For the first time, the Nuclear Security Summit will include a special session on responding to urban terrorist attacks — and a simulation of how to handle the threat of imminent nuclear terrorism.

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WASHINGTON — As President Obama gathers world leaders in Washington, D.C., this week for his last Nuclear Security Summit, tons of materials that terrorists could use to make small nuclear devices or dirty bombs remain deeply vulnerable to theft. Still, Obama’s six-year effort to rid the world of loose nuclear material has pulled bomb-grade fuel out of countries from Ukraine to Chile.

Perhaps more important, several countries are balking at safeguards promoted by the United States 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 Thursday night. Putin has made it clear he will not engage in nuclear-cleanup efforts dominated by the United States.

In addition, Pakistan’s embrace of a new generation of small, tactical nuclear weapons, which the Obama administration considers vulnerable to theft or misuse, has changed the way the administration talks about Pakistani nuclear security.

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While Obama declared early in his presidency that the U.S. believed Pakistan’s nuclear assets were secure, administration officials will no longer repeat that.

Instead, when the subject comes up, they note the modest progress Pakistan has made in training its guards and investing in sensors to detect break-ins. They refuse to discuss secret talks to persuade Pakistan not to deploy new weapons.

Pakistan, China, India and Japan are all planning new factories to obtain plutonium that will add to the world’s stockpiles of bomb fuel.

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 group, has emerged as a central worry. The country is so disorganized that many fear it is vulnerable to an attack far more sophisticated than the bombings in the Brussels airport and subway system last week.

For the first time, the Nuclear Security Summit will include a special session on responding to urban terrorist attacks — and a simulation of how to handle the threat of imminent nuclear terrorism.

“The key question for this summit,” said Matthew Bunn, a nuclear expert at Harvard and a former White House science adviser, “is whether they’ll agree on approaches to keep the improvements coming.”

The nuclear initiative has been a signature issue for 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.

In a recent report, the Nuclear Threat Initiative, a private advocacy group in D.C. that tracks nuclear weapons and materials, warned that many radioactive sources were “poorly secured and vulnerable to theft.” The report called the probability of a terrorist’s detonating a dirty bomb “much higher than that of an improvised nuclear device.”

Ingredients for so-called dirty bombs, which use conventional explosives to spew radioactive material, are still scattered around the globe at thousands of hospitals and other sites that use radioactive substances for industrial imaging and medical treatments. Fewer than half the countries that attended the last nuclear summit in 2014 pledged to secure such materials, and they, in turn, represent less than 15 percent of the 168 nations belonging to the International Atomic Energy Agency.

And while the administration succeeded in getting more than a dozen countries to give up their civilian stockpiles of highly enriched uranium, a main fuel of atomic bombs, the Nuclear Threat Initiative said in another report that some 25 nations still had such materials — enough for thousands of nuclear weapons.