Reply To: T-shirt designers unite and take over

[Axil]

Brian H wrote:

Believe me: DPF’s and fission are compatible. It’s a fusion device and fusion is done with light elements. And DPF’s are all about an attempt to “make watts, not rads”

I think you meant “incompatible”. DPFs are vastly more efficient than any possible fission rig.

It all depends on what you mean by the word efficient!

Here is how the fusion/fission hybrid neutron economy breads down.

I will address the thorium fuel cycle since it is highly proliferation resistant (I think proliferation proof) when coupled with fusion in preference to the uranium fuel cycle and its plutonium (a proliferation risk) byproduct.

An aneutronic boron reaction nets about 8 MeV of power and that’s it. On the other hand a d-d fusion reaction that breeds thorium nets about 200 MeV per initial thorium fission and that in turm will breed with 3 other secondary thorium atoms to net 800 MeV per each fusion/fission reaction.

In other words, each D-D fusion produces one neutron that breeds 4 U233 atoms.

An ideal thorium breeder reactor design will have a breeding ratio of 1.07. But due to neutron losses from the accumulation of isotopes of various kinds that poison the fission reaction, a thorium breeder will be lucky to break even. Let us assume a real world worse case breeding ratio in the range between .95 and .99.

So in round numbers, for every fusion neutron produced, about 400 thorium fissions can result. At 200MEv per fission that is (200Mev) (400) = 80000MeV per fusion neutron; as opposed to just 8Mev for boron fusion. That is an increased energy density factor of 10,000.

However, if not supplied with a small number of supplemental fusion produced neutrons, the thorium breeding nuclear reaction will eventually stop because the thorium breeding ratio is just a little less than one.

Without an occasional dose of fusion neutrons, the thorium fuel cycle will wind down to a stop.

It is not an overstatement to say that for the lack of an occasional neutron, the thorium fuel cycle is lost.

Those occasional neutrons can come from U235 and even Pu239 but fusion neutrons are clean and pure. It enables a pure thorium fuel cycle that leaves no long-lived nuclear wastes about.