The collapse of Soviet communism was the greatest advance for the cause of freedom in the late 20th century, but it left behind a legacy that could complicate the 21st century struggle to overcome terrorism. While the United States and Russia work to dismantle nuclear arsenals, terrorists and rogue states are seeking to obtain materials -- from former Cold War armaments and other sources -- to make nuclear weapons and "dirty bombs."

Securing this nuclear and radiological material is a top priority for the United States, Russia and many other nations. While much of it is concentrated in the former Soviet states, it is also found in other countries around the world. It constitutes a formidable threat if it falls into the wrong hands.

In the early aftermath of the Cold War, nuclear nonproliferation programs were appropriately focused on reducing and securing nuclear weapons and weapons material in the former Soviet Union. In 2001 President Bush broadened and accelerated these programs. Both he and Russian President Vladimir Putin have made nonproliferation a personal priority. The United States has developed much better working relationships with our counterparts in the Russian government, and we have been successful in bringing other countries into the effort.

President Bush's most recent budget request for the Department of Energy calls for $1.35 billion for the Defense Nuclear Nonproliferation Program -- nearly 75 percent more than the largest budget request of the previous administration -- and we have shortened the timetables and expanded the scope of many important nonproliferation programs.

The Department of Energy has accelerated efforts to secure 600 metric tons of weapons-usable material in Russia. By the end of this fiscal year, we will have secured more than 46 percent of this material, and in 2003 and 2004 we will have secured more of it than in any two-year period. We will finish this work by 2008, two years ahead of the schedule we inherited.

We have accelerated the recovery of about 10,000 high-risk radiological sources in the United States. We have helped the Russian navy secure its nuclear fuel and warhead sites much faster than originally planned, and have accelerated similar work at other military sites. We are installing nuclear detection equipment at international ports, airports and border crossings, and are working with the International Atomic Energy Agency on initiatives to locate and secure weapons-usable material around the world.

In the spring of last year, the Energy Department began a new program with Russia to upgrade security for its strategic rocket forces sites. By the end of this year, we will have secured two sites, and we are working to secure the remaining 15 by the end of 2008.

Other U.S. initiatives include the Global Partnership Against the Spread of Weapons and Materials of Mass Destruction, which President Bush called for and the G-8 leaders adopted. This 10-year program has brought new resources to bear and is engaging additional nations in nonproliferation efforts. The partnership already has secured almost $17 billion in pledges toward our target of at least $20 billion -- a figure we believe should be a floor, not a ceiling.

We are moving aggressively forward with these and many other programs, but we would be fooling ourselves, and endangering our citizens, to say we have done enough. This is why, in May in Vienna, the United States proposed the Global Threat Reduction Initiative to secure, remove or dispose of an even broader range of nuclear and radiological materials around the world.

Under this initiative, we will work in partnership with Russia to recover nuclear fuel of Russian origin from research reactors around the world and repatriate it to Russia for safe storage or disposition. We seek to do this not in three years, or five years, or eight but by late next year. We are also working to repatriate all Russian-origin spent high-enriched uranium fuel by 2009. At present about four metric tons of this spent fuel are at 20 reactors in 17 countries.

A second feature of this initiative is completing the repatriation of U.S.-origin high-enriched uranium spent fuel from research reactors -- about 40 metric tons in more than 40 locations around the world. This effort involves a number of diplomatic and legal challenges, but we believe most of the fuel can be repatriated in four to five years.

A third feature will be converting the cores of 105 civilian research reactors that use high-enriched uranium fuel to instead use low-enriched fuel. About one-third of the reactors are in the process of being converted or already have been converted, and we expect to finish another one-third in three to five years. The final one-third could be more difficult.

While we have developed low-enriched uranium fuel that works for some reactors, it does not work for others -- and developing a suitable fuel for these reactors will take time. Until a new fuel is available, some nations will be reluctant to give up the use of their research reactors. We have top scientists hard at work developing a substitute fuel, and they are making progress. In the meantime, critics who question our pace or commitment do not understand the technical realities or are choosing to ignore them.

The final pillar of the initiative is to identify nuclear materials and equipment not yet covered by existing programs and secure those materials and equipment as safely and quickly as possible.

With all these initiatives and other efforts across the government, President Bush is pursuing the most aggressive nonproliferation effort in history. Four years ago there was no comprehensive international effort to address radiological dispersal devices. Today there is. Four years ago there was no program to place radiation detection equipment at the world's major shipping ports. Today there is. Four years ago, there was no formal agreement to return Russian-origin spent high-enriched uranium reactor fuel to Russia. Today there is. Most important: Four years ago there was no G-8 global partnership with $20 billion in commitments for nonproliferation. But today, those programs are in place.

Securing nuclear and radiological materials is one of our highest priorities and greatest responsibilities in the battle against terror. The United States will continue to intensify its efforts to keep a legacy of the Cold War from becoming a tool of the enemies of freedom.

The writer is U.S. secretary of energy.