Thursday, October 11, 2007

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 "Nuclear Wallet Cards Search" seems almost designed to be impenetrable to Google.