In 2004, allegations surfaced that Iran was conducting secret experiments involving "high-explosive shaped charges with an inert core of depleted uranium" to test the characteristics of an implosion type nuclear weapon. [1] An ISIS analysis based on commercial satellite images referred to the site as a "logical candidate" for weaponization related research. [2] Iran denied the allegations and granted the IAEA restricted access to the complex in January and November 2005. [3] The Agency reported in February 2006 that inspectors "did not observe any unusual activities in the buildings visited," and that "environmental samples did not indicate the presence of nuclear material." [4] However, suspicions about the Parchin site persist. The exile group The National Council of Resistance of Iran has claimed that Iran constructed a sophisticated underground tunnel system at Parchin, where weaponization work continues. [5] These claims have never been substantiated.

Sources:
[1] "UN Atomic Agency Seeks to Visit Key Iranian Defense Site: diplomats," AFP, 16 September 2004.
[2] David Albright and Corey Hinderstein, "Parchin: Possible Nuclear Weapons-Related Site in Iran," Institute for Science and International Security, 15 September 2004, www.isis-online.org.
[3] "Iran Denies any Nuclear Activity at Suspect Site Parchin," AFP, 16 September 2004; "UN Inspectors Visit Sensitive Iranian Military Site," AFP, 2 November 2005.
[4] "Implementation of the NPT Safeguards Agreement in the Islamic Republic of Iran," International Atomic Energy Agency (IAEA), 27 February 2006, www.iaea.org.
[5] "Iran-Nuclear: Exile Says Iran Expands Tunnels for Nuclear Work," National Council of Resistance of Iran, 17 September 2005, http://ncr-iran.org.