“A remarkable achievement.”

That’s what President Obama said April 1 at the conclusion of the fourth Nuclear Security Summit. “As terrorists … look around for deadly ingredients for a nuclear device, vast regions of the world are now off-limits.”

Obama and more than 50 world leaders met in Washington to review progress in securing and eliminating nuclear materials. They discussed ways to advance shared nonproliferation goals and combat nuclear trafficking.

Since the first such summit in 2010, about 3.8 tons of highly enriched uranium and plutonium — enough to produce some 150 nuclear weapons — have been secured. “That’s material that will never fall into the hands of terrorists,” Obama said.

He noted that South America is now completely free of these dangerous materials. Central Europe and Southeast Asia will be free of them later this year, he added.

Still, the president acknowledged that more remains to be done to reduce the threat of nuclear terrorism.

“Realizing our vision will not happen quickly, and it perhaps will not happen in my lifetime. But we’ve begun,” he said. “Today we agreed to maintain a strong architecture … to carry on this work and to provide the resources and technical support that is needed to continue this mission.”