WASHINGTON • As United States President Barack Obama gathers world leaders in Washington this week for his last Nuclear Security Summit, tonnes of materials that terrorists could use to make small nuclear devices or dirty bombs remain deeply vulnerable to theft.
Still, Mr Obama's six-year effort to rid the world of loose nuclear material has succeeded in pulling bomb-grade fuel out of countries from Ukraine to Chile, and has firmly put nuclear security on the global agenda.
But despite the progress, several countries are baulking at safeguards promoted by the US or are building new stockpiles.
President Vladimir Putin of Russia, where some of the largest stockpiles of civilian nuclear material remain, has decided to boycott the summit, which begins tonight. Mr Putin has made it clear he will not engage in nuclear clean-up efforts dominated by the US.
In addition, Pakistan has embraced a new generation of small, tactical nuclear weapons which the Obama administration considers highly vulnerable to theft or misuse.
Pakistan, China, India and Japan are all planning new factories to obtain plutonium.
And Belgium, where a nuclear facility was sabotaged in 2014 and where nuclear plant workers with inside access went off to fight for the Islamic State in Iraq and Syria, has emerged as a central worry. Many fear it is vulnerable to an attack far more sophisticated than the bombings in the Brussels airport and subway system last week.
The nuclear initiative has been a signature issue for Mr Obama: It is among the goals he campaigned on in 2008 and part of the reason he was awarded the Nobel Peace Prize barely a year into his presidency.
But the administration's budget for aiding global nuclear clean-ups has been cut by half; some officials argue that less funding is needed, with fewer nations willing to give up nuclear materials.
"The key question for this summit is whether they'll agree on approaches to keep the improvements coming," said Mr Matthew Bunn, a nuclear expert at Harvard and a former White House science adviser.
NEW YORK TIMES