uranium.

Engineers want to get the temperature of the fuel rods down below 400 degrees, in part so they can keep the cooling water from boiling. Getting the heat down also lowers the risk that the damaged fuel rods will pass on the excess heat to the undamaged fuel rods. The damaged rods are the source of the radioactive gas leaks.

The bubble of gas surrounding at least part of the core is still in the domw of the reactor vessel. It is still the chief concern of engineers because it is blocking the cooling water from reaching parts of the overheated reactor. Depending on how much hydrogen and oxygen are in the bubble, it could also explode, break through the reactor vessel and still radioactivity over the inside of the concrete containment sheilding the reactor from the environment. Under no circumstances would this explosion, if it were to occur, be equivalent to that of an atomic bomb. It would be a conventional chemical explosion equivalent to a few tons of TNT.

There is concern about how much hydrogen is in the containment building as well. One explosion of hydrogen on the day after the original mishap rocked the building though it did not break through any part of the four-foot-thick walls. The latest measurement of the hydrogen inside the containment was that it was 1.7 percent of whatever gases were there, a little less than half the concentration needed to ignite the gas.

One plan under consideration to break the bubble is to lower the pressure in the reactor, which will expand the bubble to where it can be siphoned off through the pipe carrying the coolant water to the outside.

This plan could involve exposing the fuel rods for a short time, which could send the temperatures of some of the rods back up. It might also serve to block still more coolant from entering the reactor, which again could raise the heat inside the rods.

Another plan is to do just the opposite, to raise the pressure in the hop that the bubble will compress and disolve itself in the water. This plan is apparently a little more risky than the first ut may be easier to perform.

Computers at Brookhaven National Laboratory on Long Island, N.Y., and at the Oak Ridge National Laboratory in Tennessee have been running simulations of each plan to see which might work better with the most safety. All the plans discussed to break the bubble involve some risk of a new accident, since they involve changing pressures and temperatures inside the reactor.

Engineers last night were hoping to activate two devices inside the containment called "recombiners," which provide for controlled burning of the hydrogen inside the containment to prevent an explosion. The hope was that the two recombiners would be working by midnight.

The waste water begin pumped from the reactor was still venting radioactive gases into the atmosphere, a situation the Nuclear Regulatory Commission took two steps last night to stop. It ordered a temporary pipe laid to carry the radioactive gases from the sumps where the water was stored into the concrete containment.

A more permanent step was the ordering of four giant Calgon filters, each with 20,000 pounds of charcoal, to trap the gases before they're released to the atmosphere. New storage tanks have also been ordered to the scene to contain the radioactive waste water since the tanks already built at the site are not large enough to hold all the water being containated.

Meanwhile, California Gov. Edmund G. Brown Jr. sent a telegram to the NRC asking for a "precautionary and temporary" closing of the Ranco Seco nuclear plant outside Sacramento. Built by Babcock and Wilcox, the Rancho Seco plant could be described as a sister plant to Three Mile Island.

Speaking on "Issues and Answers" (ABC, WJLA), Rep. Morris K. Udall (D-Ariz.), chairman of the House sub-committee on energy and the environment, suggested "once the dust settles" the NRC study the safety record and performance of the eight Babcock and Wilcox plants now in operation.

"I'm not prepared tomorrow morning to charge out and say all nuclear plants should be shut down," Udall said. "But I think I'd start out at these Babcock and Wilcox plants and find out if the Three Mile Island story tells us they have the same problems and maybe close them down until modifications can be made.

"There are some nuclear plants that have very good safety records," Udall went on. "Others have had a history of problems and seem to be trying to tell us something."

One thing Udall said he was concerened about was the confusing information coming out of Three Mile Island. An aide to Udall said he'd been told different things by the NRC about the gas bubble, the amount of radioactivity released and the damage to the core.

The FBI said last night that it had looked into the possibility of sabotage at Three Mile Island and found "no evidence" of any.

Carter decided to visit Three Mile Island Saturday. If the journey was meant to boost morale in the area, it apparently succeeded. About 1,000 people lined the road from Middle-town to the power plant and waved as Carter's motorcade passed.

Before visiting the plant, which has been strictly off limits to all but key personnel, the president spent 40 minutes in Middletown beign briefed on the situation by Dr. Harold Denton, chief safety officer of the Nuclear Regulatory Commission.

At the plant, Carter and his party-which numbered 20 people-donned yellow plastic shoecovers and were each issued a radiation dosimeter, an instrument that measures exposure to radiation.

At the end of the visit, preliminary readings of the instruments showed that members of the presidential party may have been exposed to about a quarter of the dose of average radiation from a chest X-Ray.