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
Date: Jun 27, 2:27 pm
Poster: Larry Cress

On Jun 27, 2:27 pm, Larry Cress wrote:

>Now that we are making neutrons, what about using them to fission U238? Depleted uranium is fairly available (actually, almost free, in multi-ton (>40) lots, because there is a huge surplus). Using the n from the DD or DT reaction, you can hit a piece of U and generate 2 more n's, etc.

Each fission will indeed generate slightly over two more neutrons, but due to non-fission capture and degradation of the neutron spectrum to energies below the approximate 1 MeV fission threshold of U-238 by inelastic scattering, the multiplication is less than at first seems reasonable. In a small assembly, in addition, lots of these rapidly moving neutrons will be lost through leakage across the system boundary, and not contribute to further multiplication.

Los Alamos report LA-697 reports an experiment in which a massive pseudo-sphere of natural uranium surrounded a D-D source. The uranium weighed over 3 tons. A maximum multiplication of 5.7 was reported.

Increasing the energy or density of the deuterium nuclei in the confined plasma will increase the neutron yield with less effort than surrounding the device with a massive uranium multiplier.

If neutrons are what you're after, a conventional low-voltage accelerator, perhaps with a single high-voltage gap in a vacuum, will give about 300,000 D-D neutrons per second per microamp at just 100 KeV. It's not too hard to design an ion source to give 100 microamps of deuterons, or 30 million neutrons per second. Such an accelerator would be only slightly more difficult to build than a fusor. If the target is run at negative polarity, the ion source can be run at ground potential, allowing water cooling of the source and avoiding many of the "smoke test" problems to which the fusor is prone.