A new study of the operating record of nuclear reprocessing plants has concluded that these facilities--which take used fuel from atomic power stations, and separate out plutonium suitable for use in nuclear weapons--are shut down so much of the time because of accidents and technical problems that they are "not commercially viable."

The report by Dr. Arjun Makhijani, a nuclear engineer, found that the average life of commercial reprocessing plants around the world has been six years, and that they have operated at "from 10 percent to about 35 percent" of capacity--far below "the 80 percent which is generally assumed in calculations of the economics of reprocessing."

The study, which will be made public today, seems certain to become a new element in the debate over the Reagan administration's efforts to revive and bring into commercial operation a $300 million nuclear reprocessing plant in Barnwell, S.C.

In an effort to encourage private investors to put up the estimated $400 million still needed to put the Barnwell plant into operation, the Energy Department is recommending to the White House that the U.S. government agree to purchase all plutonium produced by the facility and insure new investors against any change in policy that would prevent its operation.

The Energy Department, which was provided on Thursday with a copy of the new report on reprocessing by the Washington-based Health & Energy Learning Project, yesterday declined to comment on its conclusions.

The study by Makhijani, a private consultant who holds a doctorate in nuclear fusion from the University of California at Berkeley, examines the operating records of the six plants that have reprocessed spent fuel from civilian nuclear power plants to date: La Hague, France; Windscale, England; Karlsruhe, West Germany; Mol, Belgium; Tokai Mura, Japan, and West Valley in upstate New York.

The two that went into service most recently--La Hague, which started up in 1976, and Tokai Mura, which began reprocessing fuel in 1981--have an operating record even poorer than some of the earlier plants, the report said.

"The problems of the newer plants have been at least as severe as those of the first one at West Valley" which was shut down permanently in 1972, the report said.

The La Hague plant, it said, has operated at "about 10 percent of rated capacity. Even so, it has had, on average, one serious accident about every four months from January, 1980, to June, 1982.

"These accidents include spills of large quantities of plutonium a highly toxic substance , a fire in a radioactive storage trench, and an April, 1980, total power failure which stopped the cooling of the high-level waste tanks and almost resulted in their blowing up," the report said. "Such an explosion would have contaminated vast areas in France and southern England with high level radioactive waste."

The newest facility, in Tokai Mura, began operation in 1981 and performed at about 15 percent of rated capacity in its first year, the report said. "It has already had several breakdowns, accidents and severe worker exposures to radioactivity," the study reported. The operators of the plant announced on April 15 that as the result of an accident in which radioactive matter touched the heating steam of the fuel rod melting pool, Tokai Mura would be closed "for about one year."

Makhijani said he had been unable to find an assessment of the cost of reprocessing "based on actual experience.

"On the contrary, the estimates of cost that are done are based on assumptions that have nothing to do with the operating record of reprocessing factories," he said. "These factories are assumed to work near rated capacity for 20 to 30 years."