Questions about combining Nuclear Fusion and Fission

[TheBFG81]

First of all I'd like to thank all those who stop by and take a look at this question, doubly so for anyone who leaves a useful comment.

 

I’ve been reading a few articles recently about nuclear fusion and fission and had some questions which I was unable to find an answer to. Unfortunately Math (Specifically Calculus) is not a strong point of mine so please try to keep that in mind when responding. Let me start by explaining the theoretical situation I have in my head.

 

Say we start by using a Farnsworth–Hirsch Fusor to produce some neutrons for us. And using beryllium to contain the radiation and focus the neutrons on some fissionable material what would the effect be of increasing the amount of neutrons acting on this sort of material be.

 

I see either more atoms splitting then normal causing an increase in the amount of energy produced by the material thus “burning” the fissionable material faster than normal or the neutrons joining with the materials nucleus increasing the atoms instability causing it to be more likely to release alpha or beta particles.

 

Could this be used to increase the effectiveness of certain fissionable materials? Would the beryllium keep the deep penetrating neutron radiation at bay enough to make the device safe for producing energy? Is my understanding of the situation or the results of the situation incorrect thus leading to an outcome I haven’t listed (maybe this creates a chain reaction and the whole city goes up in a mushroom cloud)

 

 

[swansont]

It's posible that with extra neutrons that you could sustain a reaction with an otherwise subcritical mass of fissile material or some amount of fissionable material. It will depend on what the material is, because some fissionable but non-fissile materials require energetic neutrons to initiate fission, and there's the issue of net neutron yield from the reaction. Adding extra neutrons generally won't change the probability of capture vs. fission (though this can depend on the neutron energy), so you will still have these losses. A neutron reflector does not scatter all of the neutrons, so you still have to shield against them, but that's not a new problem — all reactor have this issue.

 

Your concern about the whole city going up is not off-base. A nuclear explosion is unlikely (that's hard to do) but with an external source of neutrons, the fission reaction will not automatically shut down or at least regulate itself, which is something you can do with fissile materials and proper construction.

 

I think the practical objection is that the technology would be more complicated and expensive, so why bother with it?

[TheBFG81]

The hope would have been to shrink the set up and foot print a normal nuclear reactor takes up and to possibly use a less dangerous material (like maybe thorium) in the process thereby allowing one make such a device more widespread.

 

The Farnsworth–Hirsch Fusor can be made small enough to fit on a table and is apparently simple enough home armatures have built them for science experiments.

 

However if a small beryllium casing around the device wouldn't be enough to shield it then as far as I am aware the only solution for shielding neutron radiation is several feet worth of concrete and that’s not something you could easily add to your average home basement.

 

I'm pretty sure I could use this set up to change the thorium into something more useful for a nuclear reactor but would I also be able to keep it relatively safe and small enough to make it so you could provide a buildings power supply with is without making a whole other building. It doesnt sound like it.