AFTER the initial euphoria over last month's failed Soviet coup, the world is belatedly facing the vital question of who controls the 27,000 nuclear warheads in what used to be the Soviet Union.Last week, French President Francois Mitterrand injected a much-needed note of urgency into the debate by calling for an immediate meeting of Europe's nuclear powers - including the United States - to "ensure the security of the continent." President Bush can seize this opportunity to put forward a bold and innovative proposal that could greatly reduce the nuclear risk to Europe, the Soviet Union, and the US, while satisfying political leaders in both Moscow and the former Soviet republics. Unfortunately, until now the Bush administration has demonstrated a marked lack of creativity in addressing the Soviet nuclear question. Secretary of State James Baker III has called only for a continuation of the status quo - weapons continuing to be controlled by a central authority in Moscow. But Mr. Baker's proposal has two main drawbacks: First, it presupposes the continued existence of a Soviet "center," by no means a certainty. Under the Baker proposal, if central authority breaks down sometime in the future, control over thousands of warheads could be lost. Second, even if central authority is maintained, Baker's proposal would permit nuclear weapons to remain in the outlying republics, where they could be vulnerable to unauthorized use or seizure by terrorists or local military forces. From a nonproliferation standpoint, the ideal resolution would be for all the weapons to be concentrated under unified control in a single republic, most logically Russia, where 80 percent of the weapons already are located. Baker avoided explicitly calling for such a solution - entailing the return of nuclear weapons from the outlying republics to Russia - most likely out of concern the republics would regard it as an infringement on their sovereignty. While many of the republics have disavowed nuclear weapons, they well might resist a call to return the weapons to their giant Russian neighbor. The worst outcome would be for each republic to take control of the strategic and tactical nuclear weapons on its soil. Overnight, the number of nuclear powers in the world would double. Even more troubling, the newly independent republics would likely have unstable leadership and poor command and control infrastructure, significantly increasing the risk of accidental launch or unauthorized seizure. Further complicating matters, many of the republics are located near regional hot spots in the Middle East and Eastern Europe, which could draw them into military confrontations. There is a way out of this box: a "nuclear grand bargain" between Russia and outlying republics. Under such a deal, the republics would return their nuclear weapons to Russia, but only on condition the weapons were dismantled in a verifiable manner. Both sides would have much to gain. The republics would maintain sovereignty - since they would be choosing to destroy their own weapons - while reducing the risk of the weapons falling into the wrong hands. The Russian government would avert the threat of proliferation, while sacrificing little. Several hundred warheads that would be dismantled under this deal already are slated for retirement under the recently signed START treaty. As for the rest, certainly Russia's leaders would prefer these weapons be dismantled, rather than come under the control of potentially rivalrous neighbors. The US and its European allies also can provide extra incentive for both sides to come to the bargaining table, by making clear that substantial Western economic aid and technological assistance is contingent on satisfactory resolution of the nuclear question. The key to this nuclear grand bargain is quick action, before the weapons become a contentious issue in independence negotiations. Mr. Bush should accept the French offer for an urgent international meeting and utilize his exceptional diplomatic skills to build a Western consensus behind the proposal. Failure to act quickly could have tragic consequences. In the worst case, what now appears to be the fulfillment of the Soviet people's dream of freedom could turn into a nuclear nightmare.