Water vapor (circled in red) rises from the Arak nuclear complex in Iran.  This recent imagery suggests the plant is nearing operational capability, giving Tehran the “heavy water” option for producing nuclear weapons (DigitalGlobe imagery/McKenzie Intelligence Ltd assessment, via the UK Telegraph)

Tuesday’s edition of the UK Telegraph reminds us that Iran has more than one path to obtaining nuclear weapons.  Along with the uranium enrichment method–which has been the focus of world attention for years–Tehran could also produce a plutonium bomb, through its heavy water plant at Arak.

Readers of this blog learned about Arak back in 2006.  At the time, Iran announced plans to active the facility within three years, to produce isotopes “for industrial, medical and other peaceful purposes.”  The Arak complex was supposed to replace a much smaller facility in Tehran.  Never mind that Arak could produce isotopes on a scale far beyond Iran’s needs, or (as experts noted at the time), the plutonium needed for a nuclear device.

Flash forward almost seven years, and it appears the Arak facility, located about 150 miles south of the Iranian capital, has entered operational service.  As the British publication reports:

The Telegraph can disclose details of activity at a heavily-guarded Iranian facility from which international inspectors have been barred for 18 months.
The images, taken earlier this month, show that Iran has activated the Arak heavy-water production plant.
Heavy water is needed to operate a nuclear reactor that can produce plutonium, which could then be used to make a bomb.
The images show signs of activity at the Arak plant, including a cloud of steam that indicates heavy-water production.    
The details of Iran’s plutonium programme emerged as the world’s leading nations resumed talks with Tehran aimed at allaying fears over the country’s nuclear ambitions.
The new images also show details of the Fordow complex, which is concealed hundreds of feet beneath a mountain near the holy city of Qom. At talks in Kazakhstan yesterday, world leaders offered to relax sanctions on Iran in exchange for concessions over Fordow, which is heavily protected from aerial attack.
Iran insists that its nuclear facilities are for peaceful use, but Western governments fear that Tehran is seeking a nuclear weapon – or at least the ability to build one.
The striking image of steam over the Arak heavy-water complex is a vivid demonstration that the regime has more than one pathway to a potential nuclear weapon.
The Telegraph’s assessment is based on new commercial satellite imagery and analysis of activity at the Arak plant.  At one point, Iran hoped to have the plant operational by 2009, but that date was eventually pushed back to 2014.  Activity noted in the Telegraph article suggests Tehran may have accelerated that schedule, perhaps a result of problems with its uranium enrichment program, which has been hit by U.S. and Israeli cyber attacks and recent sabotage that wrecked hundreds of centrigues.  
In fact, those strikes were so successful that even Israeli intelligence moved back the window for an Iranian bomb by several years, to the 2015-16 time frame.  But with Arak entering operational service, that timeline may be revised again–and the likelihood of an Israeli attack may actually increase.  As one expert told the Telegraph:  
Mark Fitzpatrick, a former US State Department official at the International Institute for Strategic Studies, suggested that Arak could be part of a process that might trigger Western strikes on Iran.
One option for the Iranian regime would be to acquire the necessary reprocessing technology from North Korea, he said.
“By then, the option of a military strike on an operating reactor would present enormous complications because of the radiation that would be spread,” he explained.
“Some think Israel’s red line for military action is before Arak comes online.”
While Iran currently lacks the ability to reprocess uranium fuel rods (from its light water reactor at Bushehr) into plutonium, that capability could be easily acquired from North Korea.  Indeed, Pyongyang’s recently-accelerated nuclear testing program suggests it is “testing for more than one country,” an indication that technologies under development will be quickly shared with Iran.  
Mr. Fitzpatrick (and other western experts) believe the timing for an Israeli strike may be measured in only a few months.  Construction of the reactor is almost complete, and the complex should be fully operational by sometime early next year.  
Arak has always been the most heavily-defended nuclear site in Iran, with at least three surface-to-air missile sites and 50 AAA batteries surrounding the complex.  The defensive shield is the result of two factors; first, Arak’s geographic location makes it more vulnerable to attack, and unlike the enrichment facilities at Natanz and Fordow, Arak appears to be built entirely above ground, further increasing its susceptibility to air attack.  
Still, the defenses around Arak are largely outdated.  The three missile sites employ the Shahin missile, an upgraded version of the HAWK, which Iran acquired from the U.S. during the reign of the Shah.  Most of the anti-aircraft guns are optically-guided, making them ineffective against cruise missiles and other types of precision weapons.   
As Arak nears full operation, Iran may elect to further upgrade defenses with the addition of the SA-15.  The short-range SAM is Tehran’s most effective air defense weapon, with some capabilities against precision weaponry.  There have also been reports that Iran is acquiring the long-range SA-20 from Russia, but so far, there is no firm evidence that Iran has actually obtained that state-of-the-art system.
New concerns about Arak come as U.S. military strength in the Persian Gulf is on the wane.  With the delayed deployment of the USS Harry S. Truman battle group, the U.S. will have only one carrier in the region for the foreseeable future.  Many experts believe Washington would launch military action against Iran with no fewer than two carriers in the Gulf or nearby waters.  
Of course, Israel doesn’t operate under such constraints, but its own military options are limited by the distance between its air bases and targets in Iran.  By some estimates, an Israeli strike package might be limited to only two dozen aircraft, reflecting the 1,000-mile flight to Iran, and the IAF’s small tanker fleet.  That obstacle could be overcome by staging the attack from bases in neighboring countries, such as Azerbaijan.