IAEA Board of Governors that high-level IAEA delegations had met twice with Iranian officials to intensify efforts to resolve outstanding issues, but that major differences remained and Iran did not grant IAEA requests for access to the Parchin site, where the IAEA believes high-explosives research pertinent to nuclear weapons may have taken place. Iran dismissed the IAEA's report on the possible military dimensions to its nuclear program as based on "unfounded allegations." Amano called on Iran to agree to a structure approach, based on IAEA verification practices, to resolve outstanding issues. In March 2012, Iran said it would allow another inspection at Parchin "when an agreement is made on a modality plan." Not long after, it was reported that Iran might not consent to unfettered access. An ISIS study of satellite imagery claimed to have identified an explosive site at Parchin. The February IAEA report also described progress in Iran's enrichment and fuel fabrication efforts, including a tripling of the number of cascades enriching uranium to nearly 20 percent and testing of fuel elements for the Tehran Research Reactor and the still incomplete IR-40 heavy water research reactor. Though Iran was continuing to install thousands of additional centrifuges, these were based on an erratic and outdated design, both in its main enrichment plant at Natanz and in a smaller facility at Fordow buried deep underground. "It appears that they are still struggling with the advanced centrifuges," said Olli Heinonen, a former chief nuclear inspector, while nuclear expert Mark Fitzpatrick pointed out that Iran had been working on "second-generation models for over ten years now and still can't put them into large-scale operation". Peter Crail and Daryl G. Kimball of the Arms Control Association commented that the report "does not identify any breakthroughs" and "confirms initial impressions that Iran's announcements last week on a series of 'nuclear advances' were hyped." May 2012 report In May 2012, the IAEA reported that Iran had increased its rate of production of low-enriched uranium enriched to 3.5 percent and to expand its stockpile of uranium enriched to 19.75 percent, but was having difficulty with more advanced centrifuges. The IAEA also reported detecting particles of uranium enriched to 27 percent at the Fordu enrichment facility. However, a diplomat in Vienna cautioned that the spike in uranium purity found by inspectors could turn out to be accidental. This change drastically moved Iran's uranium toward bomb-grade material. Until