What To Do With Those Neutrons?

One problem bugging me about so-called aneutronic fusion is the large numbers of neutrons it actually would produce in practice. To illustrate this, let's go back to an earlier post I wrote about a hypothetical 1GW commercial fusion reactor. Remembering that 1 GW = 1 GJ/s

1 GJ/s / (8.6x10⁶ eV/reaction * 1.602x10^-19 J/eV) = 7.3x10²⁰ reactions/s

Per Wikipedia, 0.1% of all fusion reactions would end up creating a neutron anyway from the ¹¹B + α side reaction. (Update 10/17: This ends up being a 2.7 MeV neutron, well over the threshold of a fast neutron, at 1 MeV.) This means that 7.3x10¹⁷ neutrons/s will be generated from the most widely discussed "aneutronic" fuel out there! That's a simply enormous number. Contrast this with reported background radiation at 2,420m above sea level of 65±3 neutrons/cm²*h. (I'm still trying to come up with a neutron flux figure for a commercial fission reactor.) But assuming a completed ICF device is something like 3m (rounding up a bit) in approximate diameter, and that neutrons are sprayed uniformly (this may not be a good assumption), that means you now have to deal with

4*π*(3m)²*1x10⁴cm²/m²*7.3x10¹⁷n/s = 6.46x10¹¹ n/cm²*s

In a fission reactor, you can use regular water to moderate those neutrons. But what do you do with a fusion reactor?

Coming soon in part 2: how this will affect the parts of the fusor itself thanks to this neutron activation calculator.

Update 10/15: A much better nuclide decay calculator at BNL.gov. "Nuclear Wallet Cards Search" seems almost designed to be impenetrable to Google.