A Ralph Nader organization reported yesterday that more than 4,000 nuclear reactor mishaps -- 140 of major significance -- occurred during 1981 and said it is "just a matter of time" before another serious accident.

In a 32-page report, Public Citizen's Critical Mass Energy Project said the day-to-day operation of nuclear reactors "is plagued by equipment failures, human errors and design defects."

"The multitude of serious safety problems is a sure sign that without major changes, it is just a matter of time before another serious nuclear accident occurs," the report said.

The Atomic Energy Forum, the major trade association of the nuclear power industry, responded by saying the industry has an "exemplary safety record." It criticized the Nader group for using reports to the Nuclear Regulatory Commission as the basis of its study.

"To use these raw reports for the purpose of alleging laxness is a gross distortion of reality," an industry spokesman said.

Carlyle Michelson, director of the NRC's Office for Analysis and Evaluation of Operational Data, said he did not agree with some of the study's conclusions, but he called it a "significant document."

"They're grinding a particular ax, so they slant the situation somewhat," Michelson said. "But they're quite right on the significance of some of these events. Clearly if you build more and more complex machines, it's obvious you're increasing the odds of a serious mishap. With more cars on the road, there's more chance of an accident."

The Nader organization said its report was based on data, much obtained through the Freedom of Information Act, that included secret codes, lists and documents.

"The nuclear industry is in trouble," the report concluded. "It is doubtfdul whether even the pro-nuclear Reagan administration can help this dying industry, and it's clear that it should not."

The Brunswick 2 reactor of Carolina Power & Light at Southport, N.C., and Sequoyah 1 reactor of the Tennessee Valley Authority at Daisy, Tenn., topped a list of what the report called the worst reactors, based on the number of "especially significant mishaps," each with eight.

The McGuire 1 reactor of Duke Power Co., at Cornelius, N.C., led an overall listing of mishaps during the year with 220 -- far ahead of Brunswick 2 at 158.

Richard Udell, who wrote the Public Citizen's report, said a mishap could range from a minor equipment problem to loss of coolant, while especially significant mishaps were more serious in terms of safety.

He also said that worker exposure to radiation reached an all-time high of 54,555 person rems (dosages producing an effect equal to a roentgen of X-ray radiation) during 1981, with more than 80,000 employes in the nuclear power industry exposed to radiation.

The report also said nuclear plants "are unusually vulnerable to terrorist attacks and sabotage" and listed 23 threats to plants during 1981, many of them bomb threats.