The Nuclear Regulartory Commission staff yesterday tried to soften the impact of an earlier report that said operators could have prevented the accident at Three Mile Island if they had followed existing procedures.

In a memo responding to questions raised by Rep. Morris K. Udall (D-Ariz.), NRC inspection and enforcement chief Victor Stello Jr. said "undue emphasis" had been placed on operator deficiencies discussed in an August report from his office.

That report was "limited in its scope" and should be seen in the context of other studies that spread the blame over several factors, including operator actions, design, equipment perfromance and information flow, Stello's memo said.

The memo also said that "some statements and reports have suggested, contrary to our intent, that inappropriate operator actions were essentially the sole cause" of the Three Mile Island accident last March. Stello said in an interview that he meant industry and press reaction to his August report, which was called NUREG 0600. "Some of these statements have placed undue emphasis on the operator deficiencies discussed in NUREG 0600," the memo said.

Press reports at the time quoted Stello as saying that: "Clearly the accident was preventable . . . if basic equipment and procedures that were there had been followed, allowed to function or be carried out as planned." He then said the "mind set" trained into the reactor operators had caused them to overlook the proper procedures.

John H. MacMillan, vice president for nuclear power generation of the Babcock and Wilcox Co., which built Three Mile Island, was quoted in the company's September Nuclear Newsletter as saying the results of Stello's report were "very gratifying. They show that the plant's equipment, in conjunction with emergency procedures and accident analyses, were adequate to have prevented the serious consequences of the accident."

But Stello's response to Udall said that was not what he meant at all. "It is most likely that the cause of the accident will be a combination of inadequacies" by "designers, reviewers, builders, vendors and regulatory agencies," the memo said. "There is also consensus that the operators' actions were the result of inadequacies in equipment performance, transient and accident analyses, operator training and performance, equipment and system design and information flow."

In an interview, Stello insisted that the memo in no way represented a change in his position. Operators violated clear procedures before the event that could have kept it from happening at all, he said. But he agreed that correct procedures that were ignored during the event "were not as clear as they should have been."

A member of Udall's Interior Committee staff was not satisfied. "The question was whether operators acted reasonably in the light of conditions that confronted them that morning," he said. "We infer from the NRC's response that they did." This, he said, "reflects a change in the NRC's position as we interpreted it from NUREG 0600."