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hundred twenty meter contour line.” Aitan raised his left hand up in the air. His fingers were straight and his palm was face down and parallel to the table. “The floor of the main chamber is eight hundred and eighty-six meters above sea level.” He dropped the level of his hand a few inches to visualize the drop in altitude from the entrances to the level of the chamber. “The finished height of the chamber is four meters. The minimum amount of earth above the chamber is the point over the southwest corner. At that point, there is fifty-two meters of volcanic basalt. Fordow is part of a geological formation known as the Oromeieh-Dokhtar belt. It is the end result of ancient volcanic flows.” Aitan had no notes. All of the information was stored in his head. Everyone in the room knew that if Yavi Aitan read it once and committed it to memory, then the information would be locked away as if on a hard drive.
    Aitan turned his face to look at Avner. The younger man continued. “That depth is equal to one hundred fifty-five feet of earth at a minimum at the southwest corner. The maximum depth to reach the chamber is seventy-eight meters, or two hundred fifty-seven feet of basalt rock above the primary enrichment hall. If I recall correctly, the GBU-28 can penetrate, at best, only one hundred twenty feet of earth. And that assumes the bomb is dropped from at least fifty thousand feet.”
    “How long?” The words were in the direct style of Benjamin Raibani. The metronome was ticking again.
    “Eighteen months as …”
    “No,” Raibani interjected. “Once they have Fordow operational, how long to get to ninety percent enrichment?”
    Yavi Aitan pursed his lips. “That depends. Let me make a couple of assumptions.” Aitan pivoted his left forearm upward from the table on its elbow. He stuck his thumb upward. “First, assume that they enrich to twenty percent at Natanz, which I must point out they have not yet done to our knowledge.” Aitan extended his left forefinger. “Second, assume that we are right on the number of centrifuges inside Fordow and that they are all operational.” Aitan extended his middle finger. “Third, assume they are using their first generation centrifuges inside Fordow, what they call the IR-1. This last assumption is key and is in our favor. The IR-1 is not very efficient.” Aitan extended his next finger, his pinky finger now struggling to stay folded. “And finally, assume that they have a reasonably advanced implosion design that only needs twenty kilograms of ninety percent uranium.”
    Raibani interjected. “How do you know the efficiency of their centrifuges?”
    “Well, we have a lot of data on Natanz from both the IAEA and other sources. We know the statistics for the amount of base feedstock, the product, which has been three point five percent enriched uranium and the tail. We have even been able to get some of the tailings from Natanz to Dimona for analysis.” Aitan paused. The look in Raibani’s eyes asked the question that he didn’t need to verbalize. “Excuse me, I am hanging around with too many nuclear scientists and physicists these days. The tailings are the depleted uranium byproduct of the enrichment process.
    “In a nutshell, you start with natural uranium ore which is ninety-nine point three percent uranium 238 and only seven-tenths percent uranium 235. For a weapon, you want uranium enriched to about ninety percent uranium 235. At Natanz, they use gas centrifuges which spin uranium molecules at a high speed. The heavier 238 molecules migrate to the outside of the centrifuge tubes and the lighter 235 molecules migrate toward the center of the tube. You then collect the separated molecules. The enriched 235 molecules are sent downstream and the depleted 238 molecules are sent back to repeat the process. Eventually the depleted uranium 238 is no longer needed and sent to storage in steel drums. This is the tail. ”
    Aitan glanced at Zvi Avner. The defense