The Nuclear Regulatory Commission staff yesterday announced a three-month freeze on the issurance of operating licenses and construction permits for new nuclear power plants while experts assess the implications of the accident at Three Mile Island.

Harold Denton, director of the NRC's office of nuclear reactor regulation, told the commissioners it would take one month for a special 15-member task force to list the lessons learned from the March 28 near-disaster in Pennsylvania. Another month will be needed to put those lessons into the form of required changes in technology and procedure and to issue them in letters to the nuclear plants, and a third month will involve responses from the plants and a review by the NRC staff.

Denton called this period "suspended animation," noting it will affect four pending operating licenses - including the North Anna II unit of Virginia Electric and Power Co. in Mineral, Va. - and five construction permit applications in a total of six states.

The ultimate impact of the nuclear industry is not yet clear. The utilities involved stand to lose tens of millions of dollars in idled equipment and interest costs during the delay, in addition to spending unknown amounts to make the changes the NRC ultimately requires. At the moment, 94 plants are in some stage of planning or construction stretching to the end of the century.

Earlier, the chairman of a House investigating team reported that in his opinion, Three Mile Island "has proved the extreme vulnerability of nuclear power." Rep. James Weaver (D-Ore.) told a House energy subcommittee that "an accident such as occurred at Three Mile Island not only could happen again, but is likely at any time."

Either the operators on the scene did not know what was going on inside the reactor during the most crucial period, he said, or "they did know and did not tell the truth." If they did not know, he said, "it is a terrible indictment of the entire technology."

Weaver said he was "flabbergasted" to learn during his group's probe that the operators seemed to disregard a temperature reading of 2,400 degrees Fahrenheit inside the reactor and indictions of a sharp rise in pressure, later thought to be a fire or an explosion, in the reactor containment building.

"If they don't know the significance of that," Weaver said, "what do they know? I ask you, what do they know?"

He later asked NRC Chairman Joseph Hendrie if it would be fair to say that "the reactor was out of control and the operators didn't know what was going on inside it." Hendrie responded, "I think that's a fair characterization of the situation" for the first few hours of the March 28 event.

At the hearing, Commissioner James F. Ahearne said he was "in essential agreement" with Weaver's conclusions, which went well beyond the dry recital of facts presented by Weaver's full investigative team. "A sense of complacency had grown up over the years that accidents would never happen," Ahearne said, "but accidents will happen, and the system has to be designed with the assumption that they will take place."

The team report listed four kinds of failures that took place at Three Mile Island:

Design errors that caused operators to receive confusing information or none at all when they needed it.

Equipment malfunctions that worsened the situation.

Procedural errors including incomplete checklists or inadequate foresight in written insructions.

Operator errors in which decisions were made that appeared to contradict the-information available.

"I saw no operator error not closely related to design or equipment error and, therefore, it would be impossible to assume that merely more training could prevent accidents," Weaver said. "The possibilities for error are simply too numerous."

The fundamental question, he said, resolves around "the fact that the nuclear industry has had 25 years [and] billions of dollars . . . and is still brought to its knees by a valve malfunctioning and one or two inappropriate responses."

Weaver asked each of the five nuclear regulatory commissioners whether he favored a moratorium on new nuclear plant licensing and construction, and got a 2-2-1 split. Commissioners Peter Bradford and Victor Gilinsky favored the idea, Gilinsky saying, "It's clear we cannot continue [issuing licenses] on a business-as-usual basis."

Chairman Henrie said he preferred a case-by-case examination to continues, and Commissioner Richard T. Kennedy said "business-as-usual" would automatically take into account any safety issues from Three Mile Island. Ahearne, the swing vote yesterday as in many NCR verdicts, said he wanted to hear more from the NCR staff before making up his mind.

With Hendrie absent because of a respiratory illness at the later NCR meeting, however, the four commissioners gave tacit consent to the NCR staff's three-month-freeze on permits.

The freeze is less a formal decision than a description of NCR staff plans that will just happen to take about three months, Denton stressed to reporters, Commissioner Gilinksy said he would prefer that the commission give clear, formal guidance to the industry on the issue, and he asked the NCR legal staff to draw up several options in the form of resolutions for the commission to consider at its Thursday meeting.

The options, he said, should allow the commission to spell out its intentions inthe issuance of new permits. "It seems to me that no license should be issued without the apprvoal of the commission itself, and we ought to make clear pretty soon what grounds we will use in making those decisions," Gilinsky said in an interview later.

One aspect the commissioners agreed on earlier was their concern that a consultant's warning of possible trouble in Babcock & Wilcox Co. reactors somehow got lost in the NCR bureaucratic shuffle. Carl Michelson, a nuclear engineer on loan to the NCR from the Tennessee Valley Authority, told the subcommittee yesterday he had given a longhand draft of his report in late 1977 to a member of the Advisory Committee on Reactor Safeguards but that to his knowledge it had never penetrated the NCR itself.

The same report, which warned of the misleading pressure indicators that were a major problem at Three Mile Island, went to Babcock & Wilcox and was beiing discussed in an exchange of letters when the March 28 accident occured, Michelson said. CAPTION: Picture, Reps. James Weaver, left, and Morris Udall before hearing on nuclear mishap. AP