A group of scientists charged yesterday that the Nuclear Regulatory Commission suppressed criticism by its own officials that cast doubt on the validity of a 1975 report defending the safety of nuclear power plants.

The NRC's so-called Rasmussen Report found that the chances of a major nuclear accident were small and has been used as a primary defense against charges that the plants - 63 of which are in operation now in the United States - are unsafe.

The latest attack on the validity of the report came from one of its most persistent critics, the Union of Concerned Scientists, which based its findings on a review of 50,000 pages of NRC working papers obtained under the Freedom of Information Act.

The union, a 30,000-member group based in Cambridge, Mass., charged that the NRC and its predecessor agency, the Atomic Energy Commission, make a "concerted . . . effort to bias the study and to conceal evidence of deficiencies in the study's methods, assumptions and data base."

It said the AEC-NRC leaned heavily on the nuclear power industry for help in making the study and ignored criticisms raised by 12 reviewers of a preliminary draft, including senior AEC officials and independent consultants.

Although some of the reviewers praised the report in general, they criticized specific points, including the basis it used to assess the probability of accidents. One called it "gibberish." Another said the report contained "deficiencies and inconsistencies" of such magnitude that a major redrafting was warranted.

Saul Levine, director of nuclear regulatory research for NRC, said "a great deal of it [the criticism] was used in modifying the report" before it came out in final form. "Where we agreed with the opinions, we used them," said Levine, who participated in preparing the NRC report.

The union charged that the criticisms were ignored and suppressed until the union requested the working papers. Levine said the information was made public after release was requested by the union.