As important as trying to reach a negotiated settlement with Iran is, it is as important if not more important that such a deal creates confidence on both sides.
Building the Arak heavy water reactor, which could produce plutonium to make a bomb while the talks continue, fails to create confidence; it does the opposite. It creates the impression that Iran is interested in maintaining the option of making a bomb, if and when it decides to do so while the talks are taking place. What reinforces concern and urgency regarding Arak is that the Iranian regime has not allowed the International Atomic Energy Agency (IAEA) to visit the site since 2011. The IAEA has since had to rely on satellite images to assess developments regarding the site.