The federal government has agreed to open-ended funding of part of the cleanup at the crippled Three Mile Island nuclear power plant--the part that can be included under the "research and development" label, Energy Secretary James B. Edwards said yesterday.

But Edwards rejected any government role in an insurance scheme to cover future atomic accidents, and the overall effect of his announcement appeared to be confusion among the main actors.

Edwards said the government's initial commitment is $123 million over three years.

Nevertheless, nuclear critics quickly condemned any government participation in TMI's cleanup as a Chrysler-style bailout for the nuclear industry. Rep. Alan Ertel (D-Pa.), whose district includes the Three Mile Island plant, called Edwards' announcement "an attempt to confuse people" which really provides no new money and no new commitment.

Gov. Richard Thornburgh, on the other hand, was jubilant, saying Edwards' promise brought the Three Mile Island cleanup saga "light years closer" to an end.

"Our commitment is to complete the basic R&D objectives we have outlined," Edwards told the crowded Senate Energy subcommittee hearing. "The final cost to achieve these objectives will depend on the extent of core damage."

In a letter to the Pennsylvania congressional delegation and to Thornburgh, presidential counselor Edwin Meese III reiterated Edwards' remarks, saying President Reagan would ask Congress for "sufficient funds in future years" to complete the DOE research program.

Total cleanup at the Middletown, Pa., plant is expected to take at least six years and could cost $1 billion. Meese's letter promised funding for five areas: technical aid to clean up the water in the damaged plant's basement; to remove and dispose of nuclear wastes not disposable at commercial sites; to remove and evaluate the damaged reactor core; to develop special tooling for the cleanup; and to complete "other appropriate activities."

A spokesman for Thornburgh said later that the outline "leaves the door open for more substantial involvement if the situation warrants it."

Thornburgh earlier proposed a cost-sharing scheme under which the federal government would shoulder $190 million, with other shares being taken by the electric utility industry, the states of New Jersey and Pennsylvania, and the plant owner, General Public Utilities.

The fund would only cover cleanup, since GPU is nearly bankrupt, and would not pay for restarting the plant, Thornburgh emphasized. The Edison Electric Institute group of utilities has agreed to ask their state regulators for permission to provide $192 million over the next three years.

Edwards said his announcement was "consistent with Gov. Thornburgh's recent proposal," although he said it "would not be appropriate" to commit the government to a dollar figure.

Rep. Ertel, however, said that Edwards had always wanted to provide the research money and that cleanup would not be covered. Ertel has introduced legislation to set up a mandatory insurance program for nuclear utilities to cover damages over $500 million. Funded by utility premiums, the plan would also pay for about $450 million of the TMI cleanup.

Edwards, however, rejected that plan. "Private efforts are under way to establish more adequate levels of private property insurance. We believe these efforts will be successful, making a federally mandated program unnecessary," he said.

Sen. Gary Hart (D-Colo.), calling himself "the only truly free-market advocate in the room" regarding nuclear power, said Edwards' plan would set a precedent of helping future troubled reactor-owning utilities. He said the $123 million had no visible relation to the value of any research that might come out of TMI. Manufacturers and vendors of nuclear equipment should shoulder some of the cleanup costs, he added.

Committee Chairman James A. McClure (R-Idaho) said ratepayers nationwide will eventually pay the costs of cleaning up TMI, either directly or in the form of higher "uncertainty premiums" their electric utilities would be paying.