From the catwalk, the dull shine of crisscrossed pipes can be followed deep into the pit below, hundreds of them in a mammoth plumber's nightmare that is getting ready to handle the poisoned waters of Three Mile Island.

This is a giant filter, the $10 million pride and joy of the crippled nuclear power industry, and it was the star exhibit yesterday as Metropolitan Edison Co. showed off the progress it has made since the nation's worst nuclear accident shut this place down two years ago this month.

The tour was upbeat, but Met Ed executives were careful to lace the lecture with grim statistics that amount to a plea for government help. It is now clear that the most important fallout from that cold morning of March 28, 1979, was not radioactive, as the nation thought at the time, but financial. Largely because of TMI, the entire nuclear industry is on hold.

"We have to have $125 million a year to proceed as rapidly as possible with the cleanup," said Robert C. Arnold, president of the "nuclear group" that Met Ed's parent company, General Public Utilities, has organized to show its new approach to nuclear management. "At the present time, he funding simply is not available."

Cleanup costs here are now estimated at $1.3 billion, with work extending to 1986. But progress in cleaning up these days is "negligible," the holding company told its investors in February, because of delay in getting the Nuclear Regulatory Commission's approval for the cleanup plans.

Met Ed is still awaiting approval for this giant filter, called a "submerged demineralizer system," but is building it anyway in "99 and 44/100 percent confidence," Arnold said, that it will be okayed when it is finished in May. It is the apparent solution to the next step in the cleanup, the assault on 700,000 gallons of radioactive water that is now 8 1/2 feet deep in the reactor building next door and rising at 1/10th of an inch a month.

When all is ready, clean water will fill this pit, which was originally built to hold spent reactor fuel, and the radioactive water will flow underneath through the pipes and special resin filters to be stripped of the atom fragments that make it lethal.

Nothing else can be done here until the water is cleared out, but no proposal on the water's final destiny is even expected until next year.

Every week on hold brings another problem: a leak in a pipe, contamination found in the caulking or in another wall joist, an unexpected small puff of radiation inside the plant. Officials even used the recent discovery of radioactive rodent droppings as an occasion to call for a quick go-ahead on their mop-up efforts.

But area residents, many of them traumatized by the accident, remain opposed to any quick fixes. They want to make sure none of that radioactive water ends up in the Susquehanna, and they don't want the undamaged Unit One reactor opened until every aspect of it is scrutinized. The NRC promised to do that, and hearings will continue into the summer.

Area residents have already collected $1.3 million from TMI's insurance in recompense for lost wages and extra living expenses during the accident, while dozens of other damage suits were combined into one class action suit against Met Ed and several other corporate defendants that was settled out of court for $25 million. GPU has filed its own suits, including a $4 billion action charging the Nuclear Regulatory Commission with failing to spread the word about a problem in another plant that could have warned operators about the possibility of the TMI accident 17 months beforehand.

Meanwhile, GPU is almost out of money. It has already spent $209 million of its insurance payments, and the $91 million it still has coming "will cover less than one year of full effort" in the cleanup, the stockholders' statement said. After that, if Unit One remains closed, there won't be any more cleanup money. The corporation omitted its quarterly dividend in December for the fifth time in a row and canceled a planned nuclear plant in Forked River, N.J., after spending $349 million on it. "TMI demonstrates that $300 million is not sufficient damage coverage when one considers the cost of cleanup, too," Arnold said.

The consortium of banks that provided GPU $415 million in short-term loans is not likely to extend the deal, because nuclear power since Three Mile Island has a bad-risk image. Investors are staying away in droves in a tight bond market, even at astronomical interest rates. There are 85 plants being built, but no new reactors have been ordered since 1978.

"If we get no further orders, the industry only has three or four years to live," said Herman R. Hill, head of General Electric's power systems division. "In a funny way, TMI prolonged the life of the industry. It put all those people to work making and installing the new equipment the NRC required afterward."

Various financing schemes to save TMI from bankruptcy and finish the cleanup have been offered, most designed to spare the 60 nuclear-owning utilities from similar straits if an accident should strike them. The most recent is a National Nuclear Property Insurance Corp. proposed to Congress from Rep. Allen E. Ertel (D-Pa.), whose district includes Three Mile Island.

Nuclear utilities -- which means their ratepayers -- would be required to contribute $2 million each per year under this plan to build an eventual reserve fund of $750 million against the damages from a future accident. TMI would be grandfathered in and gat a $262 million grant, but the idea faces rough sledding in Congress, not to mention opposition from state utility regulators whose constituents would foot the bill.

The president's Nuclear Oversight Committee earlier recommended a package of direct loans, grants and loan guarantees, but President Reagan's crackdown on federal credit appears to have killed that idea.

The reason for all this financial pain, analysts say, is uncertainty that began before TML Long licensing times, construction delays and changing federal requirements are part of the problem. State utility regulators, reluctant to provide unending rate increases and skeptical about the need for new plants in a period of slowing growth in electricity demand, are another.

But Three Mile Island didn't help any.A Harris poll last December found people evenly split, 47 percent to 47 percent, on whether more nuclear plants should be built. Before TMI, it was 57 percent in favor to 31 percent opposed.

Trying to deal with some of the uncertainty, the NRC last week proposed a plan to speed the licensing process, but it came under immediate attack from all sides. It would eliminate the right intervening groups to demand documents at will from the NRC staff, allow oral instead of written rulings on some motions, and make other changes that consumer advocate Ralph Nader called "dictatorial procedures" leading to "an attitude of the public be damned."

After two years, many people remain suspicious of nuclear power and unsure that they know what really happened at Three Mile Island. A coalition of nine labor unions and area residents plans a demonstration; the latest of many, on the day of the anniversary. The investigators continue. Met Ed has paid $155,000 in fines for assorted violations of NRC rules, the legal maximum, but is still under fire from Rep. Morris Udall (D.-Ariz.) for its alleged deliberate withholding of information at the height of the accident.

Met Ed's reporting failures, Udall told the NRC at its budget hearing, "call into question the fundamental premise upon which the licensing procedure is founded: that licensees will voluntarily provide officials with information affecting the public health and safety."

Udall's findings, disagree with those of two other probes, which said that while certain information about the state of the reactor had not been given to officials, it had not been deliberately withheld. A grand jury continues to probe allegations that dangerous leaks were covered up before the accident.

Although no power emerges from Three Mile Island and the four giant cooling towers remain silent, 1,900 people report to work here every day to keep it running, 750 of them directly involved in cleanup operations. Yellow "contaminated area" signs still stand here and there, but it is all routine now.

"I don't think there's anything we have to be concerned about," said David Smith, a shift supervisor in the Unit One control room, "except we can't seem to get out of the limelight."