The Nuclear Regulatory Commission promised yesterday a thorough housecleaning and evaluation of all its rules and procedures in the wake of the Three Mile Island nuclear accident, admitting for the first time that its regulatory procedures "have somehow been inadequate."

Testifying before a Senate nuclear subcommittee, NRC Chairman Joseph M. Hendrie added, "We cannot tolerate accidents of this kind, and we must take whatever steps are necessary to prevent them."

The commission, he said, "must find out where our inspection and enforcement of safety-related operating requirements, our design standards and our reviews . . . have somehow been inadequate to precent the Three Mile Island accident."

He cautioned, however, that "much the greater part" of the factors involved in the incident "are associated with operating errors rather than with plant design." For that reason, he continued, the NRC does not feel it necessary to close eight other plants designed as Three Mile Island was by the engineering firm of Babcock & Wilcox.

Commissioners Victor Gilinsky and Peter A. Bradford, however, argued that it is still too early to allot blame between operator and designer. "We're not even satisfied with the chronology yet, " Bradford said.

As the commission testified, schools were reopening in Harrisburg, Pa., for the first time in 12 days. Residents who had evacuated the area within a five-mile radius of the plant returned home to check their insurance policies, and several dozen received free radiation tests by riding prone through a sensitive wholebody scanning machine. All were told their radiation level were normal.

"I feel better now," said Mary B. Stoner, 71, on emerging from the scanner. "I was really worried about it. I was out working in the yard steady for two days after the accident happened. You never know what it might do to you."

Said Dr. R.L. Gotchy, the radiobiologist operating the scanner: "We didn't expect to find anything, and we haven't. The main thing is to provide assurances to people."

Hundreds of reporters who invaded the area were mostly gone yesterday, and business was picking up in downtown Harrisburg. Several communities within the evacuated area passed resolutions Monday night asking their local power utilities to shoulder the costs of additional police and fire protection that was given the empty homes.

Cleanup within the damaged reactor itself, Hendrie testified, would cost "at least tens of millions of dollars" and could take as long as two years. Workers will not be able to enter the plant for "some months," he said.

The plant itself was proceeding in an orderly manner toward cold shutdown as engineers continued to draw gases, mostly hydrogen, out of the water circulating around the reactor, Hendrie said. Cold shutdown, the point at which temperatures within the plant are below the boiling point of water, will not be reached for four or five days, he said.

Radiation for the first time in 12 days, was low enough to let workers take a sample of the reactor coolant water and send it to be analyzed for further information about the damaged nuclear fuel rods in the reactor core NRC authorities in Harrisburg reported. It was such a sample taken the day after the accident that galvanized the NRC into major action, and the commissioners and yesterday they were not happy about that.

"Through Thursday night [March 29] we did not fully appreciate the level of core damage," Hendrie said, "and I am upset and distressed that it took us that long to get a handle on it."

"We just had a terrible time that first day getting adequate information," echoed Commissioner Gilinsky. Commissioner Bradford said the lack of knowledge about core damage for nearly 48 hours was "the single most astonishing element" in the whole crisis.

They told the senators that telephone lines were jammed between Washington, D.C., and Three Mile Island, and the senators in turn expressed astonishment that the NRC was relying on telephone communication in the first place.

Metropolitan Edison, operators of the plant, "probably should have notified us earlier" that "it wasn't just an other quiet dawn over Three Mile Island," Hendrie said. "We need to look very carefully at our early communication system."

Review of the procedures required in checking valves and gauges is also necessary, bradford said. Plant operators at 42 nuclear plants nationwide were notified yesterday that they have water-level gauges like the one that misled an operator in to shutting off emergency cooling pumps prematurely at Three Mile Island considerably aggravating the situation.

The gauge indicated a high water level over the reactor core when in fact the level was dropping, a situation Hendrie said resulted from the rapid passage of steam through the gauge area. Misreading the gauge, Bradford said, was "not necessarily an operator error" since there had not been any official stress on the need to the compare that gauge with others.

Instructions to check other instruments were sent to the 42 plants, Hendrie said, and possible design changes will be studied as well.

Design changes might include a vent at a top of the reactor which would have made it possible at Three Mile Island to get rid of the hydrogen bubble that for a while appeared to threaten an explosion, Hendrie said. Attention might be given to requiring supervisors to check the position of all valves on a routine basis, since valves left closed after routine mainteance were a major cause of trouble when they went unnoticed at Three Mile Island.

Hendrie also called for an upgrading of operator training in dealing with emergencies, as well as a "critical reexamination" of the relationship among the regulators, the plant management and their support teams during a crisis.

Metropolitan Edison "was thin in terms of its backup engineering staff" and it is not yet certain if the utility had more information early on than it gave to the NRC, Hendrie said. The subcommittee chairman, Sen. Gary Hart (D-Colo.), said he was "concerned that in a crisis situation, when people are confused, angry and frustrated . . . the question of who is in charge can be a very bitter thing."

The propsed requiring NRC operator to be in charge of every nuclear power plant, and said he would pursue this ideal with legislation, Rep. Elizabeth Holtzman (D-N.Y.), with consumer advocated Ralph Nader looking on in approval, announced legislation do just that. Her bill, to be introduced after the Easter recess, would also require emergency preparedness information to be distributed to all households within 50 miles of a nuclear power plant.

President Carter, in his press conference yesterday, said the accident had not altered his support for measures to streamline the licensing and siting procedures for new nuclear plants. "There is no way for us to abandon the nuclear supply of energy in our country in the foreseeable future," the president said. CAPTION: Picture 1, Hendrie at Senate hearing: "I am upset and distressed that it took us that long to get a handle on it." AP; Picture 2, Sens. Alan Cranston (D-Calif.), Gary Hart (D-Colo.) and Jennings Randolph (D-W.Va.) hear regulators testify. By James K. W. Atherton-The Washington Post Post; Picture 3, Dairy farmer Chris Becker is scanned for radioactivity by operator Bill Gibson. AP