These contracts let buyers lock in a price for basic materials.

VOA: special.2010.12.03

"We will set new standards, expand our cooperation with Russia, and pursue new partnerships to lock down these sensitive materials."

VOA: standard.2010.04.06

Now what you want to do is lock down the materials so they can't get to them."

VOA: standard.2010.08.06

Nations have come together to lock down nuclear materials, and America and Russia are reducing our arsenals.

WHITEHOUSE: Remarks by the President to the UN General Assembly

On nuclear issues -- our efforts to lock down nuclear materials around the world depend in large part on effective cooperation between the United States and Russia, and other countries.

WHITEHOUSE: Press Briefing

It's manifested in the progress we've made on nuclear security and nonproliferation through the New START Treaty and our efforts to lock down nuclear materials together, and it's manifested, frankly, in the fact that, even as we speak now, the Russians are hosting the P5-plus-1 talks in Moscow aimed at getting Iran to live up to their international obligations.

WHITEHOUSE: Press Briefing by Jay Carney and Ben Rhodes