I was going to discuss something like this in connection with North Korea, but Netanyahu has scooped me. What follows is not to advocate that the U.S. should scrap the Iran deal. I tend to think like Mattis.
U.S. nuclear weapons are highly optimized designs, made as well as the mind and hand of Man can do. Russian nukes are not quite as good. Of the others, not much is publicly known, but you can throw away a lot of design potential and still get a bang. Some U.S. warheads, highly optimized for yield (explosive power) per weight, have been deliberately downgraded to smaller yields. This offers more flexible escalation of force. A target can be destroyed with the loss of thousands of lives, instead of millions. Offering POTUS the choice of “an eye for an eye, a tooth for a tooth”, the barrier to use, still high and horrible, is reduced to an exchange.
If a power knows enough about nuclear weapons, it is not mandatory to test the ones it builds. The knowledge is hard to come by, but it can travel on a thumb drive. It can take these forms:
- Exact design.
- Scaleable model of the whole device.
- Partial models, models of pieces of the puzzle.
- Simulation, a laborious process requiring lots of computer power, required by the less convenient models.
These words do not convey a hierarchy. One can possess a model for part of the design, yet lack for other parts. The Manhattan Project actually had a model for the behavior of the conventional explosives used to create the implosion. John von Neumann created the model with the Taylor shock wave equation . The calculations were carried out by Richard Feynman and his crew, before the advent of the modern computer, using punch card based IBM calculators. Today, your smartphone could solve it in between calls.
But they didn’t have the neutron cross sections that vitally predict the behavior of the plutonium sphere after it is imploded. They were missing parameters, numbers that all models require, and have to be filled in by experiments. Some of the trials, using the Demon Core, were deadly.
Critical mass is the smallest weight of material that when imploded in a particular shape will undergo chain reaction. If it holds together long enough, there is a nuclear explosion. The weight is very touchy. A penny under, and nothing happens. A penny over, you lose a city. Because it is so touchy, a lot can be learned by testing with a penny under. This is subcritical.
A major step in the U.S. program was the replacement of nuclear explosions with subcritical tests, in which the explosive power is solely the effect of the conventional explosives used to implode the plutonium sphere. Because the U.S. program aims for perfection, subcritical tests continue to the current day.
In theory, with enough subcritical tests, and parameters learned from actual nuclear tests, a weapon can be designed and built without actually testing it, with confidence that it will actually work. Since both Secretaries Pompeo and Tillerson were engineers, they know the Taylor Series (not the same Taylor) of freshman calculus. If two Taylor series, one on each side of the critical mass, are matched term-by-term, the result is a crude, open-source approximation of how a rogue state could vault into nuclear prominence.
Because the U.S. nuclear program aims for perfection, we neglect the possibility that a nuclear weapon could be produced solely with subcritical testing, and have a very good chance of serving both political and military aims. Such a weapon might have a 95% chance of not being a complete dud. If an Iranian weapon were designed, without testing, for a 200 kt yield, but the blast it produces when used is only 40 kt, would it be a failure?
How does this affect a decision for the U.S. to withdraw from the Iran treaty? There have been no apparent leaks of the negotiations. One can only speculate. Naturally, speculation has a strong political element. Some claim an irrational desire by Barack Obama for a successful negotiation. But this is speculation. We have described an alternative procedure to the orthodox path to becoming a nuclear power. Iran may have done this.
During the negotiations, the intelligence community may have decided that by this alternative path, Iran could build the bomb at will. The exact date at which Iran would build the bomb could be closely bracketed with clandestine knowledge of the supply chain. During this period of scrutiny and negotiation, the only thing Iran would give up is the actual test of a weapon which, untested, has a high probability of working. And the process of miniaturization has no specific requirement of testing.
So if the US. withdraws, Iran may in short order test a weapon, already fully miniaturized for delivery by missile. Nevertheless, a U.S. withdrawal from the treaty could be justified so as to politically isolate Iran. Now, as with North Korea, one has to consider how the ecosystem of international relations will react.
This defines the dilemma. I’ll pass on the solution.