Reply To: A Big Prick in the Balloon of the Big Bang

[jamesr]

JimmyT wrote:

I have been reading about mobile and deploy-able Focus Fusion Reactors and have a couple of ideas to offer.

For some of the applications (aircraft) weight and size are a big issue, most of the weight and size are concentrated in to three areas: Shielding, Capacitors and Cooling.

Idea #1:
Most of the size and weight is in the neutron moderator (is that the right word? I mean the material that slows down the neutrons, usually water). I realized that the neutrons could be trapped inside a container filled with water and Boron10, by using a neutron reflector wrapped around the water container (http://en.wiki.org/wiki/Neutron_reflector) something like the attached pic1. The neutrons would bounce around until they where slowed down enough to be captured by Boron10. Would this work? Would the increased radiation levels cause problems? Would the extra neutrons interfere with the main reaction?

Back when Rutherford first fired particles at materials and witnessed scattering, we learned what a small part of any materials volume is occupied by it’s nucleus. Only a small fraction of the particles were deflected. Most simply went straight through the targets as though it were not even there. Yet this is precisely the way that neutron reflectors work. Some materials have bigger nuclei or have them more densely packed (Beryllium) making them more effective. But even then, only a small portion of the incident particles are reflected. These materials are only called neutron reflectors compared with the even poorer reflective ability of other materials. But don’t think of them as a mirror. Any more than you would think of highly polished chicken wire as a light mirror. It does reflect some back doesn’t it?

I find your analogy is a little misleading. Neutrons are uncharged – so their chance of being scattered is even smaller than the charged alphas Rutherford’s assistants observed coming back off gold. But mainly – it is not a surface effect – since the chances of collision are so low the neutrons travel deep into the material before scattering. The reflection comes from many scatters deep in the material turning the neutron by different angles (most small). After a while some neutrons have the chance of being turned by a large enough angle to make there way back to the surface they entered from.

The mean free path (ie average distance between collisions) of a fast neutron in steel for example is around 6cm

So think many millions of layers of very fine chicken wire and you’re a little closer.