Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Secondary Neutrons, uses of (U238)
Date: May 28, 4:45 pm
Poster: Jim Lux

On May 28, 4:45 pm, Jim Lux wrote:

>I am currently reading "Hitler's Uranium Club- The Farm Hall transcipts.". I got caught in the mindset of U-235 which was a hot topic in the book.
>U-238 (depleated uranium) is used for A-bomb casings and are indeed burned up by fast neuts.
>You would need a nice thick wad of uranium for the experiment. At 30.00/gram from suppliers and at its density, you will need some bucks.
> It can be obtain much cheaper (.10/gram) if you get a manufacturers site license. I am sure we will all rush out and let the government know all of our business and allow them to come in and look us over. (right!)

According to my handbook, the fission cross section for U238 for fast neuts is around .2 barns, and the radiative capture cross section is about .13. This is for fission neuts in the middle of a fast reactor (e.g. Godiva) which are distributed over a wide range, with only 38% above 1.4 Mev. For 2Mev neutrons, the cross section for U238 fission is .54 barns.

Grossly approximating the neutron diffusion (U diameter is about 3e-16 cm^2), I came up with: in traversing 1 cm of U238, there is a 4% probability that you'd get a fission. So, if you put a target covering, say 1/10 of the surface of the fusor, which is cooking away at 1E4 neuts/second, you should get 40 fissions/second.

I wouldn't trust the above calculations, by the way, but, even if I am off by a order of magnitude or so, it still looks interesting.

Last time I called about getting some DU for a pendulumn bob, they said that small quantities, unclad, would be hard to get, but 80,000 pound (carload) lots were free for the taking (I assume that you would need a license, though). Some fairly standard shapes are available clad in stainless at reasonable (i.e. basically the machining, <$1K) prices. But, there are other inexpensive sources of DU, airplane control surface counterweights being a notable possibility. Even though clad in stainless steel, it may still be usable. (I imagine that uncladding it would be one of those corrosive chemical, noxious gases kind of exercises).