(UPDATE: I recommend reading Nima Shirazi's blog)
I'm sure you've seen this story about a drawing of a "nuclear test chamber" in the media today (typically, the story is posted in the "blog" section of major newspaper sites since the "news" standards for the "blog" sections are lower) so I thought it would be a good time to repeat what's actually known about Parchin and this "nuclear test chamber":
1- Parchin is a non-nuclear site, and as such it falls outside of the IAEA's legal authority which is limited to nuclear sites. However the site at Parchin was already visited by IAEA experts TWICE in 2005, because Iran voluntarily opened it to inspections. The inspectors were allowed to pick any 5 buildings and go visit them, and environmental samples were taken. IAEA reports specifically thank Iran for allowing the inspections and say nothing out of the ordinary was found there...twice.
2- This story about an explosion chamber was debunked when it first came out and when the media had named a "Soviet nuclear scientists" Vyacheslav Danilenko as aiding Iran's nuclear program. Turned out Danilenko's specialty was actually in making nanodiamonds using similiar explosive chambers (which can be cylindrical or hemispherical), and nanodiamonds are regularly used for civilian industrial purposes. More here.
3- Iran has not refused a third visit but has asked for a "modalities agreement" from the IAEA, just as which existed prior to the other two IAEA visits there. This is because legally, Iran doesn't have to allow any inspections of a non-nuclear site such as Parchin at all. The job of the IAEA, according to Iran's safeguards agreement, is "exclusively" limited to measure Iran's fissile material to ensure non-diversion for weapons uses (and every IAEA report has certified that Iran is in full compliance with this ACTUAL requirement of the NPT.) If the IAEA suspects that Parchin is an undeclared nuclear site, all it has to do is present the evidence to the IAEA Board of Governors and get a 'special inspection' permit that Iran would be obliged to allow (this has happened in the past with Romania and North Korea.) However despite all the hype, the IAEA has never presented its evidence for suspicion to the Board and has not tried to obtain this special inspection permit.
4- Iran's nuclear program is entirely legal, according to even several former European ambassadors to Iran. The IAEA took pains to point out that were was never any evidence of any nuclear weapons program in Iran, either now or in the past:
The UN atomic watchdog said Thursday it has no concrete proof that there is or has been a nuclear weapons programme in Iran. (empasis mine)