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

The fourth nuclear security summit has concluded in Washington DC, with a focus of 'locking down atomic materials to guard against nuclear terrorism'. In a joint communique by participating leaders, nuclear threat was called "one of the greatest challenges to international security".

Chinese President Xi Jinping said new threats and challenges keep emerging in the security field. He makes 5 pledges to make sure terrorists cannot get their hands on material to build the ultimate weapon. That includes improve nuclear security capacity, and promote less use of highly enriched uranium. 

It wasn't all talk here at the fourth Nuclear Security Summit here in Washington. More than 50 world leaders were on hand for a milestone. A legally binding agreement to help protect nuclear material from theft.

"We expect that the treaty will enter into force in the coming weeks -- giving us more tools that we need to work together in the event of theft of nuclear material or an attack on a nuclear facility," said Obama.

The terrorist attacks in Brussels drove home the risks of such an attack. When Belgian police raided a suspect's apartment, they discovered covert video of a Belgian nuclear official. A possible ISIL terrorist cell appears to have had him under surveillance.

An international agreement to protect civilian nuclear materials from theft has been in effect since 1987, but it required an amendment to make it tougher.

Chinese President Xi Jinping has among those pressing to get enough signatures on the amendment to make that happen. Here are some of the main points of the amended Convention on Physical Protection of Nuclear Materials.

The agreement requires countries to be responsible for the safe transport and storage of nuclear materials, report suspected nuclear smuggling and - importantly - it would allow nations to update the agreement every five years.

It took more than a decade to get to this final tougher treaty. And now, it'll go into effect in 30 days.