IN September 1951, Senator Brien McMahon laid out the argument for investing heavily in nuclear weapons. ''The cost of military firepower based on atomic bombs is hundreds of times cheaper, dollar for dollar, than conventional explosives,'' he explained.

Nearly a half-century later, scholars at the Brookings Institution have released the first thorough assessment of the cost of nuclear weapons. ''Atomic Audit'' (Brookings Press) estimates that $5.5 trillion in 1996 dollars has been spent by Washington on building, delivering and defending against The Bomb -- and the meter is still ticking at the rate of $35 billion a year.

Just how much money is $5.5 trillion? It's more than has been spent on Medicare and veterans' benefits combined, though $2.3 trillion less than the total outlays on Social Security since 1940. Or, if you prefer the fanciful, $5.5 trillion stacked tightly in bricks made of $1 bills would be enough to build a wall 8.7 feet high around the Equator.

In the icy logic of economics, the benefits may still have exceeded the cost. But the very idea that nukes were initially embraced on such simplistic financial grounds is chilling testament to the sloppiness of military economics.

Senator McMahon, then the chairman of the Joint Congressional Committee on Atomic Energy, was right that a one-megaton nuclear weapon cost less than conventional bombs containing two billion pounds of TNT. But in giving short shrift to the expense of delivering the nukes, the Connecticut Democrat missed the forest for the merest of saplings.

The Brookings study, directed by Stephen I. Schwartz, estimates that building the weapons did indeed cost just $409 billion in today's dollars, 2 percent of defense spending from 1940 through 1996. But the delivery systems -- aircraft, missiles and related equipment -- added $3.2 trillion. Targeting nuclear weapons and systems to control their use absorbed $831 billion. Defenses against nuclear attack have swallowed $937 billion. The (largely future) cost of managing nuclear waste and dismantling obsolete weapons is projected at some $400 billion.

What began as a plan to substitute bargain technology for conventional forces took on a life of its own. First with the panicky military buildup during the Korean War. Then with the ''missile gap'' that became an issue in the 1960 Presidential election and morphed into a race with the Soviet Union for superior delivery systems. Still later, the Reagan Administration explicitly used military spending as a strategy for bankrupting the Evil Empire.

The Pentagon could calculate how many bombs would be needed to destroy Soviet rocket forces or to turn Moscow into a parking lot. But as long as the goal was something other than military victory, nuclear sufficiency was a subjective concept. ''The closest to one man who would know what the minimum deterrent is would be Mr. Khrushchev,'' said Gen. Thomas Powers in 1960, ''and frankly I don't think he knows from one week to another.''

Not surprisingly, though, everyone seemed to have an opinion. President Lyndon Johnson's Bureau of the Budget, preoccupied with financing the Great Society in 1964, concluded that a force of more than 450 Minuteman missiles was a waste of money. Across the Potomac, the military bureaucracy reckoned the right number was 10,000. Defense Secretary Robert MacNamara settled for an even 1,000 missiles.

With the invention of underwater-launched missiles, the Navy thought a few dozen Polaris subs would deter the Soviet Union. But the Air Force thought otherwise, and Congress conveniently decided that a ''triad'' of sea- and land-based missiles along with manned bombers was needed to keep the Russians in their trenches.

Defenders of the nuclear establishment point out that, quibbling aside, the American arsenal succeeded in deterring a catastrophic war. But in Mr. Schwartz's opinion, this is an inadequate rationalization.

For one thing, he says, conventional forces backed by a small nuclear force would have cost far less and might have been equally effective. For another, the failure to contain either the size of the arsenal or the uncertainties associated with technological change made conflict more likely. The chances of accidental war, for example, were presumably greater with tens of thousands of delivery systems (rather than, say, hundreds). The adoption of hair-trigger response protocols in reaction to the huge numbers of offensive missiles surely increased risks. And the invention of compact warheads that could fit six to a missile made the verification of negotiated arms limits almost impossible.

However one reads the nuclear record, it is hard to deny that the policy evolved with little regard for other ways the cash could have been spent. Indeed, the scale was so vast that it inspires a variant on Senator Everett Dirksen's hoary quip. A trillion here, a trillion there -- and yes, it did add up to real money.

Drawing. (Niculae Asciu)