Target: Iran

Foreign Policy- Iran's quest to develop nuclear energy dates back to 1957, when the United States began sending low-enriched uranium and nuclear technology to ally Shah Mohammed Reza Pahlavi for research purposes as part of President Dwight Eisenhower's Atoms for Peace program. But Western powers started having second thoughts about the Iranian nuclear program even before the Islamic Revolution of 1979, and Ayatollah Ali Khamenei's embrace of nuclear power in the 1990s only hardened that opposition. 

Over the past decade, the West has grown increasingly convinced that Tehran is pursuing nuclear weapons under the guise of civilian atomic work, while Iran's leaders have insisted that they are simply diversifying their energy sources and developing fuel for medical research reactors.

The goal of enrichment, as this overview of the nuclear fuel cycle explains, is to increase the proportion of uranium-235 atoms -- "fissile isotopes" that can split in a chain reaction and produce heat -- within uranium. The International Atomic Energy Agency (IAEA)'s most recent report on Iran's nuclear program found that, since 2010, the country has produced 320 pounds of fuel with uranium-235 concentrations of 20 percent -- a level that could enable Iran to quickly enrich the uranium to the weapons-grade threshold of 90 percent. (For perspective, natural mined ore has a uranium-235 concentration of 0.7 percent, and nuclear power plants typically run on 3.5 percent-enriched uranium.)

Understanding the dimensions of the Iranian nuclear program has never been more important than it is today, as Iran vows to continue enriching uranium and expanding its nuclear infrastructure in the face of harsh economic sanctions and U.N. Security Council resolutions to halt work, and U.S. and Israeli leaders, fearing that Iran is on the verge of atomic-weapons capability, threaten to check Tehran's nuclear advances by force if diplomacy fails. Here's a tour of the six facilities that you'll want to keep a close eye on as tensions with Iran escalate.

ATTA KENARE/AFP/Getty Images

ARAK

The National Council of Resistance of Iran, an Iranian opposition group, revealed the construction of the Heavy-Water Production Plant (HWPP) in Khondab, near the central Iranian city of Arak, in 2002 (see the satellite image below), and the facility became operational four years later. The plant will provide heavy water (ordinary water enriched in the hydrogen isotope deuterium) to the nearby IR-40 research reactor, which is expected to be completed in 2013 and appears to be using technology provided by Russian companies.

Iranian officials claim that the Arak reactor will produce radioactive isotopes for agriculture and cancer treatment, but heavy-water reactors can also produce weapons-grade plutonium once the plutonium is separated from the reactor's spent fuel. According to the Institute for Science and International Security (ISIS), the IR-40 will be able to produce roughly 9 kilograms of plutonium a year -- enough for around two nuclear weapons. (Atomic bombs can be made out of either enriched uranium or plutonium.)

In 2006, the U.N. Security Council called for Iran to suspend "work on all heavy-water related projects, including the construction of a research reactor moderated by heavy water." But that demand has gone unheeded, and the IAEA has not been granted access to the heavy-water reactor in Arak since 2011. Satellite imagery monitored by the agency does not offer many clues on how construction of the IR-40 is progressing because the reactor's external structure has been completed, but the images do suggest that the HWPP facility is still operating. Here's a look at what the reactor site looked like as of 2008:

HAMID FOROUTAN/AFP/Getty Images

Digital Globe-ISIS

BUSHEHR

Iran first considered building a nuclear power station near the southern port city of Bushehr in the mid-1970s with German help, but those plans fell through with the Iranian revolution. In 1995, however, Russia inked a deal with Iran to complete the country's first commercial nuclear reactor, and the Russians later agreed to supply enriched uranium to the facility. Unlike the IR-40, Bushehr uses light water (otherwise known as ordinary water) to moderate its nuclear process, which makes it more difficult to extract plutonium from spent fuel rods. Nevertheless, as part of their agreement, Iran is required to return the spent fuel to Russia.

Throughout the 1990s, Israel and the United States focused on weakening the relationship between Moscow and Tehran out of fear that Russia would help Iran acquire nuclear weapons. In 2000, for example, the CIA's A. Norman Schindler told Congress that "Russian entities are interacting with Iranian nuclear research centers on a wide variety of activities beyond the Bushehr project," and that "many of these projects have direct application to the production of weapons-grade fissile material." 

The Bushehr reactor has already been connected to the national grid and is slated to come fully online this fall, and Iranian officials are now pledging another Russian-built 1,000-megawatt nuclear power plant alongside the existing facility by early 2014. (They're also promising a smaller plant in the city of Darkhovin near the border with Iraq.) Here's an aerial view of the facility as of 2004:

But Bushehr no longer keeps Western leaders up at night as much as some of the other Iranian sites on this list. The IAEA visited the reactor twice in 2011, and Iranian authorities routinely update the agency about the plant's physical inventory. "The U.S.-led effort to prevent the civilian nuclear reactor at Bushehr from being repurposed to produce nuclear weapons-grade plutonium is an ongoing nonproliferation success story," Council on Foreign Relations scholar Micah Zenko has observed.

IIPA via Getty Images

Space Imaging Eurasia-ISIS

ESFAHAN

Iran conducts nuclear technology research at several sites near the central city of Esfahan, but it's the Uranium Conversion Facility (UCF) that has proven most controversial. The UCF converts "yellowcake" -- a solid, yellow-colored form of uranium ore that is used in the preparation of fuel for nuclear reactors -- into, among other things, uranium hexafluoride gas, which can be used to create highly enriched uranium for nuclear weapons and has been sent to Iran's Natanz plant for enrichment.

Iran began constructing the UCF in the late 1990s, after lifting the design information from a Chinese contract that was never realized because of U.S. pressure. In 2004, the country suspended its uranium conversion program at Esfahan in a deal with Britain, France, and Germany to dampen concern about its nuclear ambitions and avoid international sanctions. But Iran resumed uranium conversion less than a year later, arguing that the process was intended to produce nuclear fuel. 

In a February 2012 report, the IAEA noted that activities at the UCF and Esfahan's Fuel Manufacturing Plant, which produces fuel rods for the IR-40 reactor in Arak and the nuclear power plant in Bushehr, are "in contravention" of Iran's obligations to "suspend all enrichment related activities and heavy water related projects."

Below is an aerial view of the Esfahan complex as of 2006. The Nuclear Threat Initiative points out that Iran has constructed underground tunnels below the UCF for "unknown purposes" and deployed anti-aircraft missile systems around Esfahan to protect the facilities from an aerial attack.

Getty Images

DigitalGlobe-ISIS

FORDO

Iran may have begun work on the Fordo uranium enrichment plant -- an underground site located on an Islamic Revolutionary Guard Corps base near the holy northern city of Qom -- as early as 2006. But the facility wasn't made public until 2009, when France, the United Kingdom, and the United States disclosed its location only days after Iranian officials announced its existence. The Obama administration noted at the time that the facility, which is designed to hold 3,000 centrifuges (fast-spinning machines that enrich uranium), was too small to produce fuel for a nuclear power reactor but "may be well-suited for a military purpose."

In June 2011, Iran revealed that it was planning to triple its capacity to enrich uranium to 20 percent and that it would transfer this work from the Natanz facility to Fordo once it had achieved the higher production level at Natanz. Seventh months later, the IAEA confirmed that Iran had begun enriching uranium to 20 percent at Fordo.

During nuclear talks in Baghdad in May 2012, world powers failed to persuade Iran to grant international inspectors access to Fordo (and to eventually dismantle the site), in addition to suspending its 20-percent enrichment and exporting its current stockpile of higher-grade uranium. A day later, the IAEA reported that it had discovered uranium enriched to 27 percent at Fordo -- a startling find that Iranian officials chalked up to a "technical" glitch.

Fordo is arguably the Iranian nuclear site that spooks world leaders the most. In a recent New York Times Magazine piece on the odds that Israel will attack Iran, journalist Ronen Bergman noted that Israeli intelligence believes the facility, which is built into the side of a mountain. "is 220 feet deep, beyond the reach of even the most advanced bunker-busting bombs possessed by the United States."  

Digital Globe-ISIS

NATANZ

Fordo may be the most contentious Iranian nuclear site these days, but the larger uranium enrichment facilities near the central city of Natanz have long poisoned relations between Tehran and the West. American, British, and Israeli intelligence services discovered the plant -- built with the help of Abdul Qadeer Khan, the father of Pakistan's nuclear program -- in 2002. Iran began enriching uranium to 20 percent at Natanz in 2010 before transferring much of that work to Fordo, and the facility's more than 9,000 centrifuges now churn out lower-grade uranium.

Natanz is perhaps best known as a cauldron of intrigue. Two transformers exploded and 50 centrifuges were destroyed during the first attempt to enrich uranium at the site in 2006, prompting a spokesman for the Iranian Atomic Energy Council to declare that the components had been "tampered with." At some point between 2009 and 2010, a computer virus called Stuxnet -- which many computer security experts suspect was designed by U.S. and Israeli intelligence operatives -- may have infected more than 10 percent of the centrifuges at Natanz. In January, an Iranian nuclear scientist who worked at Natanz was killed in a mysterious car bombing.

Yet despite these vulnerabilities, the underground complex is also heavily fortified and guarded by anti-aircraft artillery. Here's what the site, which a National Defense University report estimated can "withstand aerial attack," looked like as of 2006:

Majid Saeedi/Getty Images

Digital Globe-ISIS

PARCHIN

In 2004, ISIS and ABC News fixed international attention on the Parchin military complex by publishing commercial satellite imagery (including the photo above) of the munitions center, which is less than 20 miles southeast of Tehran. U.S. officials worried that Iran was conducting nuclear-related high-explosives testing at Parchin -- an allegation Iranian officials denied. High explosives can be used to compress uranium or plutonium and spark a nuclear chain reaction in what is known as an "implosion device" -- the type of bomb that the United States dropped on Nagasaki, Japan in 1945.

A 2006 IAEA report allayed concerns a bit, noting that inspectors had not observed "any unusual activities" or detected "the presence of nuclear material." But the IAEA and Iranian authorities have since sparred about whether international inspectors should be granted further access to the site, as suspicion mounts that Iran is trying to clean up evidence of nuclear weapons research at the site.

Those concerns have largely been fueled by a 2011 IAEA report indicating that Iran had constructed what appeared to be a high-explosives chamber at Parchin in 2000 to hold "a large cylindrical object" in what amounted to a "strong" indication "of possible weapon development," and by the emergence of satellite images in March 2012 that showed trucks and earth-moving vehicles at Parchin, in what nuclear experts claimed could be an attempt to destroy the remnants of experiments with a neutron device that could trigger a nuclear explosion.

In late May, new satellite imagery of Parchin suggested that two small buildings had been demolished, potentially by a bulldozer. The image below catalogues the evidence (the building with the white roof right below the two razed structures is the suspected high-explosive testing chamber):

In the months ahead, these aerial shots of inscrutable buildings and grainy vehicle tracks could very well play a major role in the high-stakes standoff between Iran and the West.