Full coverage: Xi Visits Czech Republic, Attends Nuclear Security Summit

As the Nuclear Security Summit gets under way in Washington, CCTV's Nathan King takes a look back now at global efforts to secure global stockpiles of nuclear material as well as the challenges still ahead.

The nuclear bomb - mankind's ultimate weapon - held by only a few nations in the aftermath of the Second World War.

As the number of countries with nuclear weapons expanded, so did worries. First, over nuclear proliferation among states. And later, over the transfer of nuclear technology to terror groups.

The end of the cold war nuclear stalemate and the challenge to secure the Soviet Union's stockpiles, clandestine programs in Libya, South Africa and elsewhere added to these fears as did allegations against this man-AQ Khan, the so-called father of the bomb in Pakistan. Khan was accused of selling nuclear secrets to Iran the DPRK and beyond. 

And while terrorists have yet to use a nuclear device - that's not for the lack of trying.

The Aum Shinrikyo cult in Japan - which carried out a nerve agent attack on the Tokyo subway - had nuclear plans, as did Al-Qaeda. Belgian authorities suspect the terror cell that plotted the recent attacks in Paris in Brussels wanted to build a 'dirty bomb.'

"The bombers in Brussels had the intention at least of trying their hands on some radioactive material so I think people take it pretty seriously," said Thomas Karako, senior fellow, Center for Strategic & Int'l Studies.

This nuclear security summit is part of the international architecture to stop that from happening.

The U.S. hosts say the international community has made great strides in securing more than 2,000 metric tons of nuclear material held by the world's civilian and military programs.

The steps include: Eliminating all nuclear materials from 14 countries; Removing and downgrading nuclear fuel from more than 50 facilities in 30 countries; Boosting border security, and combatting nuclear smuggling; Incorporating international treaties into domestic laws and encouraging international inspections.

But while nations come together to combat the threat of nuclear terrorism, there are national security issues sometimes get in the way.

Many countries are reluctant to allow full access to inspectors, which could reveal state and commercial secrets, and even strategy.

"The extent of state to state foreign assistance is something that all too easily to look past. Sometimes we think that a nuclear program or a missile program merely is defined within the borders of a particular country. And that, of course, is not the case," Thomas Karako said.

Organizers say world done good job reducing nuclear material risk of being taken by terrorists. Lot's more need to be done, espceially because of surveillence industry is booming creating more nuclear material needs to be secure.