MORE THAN TWO years after the nuclear accident at Three Mile Island, the cleanup of Unit 2, the damaged reactor, is still in its earliest stages. Nearly $300 million has been spent. General Public Utilities Corp., the plant's owner, says it will soon run of funds. Unit 1, the undamaged companion plant on the same site, has not been turned on, costing GPU $14 million per month in replacement power. Bankruptcy of the utility because of monumental cleanup costs is still a real possibility.

Though he has no direct jurisdiction over the cleanup, Gov. Richard Thornburgh of Pennsylvania has responded to the lack of urgency displayed by industry and the federal government by proposing his own plan for raising the necessary funds. The details of the plan -- involving voluntary contributions by GPU, the federal government, the nuclear industry and the states of Pennsylvania and New Jersey -- are less important than the sum thought to be needed: $760 million. And that breathtaking estimate is probably too low.

Yet, even if the money were available, cleanup would by no means be assured. Cleansing the nearly one million gallons of radioactive water in the plant before leaks occur is no small task, but is the easiest part of the process and should be under way soon. Removing and finding a safe way to store the damaged ractor fuel is more difficult. Though no one yet knows for sure what condition the fuel is in, it is likely to be more damaged than anything dealt with before. The biggest job, one that nobody has even begun to think about, will be finding an acceptable resting place for the huge reactor vessel, and devising a safe way to get it there.

Thornburgh's plan should be taken as a remainder that the cleanup of Three Mile Island is in danger of being forgotten by just about everyone outside of Pennsylvania. It is natural for the industry and its federal regulators to want to focus on the future, on positive efforts to ensure that such an accident never happens again. The damaged plant is, nonethless, a national responsibility, one that Congress, the executive branch and the industry must cooperate in solving. Turning on the undamaged Unit 1, as soon as the Nuclear Regulatory Commission completes the hearings now in progress, will help. But so long as cleanup continues to inch along with no prospect of the necessary funding, and with major technical problems unresolved, the nuclear industry cannot expect a financially or politically stable future.

After nearly 30 years, the nation still has no policy for dealing with radioactive waste. Nuclear waste has always been an afterthought, something that someone else would deal with sometime in the future. Three Mile Islands as a reminder that the future cannot be put off indefinitely.