AUDIE CORNISH, HOST:
Loose nukes and dirty bombs are the talk of Washington today as President Obama hosts a summit with more than 50 world leaders. They're here largely because of a challenge Obama issued in 2009 barely two months into his presidency.
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BARACK OBAMA: We must ensure that terrorists never acquire a nuclear weapon. This is the most immediate and extreme threat to global security.
CORNISH: Since then, there have been three Nuclear Security Summits. NPR's David Welna reports on the expectation for this one which will be the last.
DAVID WELNA, BYLINE: To hear the White House tell it, the past three summits have helped secure some 2,000 tons of highly enriched uranium and plutonium.
BEN RHODES: Because of these efforts, it is harder than ever before for terrorists or bad actors to acquire nuclear materials, and that, of course, makes all of our people more secure.
WELNA: That's deputy national security advisor Ben Rhodes. Others are not so excited about yet another Nuclear Security Summit.
SHARON SQUASSONI: I think there is a little bit of what we would call summit fatigue.
WELNA: That's nonproliferation expert Sharon Squassoni. At a briefing on the summit at a Washington defense think tank, Squassoni said there has indeed been a lot of progress in securing the most-vulnerable nuclear materials. But the job isn't done, and political pressure to get it done is waning.
SQUASSONI: We know what to do. The question is, do we have enough willpower and money and attention to do it?
WELNA: Whatever is accomplished at this summit, Russia won't be a part of it. Russian President Vladimir Putin is boycotting the gathering due to what Moscow officials call a shortage of mutual cooperation in planning the meeting. White House Adviser Rhodes calls it a missed opportunity.
RHODES: Frankly, all they're doing is isolating themselves in not participating as they have in the past.
WELNA: Putin's snub of this summit is widely seen in Washington as payback for the sanctions the U.S. imposed on Russia after the last summit for its actions in Ukraine. At the same summit briefing, Russia expert Olga Oliker said having one of the world's top nuclear powers sitting this out is, in fact, a setback.
OLGA OLIKER: One could argue that without Russia there to weaken some of the statements as it has done in the past, you could probably get more forceful results. I mean, there's progress that could be made on a number of areas, but there are a lot of areas where Russia's absence is sort of a missing elephant in the room.
WELNA: Another nuclear-armed nation, North Korea, has never attended these summits. Georgetown University Korea expert Victor Cha expects considerable talk about North Korea's nuclear ambitions during the two-day summit and possibly another missile launch or nuclear test.
VICTOR CHA: It would not surprise me if there was activity by North Korea during this summit because of all the attention focused on what's happening here in Washington. The North Koreans always like to draw a little bit of attention to themselves.
WELNA: There will also be a special session tomorrow on what to do about the Islamic State - again, White House Adviser Rhodes...
RHODES: Having this many leaders together at once provides us an important opportunity in the wake of the recent attacks in Brussels and other countries to address how we can enhance our capabilities to work together to confront the threat posed by ISIL.
WELNA: No more Nuclear Security Summits are planned after this one.
MATTHEW BUNN: The real question for this nuclear security summit is, will the leaders take enough action to put us on a path where nuclear security will continue to get better?
WELNA: Matthew Bunn is a nuclear proliferation researcher at Harvard.
BUNN: Or once we're not meeting at the summit anymore, will attention turn elsewhere and progress stall and eventually erode, leading to a higher danger of nuclear terrorism?
WELNA: Much depends, Bunn says, on who's the next president. David Welna, NPR News, Washington. Transcript provided by NPR, Copyright NPR.