The Department of Energy is 20 percent too low in its calculations of the amount of uranium ore needed to keep a nuclear power reactor operating, the General Accounting Office revealed yesterday.

J Dexter Peach, Director of GAO's energy and minerals division, said in an April 10 letter to Energy Secretary James R. Schlesinger that DOE has been calculating uranium ore demand at the mill and not at the mine level.

"No correction is being made for milling losses," Peach's letter said.

DOE now estimates that each reactor generating 1,000 megawatts of electricity will use 5,500 tons of "yellowcake," or uranium oxide, extracted from the ore, over its 30-year lifetime. If milling losses are included and asssumed to be steady at their current 8.2 percent of yellowcake output, Peach said, the yellowcake demand should be 6,000 tons.

However, he added, the quality of uranium ore is steadily declining and the percentage of mill losses is rising. If today the industry extracts 3.1 pounds of uranium oxide from every ton of ore, that is one-third less than in 1966. "It appears quite likely that the grade of mined uranium ore will continue to decline," he said. "As this continue to declines, mill losses would continue to grow and the discrepancy between demand at the mill and demand at the mine will increase."

Mill losses will probably reach 17 percent by the 1990's, Peach continued, which means that currently operating reactors will use 6,600 tons of ore over their lifetimes, 20 percent more than the current DOE estimate.

"Such a difference could have a significant impact on any assesment of the adequacy of the uranium resource base," Peach's letter said.

He recommended that future estimates he based on demand at the mine and not at the mill, and that past analyses be redone to see if their conclusions are affected.

Spokesmen for both the Department of Energy and the Atomic Industrial Forum, the nuclear industry trade association, declined to comment until they had seen copies of the letter. A DOE official, however, said current reserve estimates showed enough U.S. uranium available to fuel 440 recators of 1,000 megawatts each over 30 years.

Other DOE figures show a known reserver of 370,000tons of yellowcakes available at $15 per pound and climbing to 890,000 tons that would be available at $50 per pound. Possible and speculative reserves at those prices could reach 1.6 million and 4.4 million ton respectively, the DOE figures showed, depending on other energy factors.