It is the world's worst post-9/11 nightmare: terrorists with a nuclear weapon threatening Washington, Beijing, Moscow, or Paris. Already Islamic State (IS) has used chemical weapons, smugglers were caught in Central Asia trafficking in highly enriched uranium (HEU), and North Korea has sold nuclear equipment to Syria, according to media reports. 

Since the end of the Cold War, the US and Russia have reduced their nuclear arsenals by nearly 90 percent and melted down several tons of fissile material. Yet there remain some 1800 metric tons of weapons-usable material scattered in hundreds of civilian and military facilities around the globe - enough for thousands of nuclear weapons.

These ominous realities led the UN Security Council to pass Resolution 1540 in 2004. In that resolution, the world community affirmed that the proliferation of nuclear, and other weapons of mass destruction are a threat to international peace and security and obligates all states adopt legislation to prevent it and establish domestic controls to prevent illicit trafficking.

This threat and the global consensus to stop it is the rationale behind the Nuclear Security Summit that US President Obama initiated in 2010, with the fourth and final one to start today.

Since the first nuclear summit, 2,697 kilograms of nuclear material has been moved or melted down to low-enriched uranium (LEU), civilian-use HEU has been removed from 14 countries, the security of fissile material has been enhanced worldwide, and radiation detection equipment has been installed at 250 international border crossings, airports and seaports to combat illicit trafficking.

This progress is the result of the Summit keeping a spotlight on the issue and peer pressure on all 53 participating nations to devise and implement national action plans. But there are key challenges ahead. The creation of a robust global nuclear security architecture remains to be achieved. And what happens after this summit to sustain the momentum created by this global effort?

Among the challenges ahead is how to remove, or convert to LEU, HEU from civilian facilities. There are 61 metric tons of civilian HEU at over 100 facilities in 25 countries. One emerging risk that requires new safeguards and new cooperation is that of cyber-attacks on nuclear facilities, a threat that has been under-appreciated.

The US and Russia need to renew their commitment to repatriate HEU supplied to third countries. Russia will not attend the summit, the result of mistrust and confrontation with the US. But nuclear security is a global issue transcending national competition: Moscow faces the same threat from IS.

Among nuclear weapons states, enhancing nuclear security remains an issue. Some call for more transparency and the adoption of best practices, information sharing and common standards adopted by all nuclear weapons states. 

The amount of HEU that fits into a five-pound bag of sugar is enough for a nuclear weapon that could kill tens of thousands. Even medical isotopes in terrorist hands could lead to a "dirty bomb" (the mixing of radioactive material with conventional explosives).  

Countries must create roadmaps for progress in their action plans going forward. Here, public-private partnership with governments and the nuclear power industry is critically important.

In the absence of future nuclear security summits to serve as action-forcing events, what mechanisms will be employed? Fully adopting UN Conventions like that on Suppressing Acts of Nuclear Terrorism, adopting the 2005 amendment to the Convention on Physical Protection of Nuclear Materials and nuclear security training for all nuclear operators, and security services are key tasks.

To maintain the momentum of the Summit institutionally, several organizations must take ownership of issue: the UN Security Council, the IAEA, and INTERPOL and regional groupings like the ASEAN Regional Forum should make it a priority.

For Northeast Asia, a new mechanism for nuclear cooperation could be created. South Korea's call for a five-party consultative process could put nuclear security, managing spent fuel, and accident response issues on its agenda. This goes well beyond North Korea. Europe has had EURATOM as a means of cooperation, so why not a Pacific "PACATOM," since all five parties have robust civil nuclear industries. 

Creativity, commitment and sustained focus will be key to meeting the challenge of nuclear security.

The author is a senior fellow of the Brent Scowcroft Center for International Security at the Atlantic Council and its Strategic Foresight Initiative. opinion@globaltimes.com.cn. Follow him on Twitter at @RManning4