Yesterday we discussed which nuclear weapons will be affected by the New START agreement, which was ratified by the Senate. Today we examine another worrisome type of nuclear material — the unaccounted-for kind, which terrorists have the best chance of acquiring.

A final vote on the New START Agreement is expected this week, after 67 Senators — exactly the two-thirds majority necessary to ratify the treaty — voted yesterday to break a GOP filibuster of the bill. The treaty would significantly decrease the number of nuclear weapons held by the United States and Russia. We're looking at which specific classes of nuclear weapons would be decreased by START, and why we worry about these weapons instead of others.

North Korea seems to have surpassed Iran, in its efforts to develop the advanced technology that produces nuclear material and weapons. According to David Sanger, chief White House correspondent for our partner The New York Times, senior American officials were recently stunned to see that North Korea is far more advanced in its nuclear ambitions than anticipated. The officials visited a new plant at Yongbyon, where North Korea's main nuclear complex sits.

American and European diplomats are meeting in Geneva today in hopes of building a framework for future talks on tempering Iran’s nuclear ambitions. But yesterday, Iran's Atomic Energy Organization announced that it had produced yellowcake uranium from domestically-mined ore — a breakthrough that eliminates Iran's reliance on imports for their nuclear goals.

On of the United States' largest concerns in the Middle East has long been the relative security of Pakistan, and more specifically, the security of that country's nuclear weapons. America provides millions of dollars in aid there, and the Pakistani government is considered an ally in an unstable area of the world. But in a country where the Taliban and other organizations have infiltrated the state military, it's a constant balancing act. 

Secret diplomatic cables obtained by whistle-blower organization WikiLeaks shed new light on the global nuclear standoff with Iran. The documents reveal for the first time that the U.S. believes Iran has obtained nineteen powerful, Russian-designed missiles from North Korea. Their range is long enough to strike Western Europe. Will this change the way the country is seen and dealt with by its neighbors?  

A new version of the venerable START arms control agreement (Strategic Arms Reduction Treaty), which was signed by President Obama and Russian President Dmitry Medvedev in April of this year, is the grandchild of the bilateral nuclear arms reduction treaty that ended the Cold War. But while the fighting between the superpowers may be on hold, a war of words is on between Democrats and Republicans. The Obama administration is pushing the lame-duck Congress to ratify the new bilateral treaty. But the lead Republican negotiator, Sen. John Kyl of Arizona, has declared that there is not enough time to reach an agreement this year on his preferred treaty component: a nuclear modernization program for which the Obama Administration has committed $84 billion.

In 2003, Valerie Plame Wilson went from being an undercover CIA officer specializing in nuclear proliferation to a reluctant celebrity when members of the Bush administration outed her to the press. She has stayed mostly out of the public eye since, but now she’s lending her expertise and her voice to "Countdown to Zero," a new documentary about nuclear weapons by many of the same people who made "An Inconvenient Truth."

BP hit another bump in the road in its attempts to the cap the oil leak when a saw got suck in a riser pipe on Wednesday. This setback comes after many failed attempts to plug the well, including the first containment dome, the "top kill" approach and the "junk shot" technique. The current operation, known as the Lower Marine Riser Package, could capture most of the leaking oil but could allow for some to continue to escape along the margins of the apparatus.

With so many failed attempts, some are calling for a radical last-measure solution: using a nuclear explosive to destroy the well and stop the leak. The option would be politically and environmentally risky, but it may not be entirely out of the question. President Obama has already dispatched a team of five nuclear physicists to the Gulf, though there have been no mentions of considering a nuclear approach.

The ongoing back-and-forth between the U.S. and Iran over the latter's nuclear ambitions is often compared to a chess game. But there's a notable difference: a chessboard only has two sides. Yesterday's announcement that Iran would trade in some of its low-enriched uranium for fuel rods to power a reactor that makes medical isotopes - a swap engineered by Turkey and Brazil - had some observers asking what Iran is up to.

We take look at what's ahead this week, with Marcus Mabry of The New York Times and Latoya Peterson editor of the blog, Racialicious. Volcanic ash continues to keep most European flights grounded affecting attendance at everything from the Major Economies Forum on Energy and Climate to the Tribeca Film Festival, both of which take place this week. Meanwhile, Washington is still buzzing about the Iran memo sent by Defense Secretary Robert Gates. And in cultural news, Wanda Sykes' show is up for renewal and Green Days' "American Idiot" opens on Broadway.

President Obama is in Prague today, where he signed the START Treaty along with Russian President Dmitry Medvedev. The treaty will reduce the number of nuclear warheads between the two countries by almost 40 percent, from 2,700 to 1,550.