The cleanup of last year's accident at the Three Mile Island nuclear power plant is going so slowly that risks of more problems there are going up instead of down, according to a report yesterday of a special Senate investigation.

The one-year probe, the last of the official Three Mile Island accident studies, also found no evidence of a coverup during the early stages, despite much contradictory testimony about who knew what and when.

"This report is neither a death sentence for nuclear power nor an absolute absolution," said Sen. Gary Hart (D-Colo.), chairman of the Senate subcommittee on nuclear regulation, which conducted the investigation.

Instead, he continued, the report assigns more blame than previous studies did to the utility, the reactor designer, the builder and the Nuclear Regulatory Commission (NRC) and less to the reactor operators who were on duty in the control room.

"It is inappropriate and unfair simply to blame these personnel for the TMI accident," the study said. The underlying causes were training lacks, control-room and plant design, faulty instruments and deficient training and emergency procedure, the report concluded, agreeing in that generalization with the earlier Rogovin report to the NRC and the so-called Kemeny Commission report to the President.

The 423-page report is the first to identify uncertainty as a condition particular to a reactor accident and one, it said, that ought to be recognized as likely to recur in the future.

"If there is prolonged, substantial uncertainty about whether the core of a nuclear power plant is covered (with cooling water), the affected state (government) should promptly consider the need for evacuation," Hart said. "The surrounding community is unlikely to want to wait around to test the accuracy of (confirmatory) calculations."

The task of cleaning up after the March 29, 1979, accident is so enormous that "it can, in fact, be considered a continuation of the accident," the report said.

But there is still no careful, overall plan for the cleanup procedure, "in part because of technical, regulatory, legal and financial uncertainties," the report noted. The NRC developed three schedules and Metropolitan Edison Co., which owns TMI, put another one together, but none has been approved formally so far by the NRC.

Venting of radioactive krypton gas from the reactor building began last weekend, but that is only the first step in what may be a five-year process costing $500 million or more, the study said. Meanwhile, "the likelihood of further accidents accumulates with time," and those most at risk are the plant employes working on the cleanup, it concluded.

Among the possibilites are a restarted chain reaction within the reactor core, which is damaged to an unknown degree now, according to the investigation. "Further deterioration can be assumed," the study said.

More likely, however, is an occasional release of radioactivity like the two that occurred in February. "The hazard to (workers) of accidental overexposure will be present as long as areas of high radiation are widespread," the report said.

The study documented several moments during which various controlroom operators or Nuclear Regulatory Commission officials suspected that the reactor core had become uncovered, a situation that always has been the engineers' worst-case nightmare. However, none of the people involved either pursued their worries fully or managed to communicate them to others.

"At the worst there was confusion and unintentional misinformation, but there was no coverup," Hart told a press conference. He said testimony was often contradictory about what was known when and communicated to whom and that it appeared some of the operators and regulators "just didn't want to believe the worst."