by former Senator Sam Nunn, Co-Chairman of the Nuclear Threat Initiative

The gravest danger in the world today is the threat of a nuclear attack.

Whether launched by a state or a terrorist group, a nuclear explosion in a major city could kill hundreds of thousands, close borders, erode civil liberties, slash trade and travel, and change the world as we know it. No country would escape the consequences.

Preventing the spread and use of nuclear weapons should be the top security priority of the 21st century. But this is not something that can be done by any one nation; it has to be done by many nations, working together.Let me offer an example. Today, as energy needs rise and the pace of global warming increases, more and more nations are interested in using nuclear power to generate electricity.

That could be a good thing -- or it could be very dangerous. The process used to make nuclear fuel can also be used to make the key ingredient of a nuclear-weapon. If every nation that wants to use nuclear power decides to make its own nuclear fuel, the world could see -- over the next decade or so -- dozens of new nations capable of making not only nuclear fuel, but also nuclear weapons.

So how can the world accommodate more nations using nuclear power without creating more nations who can produce nuclear bombs?

There is only one answer: through intensified international cooperation. If nations that already have nuclear know-how can come together to guarantee a supply of nuclear fuel to nations who need it but can't make it, then nations who need nuclear fuel may be persuaded to elect to import it, instead of building the capacity to make it on their own.

That is why The Nuclear Threat Initiative, with Warren Buffett's backing, has pledged $50 million to help build an international fuel bank that will be available as a last-resort fuel reserve for any nation that is meeting their nonproliferation commitments that chooses to rely on international fuel markets rather than choosing to develop their own fuel supply facilities.

It is important that we build such a fuel bank, and the technical and financial means are there to make it possible. What is missing is the collective will to bring about the international cooperation that can make it happen. Cooperation is the essential challenge of 21st century security. Along with well-trained troops, top weapons systems, and effective intelligence services, one of our greatest security assets will be our ability to cooperate with other countries to achieve our common security goals. I believe we are in a race between cooperation and catastrophe - and cooperation is moving forward far too slowly.

We should consider two questions: If, in the years ahead, the world experiences a nuclear catastrophe, what would we wish we had done together to prevent it? Why aren't we doing that now?