On the day of the accident at Three Mile Island, the Nuclear Regulatory Commission was left unaware of two critical pieces of information that would have eliminated much of the confusion, indecision and helplessness that marked the first three days there.

The staff of the Senate subcommittee investigating the accident submitted evidence yesterday that the NRC was never told on March 28 that temperatures inside the nuclear core had climbed to more than 2,500 degrees and that there had been a hydrogen explosion inside the containment building, implying that the overheated uranium fuel was badly damaged and no longer covered by cooling water.

"The key missing piece of information was the pressure spike [of a hydrogen explosion] that indicated major fuel damage at 1:50 p.m. on Wednesday, March 28," Paul L. Laventhol, staff director, told the Senate subcommittee on nuclear regulation."It was never communicated to the Nuclear Regulatory Commission in Bethesda."

The way the subcommittee staff pieced it together from testimony under oath, the hydrogen explosion produced a "thump" that was registered on a strip chart inside the control room at TMI at 31 pounds per square inch.

"This is a very large pressure spike, capable of doing significant damage to normal structures," the staff report on the incident said. "The plant is capable of withstanding pressures of more than 60 pounds per square inch."

At the time the pressure spike was recorded, spray pumps came on automatically to wash down the walls inside the containment. The staff report says that Brian Mehler, shift supervisor for Metropolitan Edison Co. inside the control room, explained this to an NRC inspector who was also inside the control room.

"He asked me why I was concerned because the spray pumps were running," Mehler told the subcommittee staff. "I told him they would only start at 30 pounds [of pressure]. I walked over to the chart and looked at it; 31 [pounds], it was straight up . . . I showed it to him. He didn't say anything. He didn't know what was going on. All he did was write down what we told him."

Another NRC inspector on site at Three Mile Island, James Higgins, told the subcommittee staff his first knowledge of the spike came on Friday morning, March 30, though he admitted he might have looked at the chart and missed the spike and the significance of it.

"The spike could have been there and I would not have considered that significant," said Higgins, who added that he was preoccupied with informing NRC headquarters about radiation escapes. "It was always the type of thing where I had a backlog of about 40 questions I was supposed to answer for the people in Washington."

Of almost equal importance, the subcommittee staff said, the Metropolitan Edison Co. supervisors never told the NRC that temperature readings inside the core had reached more than 2,500 degrees on March 28, a piece of information that also would have indicated the damage to the core was extensive and serious.

The staff report quoted Gary M. Miller, station manager at TMI, as testifying under oath that he felt he did not have to pass along core temperature readings to the NRC, in part because he was too busy and in part because NRC regulations did not require it.

"He conveyed to the investigation staff the impression that on the morning of March 28, he felt that relaying plant parameters to the NRC was secondary to coping with ongoing reactor problems," the Senate staff report said. "He said that he was following Met Ed's emergency plan which called for communicating radiation information."

The report concluded that if the temperature readings of 2,500 degrees and the hydrogen explosion had been reported to the NRC on March 28, it would have been more quickly aware of the damage to the core and might have moved faster to strengthen communications with Three Mile Island, to get more people there, and to talk about possible evacuation.

"Although major damage to the nuclear core occurred during Wednesday, the executive management team of NRC did not formally discuss evacuation of the area around the plant on that day," the staff report concluded. "It was not until Friday that the senior staff turned serious attention to the issue of evacuation."