EVERYBODY KNOWS that three decades' accumulation of radioactive waste is being stored, temporarily, in pools at the reactors that produced it. Everybody knows that it would be much safer to store the stuff in sealed canisters buried deep underground. But none of it has been buried yet, because the country hasn't been able to make up its mind where to put it. Several years ago Congress passed legislation that, by a slow and cautious process, would force a decision. This week President Reagan took a first gingerly step when he identified three possible sites.

Nothing will happen quickly. The three states chosen -- Texas, Washington and Nevada -- will undoubtedly attempt, in court and in Congress, to block further action. Meanwhile the Energy Department, having already done a good deal of drilling and testing at these sites, will now sink mine shafts and begin opening caverns to allow geologists to inspect the rock directly. That will take five years. If all goes well, Mr. Reagan's successor will make a final choice and, in 1998, burial of the waste will begin.

When Mr. Reagan narrowed the choices to the three western sites, the administration also announced that it was deferring the search for a site in the East. That's an important decision, and probably a mistake. The law specifies that there are to be two. The administration argues that the first site will not be filled until well into the next century, and the search for a second can be deferred until the 1990s. But one reason for developing a second site is to have the assurance of an alternative if the technicians run into trouble at the first. The congressional Office of Technology Assessment made that point in its study last year of radioactive waste disposal. But the administration has decided that the political costs of a second site outweigh the benefits of a fall-back. All of the incentives in waste disposal are weighted toward procrastination.

And since the East enjoys the benefits of nuclear power, should not the East accept some of the responsibility for nuclear waste disposal -- particularly since it can be managed with negligible risk to public health? The answer to that question is usually a denunciation of nuclear power in general. But the alternative to nuclear power is coal. While nuclear waste can be safely buried, the wastes from coal combustion are pumped into the air that you breathe. Some are highly toxic and, while technology can reduce them, it can't eliminate them from coal smoke. The prevailing view seems to be that coal is safe but nuclear power is bad for your health. The past several decades' experience suggests that precisely the opposite is true.