As they begin their investigations of the nuclear accident at Three Mile Island, Sen. Gary Hart (D-Colo.) and Rep. Morris K. Udall (D-Ariz.) are taking what appear to be strikingly different approaches.

Hart says the Senate investigation will emphasize the accident itself, and his Senate subcommittee will explore the role of nuclear power in the United States as a secondary objective. On the House side of Capitol Hill, Udall's investigation will focus on where nuclear power fits into the nation's energy policy, and Three Mile Island will serve more as a backdrop to the study than anything else.

"Mo [Udall] has indicated that his investigative part is going to be minimal and his policy study is going to be the major part," Hart said in an interview. "We're going to put heavy emphasis on the investigation because it's the test case. Ten years from now, people will be gauging their views on nuclear power by their position on Three Mile Island."

Behind the different approaches to the investigations lie what appear to be different attitudes about the role of nuclear power in the nation. Hart says he sees no alternative to the use of the nuclear power for the next 25 years. Udall questions whether nuclear power has any real future in America.

"I'm not an enthusiast for nuclear power," Udall said in a recent interview. "I have serious questions about how much further into it we ought to go, and you really have to reckon with the fact that large numbers of American people have now given up on it. They now think that maybe it poses so many dangers they aren't willing to have any more of it."

Hart looks at nuclear's future a little differently: "Solar can't do it and I'm a solar advocate. Conservation can't do it and I'm a conservation advoctate. I think the antinuclear people have an obligation to tell us where the energy's going to come from. Where are you going to take up the slack? Unless the American people are willing to drastically alter their lifestyle, you can't close these reactors down."

Despite their differences, Hart and Udall share the belief that the nation has waited too long to agree to a plan for disposing of the radioactive wastes that build up in nuclear power plants. Udall calls that problem the "Achilles heel" of nuclear energy. Hart says it's the overriding issue facing nuclear power for the next 10 years.

"I've come to the conclusion that we're going to force the technology by using a legal ax," Hart said. "That's my proposal to shut everything down in 1985 unless the federal government has got a viable proposal to dispose of the waste."

Udall says he polled the governors of all 50 states and not one of them wants the wastes buried in his or her state. Hart says that one way to get around that is to deny the states the right to ban wastes from their states.

"The way you solve it politically and legally is just don't give the [the states] the authority to veto it," Hart said. "I think what we've got to say is that each state shall work out some mathematical formula that says each state shall be responsible for storing the wastes it generates."

Another issue Hart and Udall agree on the evasive issue of what Hart called the "hidden costs of nuclear power." Hart said he meant the costs of transporting and burying the wastes and the costs of decommissioning nuclear power plants at the end of their 30-year lifetimes.

"Part of the fallacy of nuclear economics has been that we don't calculate these costs, don't pass them on to consumers, don't amortize them," Hart said. "I've come to the conclusion that these costs have got to be factored into the costs of producing nuclear power."

Hart and Udall also agreed that nuclear safety standards have to be tightened, mostly through strengthening the role of the Nuclear Regulatory Commission. They both advocate on-site inspectors, mandatory evacuation plans, tougher penalties for rule violations and stiffer siting regulations.

"My own view is that there will be no more reactors after the 165 built or being built that will be sited within so many miles of a city," Hart said. "I know this will add to transmission costs but I'm convinced it's a significant way to decrease the risk." CAPTION: Picture 1, Udall wants to explore nuclear power's future . . .; Picture 2, . . . as Hart emphasizes the ppast: Three Mile Island