THROUGH a series of legislative, regulatory, and commercial decisions, the United States is establishing the course of the next 40 years for its nuclear power industry, say energy experts.Nuclear power remains an emotional subject for many people. Restoring battered public confidence is essential, say boosters and critics of the industry. That is why the shutdowns of the Yankee Rowe and Maine Yankee reactors last week were significant steps forward in restoring that confidence, these experts say. The US still has a long way to go in developing an overall energy policy, says David Rossin, former assistant secretary of energy for nuclear energy in the Reagan administration and president-elect of the American Nuclear Society. Nuclear power must be seen as only one of the elements of a national energy strategy, he says. Merely providing better information does not mean more nuclear plants should be or will be built, or that older plants will be relicensed, says Robert Pollard, a nuclear safety engineer and a former Nuclear Regulatory Commission (NRC) safety official. Nuclear power generates 20 percent of the nation's electricity; in nine states, the industry provides one-third of the electricity used. Of the 110 reactors now in operation, 66 will need their operating licenses renewed by the NRC in the early part of the next century. Given the long lead-time in the licensing process - at least 10 years - and the fact that no new nuclear plant has been ordered since 1974, industry sources say the US must squeeze a longer life out of the existing plants. A commercially viable and publicly perceived safe way to build and operate new plants is also needed, they say. How many old plants will actually apply for license extensions? The NRC's best estimate is 70 percent. There are two separate safety issues in the relicensing process, says Mr. Pollard, who also represents the Union of Concerned Scientists on nuclear issues. Are the plants as safe now as when they were first built? Should the plants that apply for license renewal have to meet new, and tougher requirements established since their original license was granted? The latter question addresses public confidence, says Jim Sill, spokesman for Enertech, a privately held company that makes safety equipment for nuclear power plants. The greatest obstacle in building a new plant is uncertainty in "a definite time frame with set costs," says Steve Unglesbee, spokesman for the US Council on Energy Awareness, the nuclear industry's lobbyist group. "When 30 to 40 percent of a utility's construction cost is interest," the delay in a licensing process affects the decision to build or not to build. Construction of a new plant should not begin until all the clearances are in place, and with the public having had ample time to comment on and review the process, he says. "We have to have a fundamentally different reactor design that can be simply explained to the American people in terms they understand, and that they understand to be safe," says Pollard. Mr. Rossin agrees that the absence of a generic design for a reactor approved by the NRC is one of the major obstacles to new nuclear power plants. Just as important though, he says, is the NRC's establishing a licensing process that will stand up to court challenges and political pressures. Still to be resolved in any nuclear power equation is the disposal of radioactive waste. The industry sees the technical side of the waste matter as resolved. The political side of the problem is still an open question, says Mr. Sill. Mr. Unglesbee cites successful waste reprocessing and storage programs in England and France, as well as in the United States. Opponents of nuclear power contend the public will remain opposed to nuclear power until there is a resolution of the disposal problem. The Union of Concerned Scientists, a group that is critical of the power industry's interpretation of data, does not think there is a permanent solution to nuclear-waste disposal at present.