AT ITS HANFORD reservation in the state of Washington, in underground tanks, the federal government stores 64 million gallons of highly radioactive liquid waste. It is the sinister byproduct of the manufacture of nuclear weapons, and some of it goes back to the mid-1940s. For years there's been rising anxiety about these tanks and their contents, but until recently the chief threat seemed to be leakage. Now the Department of Energy reports that these wastes generate combustible gases that, under certain circumstances, could explode.

The possibility of an explosion may have been known to the people running Hanford as early as 1979, according to the Energy Department official now in charge of waste management. Incredibly, through the following decade, they did very little about it. The congressional reaction to this revelation is predictable and deserved. It's heartening to see that there's equal outrage among the new management of the Energy Department as it confronts this latest in a long series of alarming revelations about the way the nuclear weapons plants have been run.

Over the years, it is now obvious, these plants operated with very little supervision from their nominal bosses here in this city and very little communication with them. As long as they produced the weapons as ordered, very few questions were asked from the top. A thick veil of security shielded them from any other inquiries. For decades, nobody here in Washington wanted to deal with the growing obsolescence of the plants' equipment or the accumulation of wastes.

That began to change at the end of the Reagan administration, and the present secretary of Energy, James D. Watkins, is now undertaking a gigantic attempt to clean up these facilities. It's worth noting that he is vigorously using these scandalous disclosures of past neglect as a means to impose central control on the outlying baronies of his empire and to force them to a standard of accountability that until now has been unknown in the weapons plants.

In the case of Hanford, the immediate question is the disposal of those millions of gallons of hot waste. That's one of the most difficult of the decisions awaiting Secretary Watkins. The present plan is to reduce them to stable glass, to be sealed in steel canisters. But, as the department cautiously acknowledges, that won't happen for some years. It wants to try the technology first at the Savannah River plant in South Carolina to demonstrate that it works, before applying it to the much larger quantities of waste in Hanford.