The financial fallout from the Three Mile Island nuclear accident was toted up for the final time here last week, and all that remains is for the Pennsylvania Utility Commission to decide who pays the bill.

At stake in the verdict, expected next week, might well be the financial future of nuclear power -- at least as far as that future depends on Wall Street.

Gerold Gornish, attorney for Citibank and Chemical Bank of New York, summed it up: "Banks are concerned about viability."

He was one of a dozen lawyers reiterating as many points of view before a utility commission hearing on whether what happened at Middletown, Pa., a year ago should be treated as a fluke, a product of gross incompetence, or something predictable that could, and probably will, happen to any other nuclear utility.

If it could happen to anyone, antinuclear groups argued, then the banks, investors and utilities that boost nuclear power ought to expect -- and therefore pay the costs of -- accidents in the same way they shoulder the risks of other investments.

Clearly, said the Limerick Ecology Action Group of Phoenixville, Pa., if that had been the original practice, "Tmi" would never have occured, because no one would have invested in such a risky venture."

On the contrary, argued Samuel Russell, attorney for Metropolitan Edison Co., which owns Tmi: Those who get the electric service must pay for its cost. The accident in March 1979 should be treated as another cost, a fluke event, because forcing shareholders to pay instead of customers would only put Met Ed out of business, and then who would supply electricity? w

"The necessity for maintaining credit from the banks," he said, "is the detonator . . . Unless the banks hang in and maintain our credit, we are in trouble in June."

The name of the game here is confidence. "We're not trying to threaten," said Gornish, "but banks have the responsibility to act prudently as bankers . . . and the question is whether it is prudent to continue lending money [to Met Ed] unless there is some assurance of continued long-term viability."

Without the $400 million loan floated by Chitibank and Chemical and 38 other banks last year, Met Ed's parent company, General Public Utilities, would have gone under when the utility's stock plummeted. The specter of a similar fate haunts the nation's 60 nuclear-owning utilities, which are watching this case closely for clues to their own future in the event of an accident close to home.

The PUC is formally deciding three questions:

Were the events at TMI so indicative of bad management and procedure by Met Ed that the commission should begin procedures to lift its operating license?

Should the customers continue to pay construction and maintenance costs for TMI's undamaged sister reactor, Unit One, which was shut down a year ago although nothing is apparently wrong with it, or should it be lifted from the utility's rate base?

Should the customers pay for the more costly oil and coal-generated power that Met Ed bought to replace the lost nuclear power?

The most critical issue is probably the least obvious item, the question of Unit One. Met Ed didn't want to build TMI One in the first place, Russell said in his summary, but did so at the utility commission's urgings and made it a model plant. It's operating performance "has been outstanding among American nuclear reactors," he said.

Consumers are now paying off the $210 million cost of building that plant, and to argue they should stop because the federal Nuclear Regulatory Commission shut it down is senseless, Russell continued. "The retention of TMI One in base rates is a critical issue to the banks," he said.

Gornish agreed. "The only way to have long-term viability is to have earnings, and the only way earnings is possible is through having TMI One in the rate base," he said.

Without those earnings, there will be no money for cleanup of Unit Two, where the accident occurred, and no money to pay off debts, which means "a totally unacceptable risk of bankruptcy," Russell said. Unit One will reopen someday, he continued.

Those arguments are "nonsense . . . irrevelant," argued the citizen's group, Three Mile Island Alert. If shutdowns come with nuclear power, the investors should pay for them, said attorney Mark P. Widdoff. "Some indication that this commission still cares . . . would mean a great deal to people in this area."

The licensing question also sends shudders through the nuclear community. A permit revocation would be the first ever for a major utility, which could set a precedent for other accident sites. "If we decide not to do anything, it would say we don't see the range of problems Met Ed faces as a threat to its continued ability to serve the public," said Steven A. McClaren, PUC staff attorney.

The questions of replacement costs has been partly decided. The PUC authorized about two-thirds of what Met Ed said it needed to get through 1980 so as to keep the company going during this proceeding. But last year's replacement power costs, about $90 million, still need to be paid, and the interim allowance doesn't cover current needs.

The PUC leal staff counseled compromise, a removal of TMI One from the rate base with higher rates to compensate temporarily and keep the company alive. Killing it with a permit revocation or no rate relief would be "irresponsible," but what Met Ed had asked was "patently arrogant," general counsel Joseph Malatesta told the commission.

"A starving man does not demand caviar . . . The very most they [Met Ed] can ask is salvation from disaster," he said.

Consumer advocate David M. Barasch more or less agreed, but recommended taking the Unit One continuing costs out of last year's unpaid replacement power total, in effect giving customers a double break.

McClaren said in an interview that the PUC is well aware its verdicts will assign these new risks either to stockholders or ratepayers. "The choice is there. You have to go one way or the other," he said.