CONGRESS SHOULD speedily grant the request of the presidential commission on Three Mile Island for sweeping investigative powers. The need for a through report is obvious. So is its need for the tools it has requested - the right to take testimony under oath, to subpoena witnesses and to seek immunity for witnesses from criminl prosecution.

While congressional investigations into the accident are already under way, it is to this commission that the public will probably look for a definitive report on what happened at Three Mile Island - and why. To attain credibility, the commission must be able to ensure that everyone who knows anything about Three Mile Island tells it the truth. Even in its preliminary work it has encountered witnesses whose stories directly conflict. That is why it will need to take testimony under oath, with a perjury charge the price of falsehood; and that is why it may need to clear away, through immunity grants, objections witnesses may have to particular questions. While no one anticipates the development of criminal charges from the accident, the commission should have in hand all the legal powers it needs before it gets further into its work.

The questions the commission is going to attempt to answer, according to Chairman John Kemeny, are broad - perhaps too broad. In addition to looking into the causes of this particular accident, the dangers to public health it created, and the quality of information about it that reached the public, the commission, Mr. Kemeny says, will also explore the psychological impact the accident had on the public and the question of nuclear safety "in the broadest sense."

The psychological impact of this incident is an interesting question. But it does not go to the central purpose of this inquiry, which is to learn whatever can be learned from Three Mile Island about the future safety or danger of nuclear power plants. And if the commission takes on too large a part of the ultimate question - nuclear safety "in the broadest sense" - it will risk undermining its fact-finding mission. Whatever the commission recommends will have a substantial impact on the future of nuclear power, but it is not, and should not be, the vehicle for an inquiry beyond this one episode.