In a world awash in excess nuclear bomb fuel, a few kilograms could spark an explosion on the scale of the attack on Hiroshima.

Russian stocks of nuclear weapons-grade material include more than 1,000 tons of highly enriched uranium and nearly 150 tons of plutonium. Over the past decade, numerous thefts of such material have been detected, and we now see reports that Osama bin Laden has tried to obtain nuclear weapons.

Fortunately, none of these attempts has produced catastrophe. But we must seize this moment to adopt a new approach. "So far, so good" is not good enough, since a plutonium ingot the size of a soda can could fuel a nuclear explosion that could kill 50,000 people.

Worse, terrorists do not need a sophisticated device to succeed. They could simply surround a conventional explosive with a blanket of radioactive material to disperse radiation in a city-killing many, sickening more and terrorizing all.

Americans and Russians are working together to reduce the threat.

Over the past decade, the Nunn-Lugar Cooperative Threat Reduction Program has deactivated thousands of warheads and missiles while funding peaceful work for former weapons scientists. The United States contracted to buy 500 tons of Russian highly enriched uranium, to be blended down to nonexplosive uranium for sale in the commercial market.

Bilateral cooperation has produced significant improvements in the security systems protecting nuclear materials throughout Russia.

But we need a comprehensive approach that would move beyond locking up materials to the actual reduction of the vast stocks of plutonium and uranium that could be fashioned into weapons.

To date, the uranium deal has eliminated the equivalent of 5,000 warheads. Getting rid of plutonium, which remains highly toxic and radioactive for millenniums, has proved more difficult.

Despite years of discussion, governments have yet to construct a viable plutonium disposition program. Plutonium is more expensive and more difficult to handle than other fuels, so some incentive would be needed to persuade reactor operators to use it.

One option would be to offer government support or tax breaks to promote improved reactor designs or fuels that utilized weapon-origin material. Reactors don't emit greenhouse gases; governments could reward that. Another option would harness market forces by offering to lease weapon-origin fuel to utilities. The fees utilities would pay to avoid the expense and liability of having to dispose of spent fuel could be enough to subsidize the use of plutonium as fuel.

Why should governments or companies support plutonium burning? Because we should not allow the pure, easily handled plutonium left over from the Cold War to remain in its metallic form, ideally suited for terrorist use.

Irradiating it in reactors consumes some of the plutonium, with the balance embedded in spent fuel that is so hot, in temperature and radioactivity, that terrorists could not handle it for many years.

Some have argued that plutonium disposition is too expensive, so we should focus on the easier task of storing it securely. This is a false choice; we need both safe storage and ultimate disposition. Our generation produced this plutonium; it would be immoral to shunt to our grandchildren the responsibility to eliminate it.

Others worry that burning plutonium from weapons would lead to its widespread commercialization. But it is unlikely that a program aimed at reducing total quantities of weapons-usable plutonium would stimulate increasing those quantities through the costly separation of plutonium from spent fuel.

Integrating plutonium from weapons into the marketplace would give Russia an incentive to ensure that these materials are protected and accounted for as a revenue-generating asset, rather than depending on continued U.S. handouts for safe storage.

Adding a commercial component to burning plutonium also would free more public money for other fronts in the war against terrorism.

A public-private partnership to get rid of Cold War plutonium would be a difficult challenge. But the costs of failure are unacceptable. Sept. 11 proved that those costs are real.