As nuclear negotiations enter a decisive phase, Iran is making halting progress addressing what is likely to be a key element of a long-term agreement: resolving international concerns about past research that would be necessary to develop the most common form of nuclear weapon.

According to the latest report by the International Atomic Energy Agency (IAEA), Iran has offered a possible if unlikely explanation for one of its past activities and continues to stonewall on two others.

The three areas are all pertinent to the development of an implosion-type nuclear bomb, which is what US intelligence and the IAEA believe Iran was at least researching before 2003. According to the IAEA report, Iran has provided additional clarification about its work on so-called exploding bridge wire detonators. Iran insists its work in this area is tied to the oil industry, not nuclear weapons, an explanation that is possible but not consistent with what the IAEA believes, according to Jon Wolfsthal, deputy director of the James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies. 

The other two areas involve initiation of high explosives and calculations related to neutron transport and their application to compressed materials — all elements required to produce a warhead small enough to fit on top of a missile.