Monday, May 16, 2016

Compression and Criticality - a most delicate affair

So as we saw in the previous post on the issue of proliferation of knowledge relating to thermonuclear weapons design, we must proceed down two simultaneous but distinct lines of inquiry viz.

a) Has enough information leaked out about a TN device that it is possible for people to design it with minimal effort?

b) How does one live in a world where this information is no longer secret?

Let me focus on the first question in this post. There are three ways to ask this question

(a1) Can a single hyper empowered individual design a TN device from available knowledge?
(a2) Can a small group of individuals design a TN device from available knowledge?
(a3) Can a large group of individuals (corporation or state) design a TN device from available knowledge?

Now before I get into specific responses to a1, a2, a3, I want to touch upon the bigger underlying technical issue for designing a system based on complex nuclear reactions. 

At its core - any reaction ("chemical" or "nuclear")  needs to have reactants mix intimately with each other, and collide with sufficient energy to clear any energy barriers to making products. The products of the reaction typically have to be transported out of the way of newly arriving reactant species and that way the reaction can proceed in a continuous fashion till it actually runs out of reactant material. 

The big difference between "chemical" reactions (which involve changes in the electronic configuration of atoms and molecules) and "nuclear" reactions (which involve changes in the nuclear configuration of the reacting atoms) is that the amount of energy needed to make a nuclear reaction occur is much much higher than a chemical reaction. 

Atoms consist of a positively charged nucleus surrounded by a cloud of electrons. If the two nuclei have to react one must push the nuclei together and overcome the electrostatic (Coulombic) repulsion between the electron clouds and between the nuclei. This is requires applying a very large amount of force to the nuclear material.

The only reliable and demonstrated way of doing this has been to use compression. By putting pressure on the material you want to drive a nuclear reaction in, you can push the nuclei close to each other and overcome the large electrostatic repulsion between them. With the reactants now in place, the material reaches "criticality", i.e. the reactions can now proceed in a manner dictated by the local availability of the reactant species and the local outflow of the reaction products. 

The typical compression idea relies on detonating an explosive charge and then using the blast wave from that to compress a nuclear material. The explosive charge initiating the compression could be nuclear or conventional explosive but the key thing is that the blast wave produce a very symmetric shape inside the nuclear material under compression. 

If the material is not compressed for long enough for the reactions to actually take place then no significant yield can be achieved.  If the compression is not symmetric, then the nuclear material being compressed simply squirts out the weakest point in the compression front. 

Another added element of complexity is that as the reaction proceeds a lot of energy is created and it tries to leave the reaction volume. This creates a second blast wave that travels in a direction opposite to the one initiated by the compressing charge.  This wave interferes with the wave created by the first compressing charge and the resulting hydrodynamics can significantly alter the effectiveness of the compression event. As a result of this a very complex timing issue comes into play - essentially the first compression event has to line up very precisely with the anticipated rate of desirable nuclear reactions.

Compression and criticality is not exactly a marriage made in heaven. It takes a lot to get it right. This barrier exists purely in the physics of the system we are talking about, it can be overcome by sustained engineering efforts but those cost money and time. 

The ability to achieve a deliberate balance between compression and criticality in a secondary burn, requires resources that likely outside the hyper-empowered individual described in scenario a1.  Some of these resources may be available to the small group in scenario a2 but not all of them. The required resources should be available to the state-sized entity of scenario a3

One of the most crucial resources needed to design a well balanced compression-criticality based system would be simulation resources. Something like a large IAAS cloud platform could provide the level of computational power needed but one would still need to write a reliable neutron transport code and a reliable hydrodynamics code. The only highly reliable and tested codes for such things remain under lock and key at some national labs.  Codes of unknown reliability are available on the internet. As yet there is no evidence of an effort aimed at bench marking these "open source" codes against any other codes. 

So the only way that either the hyper empowered individual (a1) or the small group (a2) get access to actionable knowledge on reliable compression and criticality is via a state size entity (a3).  Such a knowledge transfer can occur voluntarily from the large entity as part of a strategic policy decision, or it can occur via hacking or infiltration. 

I will get into how one can possibly cope with this state of affairs in the next post. 

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