Greetings from Beijing
We now have David Albright’s essay on the advanced nuclear weapons design, as well as stories in the New York Times and Washington Post. (David Sanger, Nuclear Ring Reportedly Had Advanced Weapon, June 15, 2008; David Sanger and Bill Broad, Officials Fear Bomb Design Went to Others, June 16, 2008; Joby Warrick, Smugglers Had Design For Advanced Warhead, June 15, 2008.) There is a lot to sort through.
I have only a few comments to add to James’s excellent post.
This is the second design peddled by the AQ Khan network — and it seems to be considerably more advanced that the Chinese design found in Libya. (If you are interested in the Chinese design, which had a yield of 10 kilotons and mass of 500 kilograms, check out my post: More on Libya’s Bomb Design, October 8, 2005.)
According to Sanger, the design “is half the size and twice the power of the Chinese weapon…” and, writing with Bill Broad, and happens to “bear a strong resemblance to weapons tested by Pakistan” in May 1998.
The statement about 1/2 the size and 2x the yield is probably not true — though it may not matter very much. What does seem likely is that the device is small enough for the Nodong family, which includes Pakistan’s Ghuari and Iran’s Shahab.
1/2 the Size
In the second story, Sanger and Bill Broad describe the device as “miniaturized — using about half the uranium fuel of the older design to produce a greater explosive force.”
Warrick describes the Chinese design as “bulky and difficult to deliver”; Sanger and Broad, “unwieldy but effective.” That language suggests the issue is not mass, but diameter.
Note that half the diameter, half the mass and half the fissile material are all very different things. For example, a bomb with 1/2 the diameter would have 1/8 the mass.
I find the fissile material statement plausible — Hibbs and Albright claim that Pakistan used about 15 kilograms of HEU in its design. Cutting that number in half is quite plausible — Paine and Cochran calculated that a state with medium technical skill could squeeze 10-20 kt out of 7-9 kilograms of HEU. So, that’s in the neighborhood.
But reducing mass — and more important diameter — is more about reducing the amount of conventional high explosive (HE) that surrounds the fissile material. (See my discussion on this topic in the context of North Korea.) A physics package of this sort is, by mass, probably 90-95 percent explosives.
A desire to reduce the amount of explosives would explain the emphasis on advanced electronics in the Sanger and Broad story — less jitter, less explosive:
Clearly, someone had tried to modernize it, to improve the electronics,” one said. “There were handwritten references to the electronics, and the question is, who was working on this?”
If the device is, say 60 centimeters instead of 90 cm, than it might weigh as little as 200 kilograms instead of nearly 600.
You may remember the issue of electronics from James Risen’s State of War — the allegation that the US paid a Russian to give the Iranians a faulty firing set design to retard their program. Same thing, as far as I can tell from the news reports.
Oh and 60 centimeters isn’t an arbitrary number. More on that is a moment.
2x the Yield
Alright, this one I have more trouble with.
Pakistan’s nuclear tests on May 28 and May 30 had yields of 9-12 kt and 4-6 kt, based on the seismic signals. (The announced yields were 40-45 kt in five devices and 15-18 kt in the sixth device.)
In other words, I don’t understand how a Pakistani device that was tested can have twice the yield of the 4th Chinese test, because those two events had about the same bang.
Indeed, Warrick — in contrast to Sanger — implies that the yield is the same:
The lethality of such a bomb would be little enhanced, but its smaller size might allow for delivery by ballistic missile.
“To many of these countries, it’s all about size and weight,” Albright said in an interview. “They need to be able to fit the device on the missiles they have.”
I lean toward thinking the yield of the device is about the same, though I am open to be persuaded otherwise.
Implications for Iran
First, let me begin by noting that the Chinese design was deliverable by a missile — it was tested on a DF-2. But that’s just being pedantic — the DF-2 had significantly more throw-weight than the Nodong/Ghuari/Shahab. The 1966 Chinese design, as I have said before, wouldn’t be much good for Iran.
Some of you may recall the question that David Albright raised about the so-called Laptop of Death — that the RV was designed to accommodate a warhead of only 60 centimeters in diameter:
Another important question that is sidestepped by the misleading use of warhead in the article is whether Iran can build the relatively small nuclear warhead able to fit into the triconic re-entry vehicle apparent in photos of a 2004 flight test. Based on publicly available photos of the 2004 test launch, the nuclear warhead would require a diameter of about 600 millimeters. Achieving such a diameter would be challenging for Iran. For example, the diameter of the warhead in the design provided to Libya (and perhaps to Iran) by A.Q. Khan was about 900 millimeters. A legitimate question is whether Iran could successfully build such a small nuclear warhead without outside help.
Obviously, then, the 1998 Pakistani design may be helpful for a state like Iran.
On the other hand, it isn’t clear to me that simply receiving a design, without any domestic testing experience, is enough to have confidence. As North Korea discovered in trying to jump to a weaponizable device, there are apparently still a few tricks of the trade.
Still, interesting stuff.