Olli Heinonen, senior fellow at Harvard’s Belfer Center for Science and International Affairs, and former deputy director-general for safeguards at the International Atomic Energy Agency

The recently concluded first step agreement slows down Iran’s nuclear program. This is important. It is also important to recognize that it is not a rollback.

Let us look at the current facts on the ground. With Iran’s inventory of 20 percent enriched uranium, it would take about two weeks using 6,000 IR-1 centrifuges, operating in tandem cascades, to produce enough weapons-grade material for one nuclear device. If Iran uses 3 to 5 percent enriched uranium as feed material at all its currently installed 18,000 IR-1 centrifuges at Natanz and Fordow, the same result would be achieved in two months.

The current agreement retains Iran’s fleet of more than 18,000 IR-1 centrifuges. Operational restrictions are placed that allow 10,000 centrifuges to continue to enrich at up to 5 percent at any given point of time. These measures, together with a cessation of 20 percent enriched uranium production and conversion of the 20 percent-level stockpiles to oxides, extend the current breakout times by about two months.

The enrichment and inspection measures in this agreement cover Iran’s declared facilities. The presence of any undeclared facilities, however, changes the picture. The International Atomic Energy Agency (IAEA) remains unable to provide credible assurances on the absence of undeclared nuclear facilities and activities.

The present deal—freezing construction of the Arak reactor and its fuel production—is welcome. If left unchecked, the reactor, which is envisaged to start operation by the end of 2014, would be able to produce more than one bomb’s worth of plutonium annually. At the same time, the deal is silent on the manufacturing of remaining key components of the reactor and its continued heavy water production. Technically, such efforts are not reasonable if the goal is to either dismantle the reactor or modify it to a more proliferation-resistant and smaller light water reactor as an alternative path of producing medical isotopes.

There are many strong views being aired that positively or negatively view the current interim deal. There also remains ample language in the current agreement that leaves space open for interpretation—another challenge for its implementation. Still, the agreement is where we are, and the proper focus should be using this first step to secure a more long-lasting deal that significantly rolls back Iran’s nuclear program.

While the final taste is in the end-goal pudding, we already need to start pre-tasting its recipe.