President Carter's energy plan means a "nearly inevitable" reliance on nuclear power and the building of reactors "at a rate far exceeding the pace achieved in the last few years," a leading enviromentalist said yesterday.

Barry Commoner, a biologist at Washington University in St. Louis, said Carter's proposal, "despite claims to the contrary, does very litte" to boost use of solar power for energy.

Commoner, a long-time opponent of nuclear power and a proponent of solar energy, said that under Carter's plan "nuclear power plants would represent a mahor part - perhaps one -half - of the nation's generating capacity" by the end of the century.

Asked to comment, Robert W. Fri, head of the Energy Research and Development Administration, said "Commoner's characterization is unbalanced." The President's plan stresses energy conservation, increased use of coal, and steps to ensure the safety of nuclear reactors, and it gives "a substantial increase to solar energy research," he said.

Members of the President's energy policy staff, headed by James R. Schlesinger Jr., have made a concerted effort over the last few months to elicit environmentalists' support for Carter's energy initiatives. Thus far, Carter's energy plan has drawn little fire from environmental groups.

Commoner told a National League of Cities energy task force yesterday that the Carter plan would mandate constructing 20 nuclear powere plants a year over the next 14 to 15 years.

A member of the Carter energy staff said Commoner's "comments are off base . . . our best estimate is that the building rate will be 10 to 12 plants per year by 1985." Afterward, depending upon success in meeting Carter's goals on conservation and coal use, the number of additional nuclear power plants would decline, energy officials said.

Administration officials declined to commnet directly on Commoner's projection's for nuclear power's share of U.S. generating capacity by the end of the century. "That is a possibility we are trying to avoid," said Gus Speth,a member of the Council for Enviromental Quality.

"I see a very cautious approach to nuclear power coming out of the Carter plan, yet an unmistakeable desire to continue to rely on it," Speth said.

He and other administration officials said Carter's nuclear policy is consistent with his campaign promise to use nuclear reactors as "a last resort." These officials cited the President's decision to cut funding for the $2 billion Clinch River plutonium breeder reator project near Oak Ridge, Tenn.

Commoner said that under Carter's energy plan, "solar energy would constitute only 1.5 cent of the additional energy supplies," to meet added energy demand by 1985.

The United State faces a choice between two types of renewable energy sources - solar energy and nuclear technology including the breeder reactor which produces more fuel than it burns, Commoner said. "By implementing the plan, we would have made an unwitting choice - which may already be anticipated in the thinking on its authors - to pursue the nuclear route," he said.

Commoner also criticized a Carter proposal to streamline the nuclear licensing process, which now takes up to six years. "If you cut down on hearings, you will dwonon environmental and safety standards," he said.

According to the Nuclear Regulatory Commission, which licenses nuclear power plants, 63 nuclear reactors are operating, 71 plants are under construction, and an additional 66 reactors are awaiting construction licenses. The 63 reactors provide 8.7 per cent of the nation's electrical generating capacity.

The Atomic Industrial Forum, a nuclear industry trade association, projects that by 1985 the United States will have 175 light water reactors in operation, which could generate 2.1 per cent of the U.S. electrical supply.

"If we go the nuclear route," Commoner said, "we will have out country powered by huge nuclear plants protected like military installations."

"What is called for its a new beginning - not a modification of Carter's plan" he said.