The governor of Pennsylvania, who wrestled with problems of public safety after the nuclear accident at Three Mile Island in late March, and two leading scientists have gloomy assessments of the future of nuclear power in the wake of the accident.

Republican Gov. Richard Thornburgh told the nation's editors he had not yet made up his mind whether the country should continue to build nuclear power plants, but said his "own level of skepticism has been raised considerably."

Thornburgh predicted that the nuclear issue would be important in the 1980 presidential election as well as the "1982 gubernatorial election in Pennsylvania."

Norman C. Rasmussen, who several years ago reported to the Nuclear Regulatory Commission that the chance of a major meltdown at a nuclear plant was slim, told the American Society of Newspaper Editors that nuclear power is probably less risky than coal burning and is the only other energy option to oil for the rest of the century.

But Rasmussen, a professor at the Massachusetts Institute of Technology, conceded that the "unique nature" of the risk involved in nuclear energy may make it unacceptable to the nation.

Robert D. Pollard, an antinuclear activist with the union of Concerned Scientists, said the only question is "how fast we should do away with nuclear power as an energy source."

Pollard, who resigned from the NRC three years ago, said that while it is technically possible to make nuclear plants safe enough, from a practical point of view it cannot be done, because to make plants safe enough would cost so much money that nuclear power would "be rejected on an economic basis."

He said that industry and government regulators recognize this and that the NRC and its predecessor, the Atomic Energy Commission, have never taken important safety steps because they would be too expensive.

Rasmussen said that the Three Mile Island incident will undoubtedly lead to corrections in plant design and operating procedures that will make nuclear plants safer, but he disagreed with Pollard's contention that there were dozens of unsafe designs and practices at each plant today.

Rasmussen said that the nation either must find substitutes for oil or do without energy. He said the economic consequences of declining power availability would be disastrous as well, especially for the poorer segment fo the population.

"Solar power, geothermal power or woodburning" may eventually substitute for oil, Rasmussen said, "but not in the short term." He said the nation is decades away from commercially viable solar or geothermal power generation.

"Some say we should switch entirely to coal," the nuclear engineer said, but he noted that even with the most sophisticated clean-up devices, coal burning plants still toss 8 million tons of carbon dioxide into the atmosphere each year.

Many scientists say the carbon dioxide warms the earth's climate, which could lead to a melting of the polar icecap raising the oceanlevels by several hundred feet.

For coastal cities, he said, that would be "no less serious than a nuclear event."

Pollard said that the "engineering feats necessary to make coal mines safe and clean-up coal emissions are far simpler tasks than trying to make nuclear plants safe enough."

Rasmussen said that all alternatives carry risks. He conceded that the nation might not want to take the kinds of risks associated with nuclear power, but said that he felt the risks involved were small.

Pollard said that the risks involved in a nuclear accident are "far different from any other accident the public is familiar with." He agreed that the nation must "weigh the cost and benefits, but I don't believe we have the opportunity to understand what the risks and costs are" in nuclear power.