I WANT to thermalize the fission products. I DON'T want to try and collimate relativistic ions; it is far far easier to dissolve the lithium salt in water and let the water serve both as a thermalization medium and as the pressurized reaction mass. The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.
Quote from: sevenperforce on 03/26/2016 11:08 pmI WANT to thermalize the fission products. I DON'T want to try and collimate relativistic ions; it is far far easier to dissolve the lithium salt in water and let the water serve both as a thermalization medium and as the pressurized reaction mass. The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.If you thermalize neutrons and fission products, you get a NTR and/or NSWR. If you don't thermalize them, you get a fission fragment engine (which is a good engine... if you can overcome engineering/materials challenges).
A neutron multiplier without a radioactive byproduct, if it exists it would certainly be desirable. From what I can find Beryllium dose multiply neutrons by splitting into 2 alpha particles + 2 neutrons and by transmuting into carbon + 1 neutron upon absorbing alpha particles so it seems the High Flux reactor noted earlier is likely making significant use of such multiplication.The only other elements I can identify that are good multipliers are Lead, Bismuth and at a much reduced rate Zirconium which has the advantage in a much higher melting point. All of these can expel 2 or 3 neutrons when absorbing a thermal neutron WITHOUT fission though they do form radioactive isotopes like Polonium. But all are considered inferior to Beryllium on Earth before mass has even become a concern as it would in an engine, only the toxicity of Beryllium and it's cost are an issue, and these are likely to be surmountable in a rocket engine.In addition I'm reading that carbon is considered a good neutron reflector and it's low density would be desirable in keeping the engines power to weight ratio up. I'm envisioning an engine with a conventional combustion chamber throat and funnel with one or more hollow central channel reactors injecting lithium salt into the combustion chamber.
Are there any fissionable isotopes which produce a chain reaction without nasty nucleotide products?
It's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium.
Quote from: sevenperforce on 03/30/2016 01:24 pmIt's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium.You are still fixated on lithium.What "high energy of lithium"? It gives you 4.78 MeV from one Z=6 nucleus fissioning, a bit below 0.8 MeV per nucleon.Fission of usual suspects U/Np/Pu gives ~200 MeV per Z~=235 nucleus, which is about the same power density, actually a bit better: 0.86 MeV per nucleon.
You guys must be missing something about Lithium being such a nice fission fuel, since none of the existing fission reactors or bombs use lithium in any way. If it would be usable, surely it would be used by military instead of, or as an additive to the heavy and more expensive Uranium.
I think I see the flaw with the proposal. To fission Lithium, you need a neutron source to generate *one neutron for every Lithium atom*:Li-6 + n -> T + He-4 + 4.7829 MeVThen you need to avoid thermalization of fission fragments and you need to collimate them into a directional beam. How far away form 100% efficient that will be?You can save yourself a lot of trouble if you just use your neutron source as a rocket engine. Must be about the same ballpark wrt trust....Which hints that the showstopper here is that it's hard to generate that many neutrons. What do you propose to be your neutron source? What's its luminosity?
Neutron Induced Lithium Fission ReactionThe showstopper is, of course, the need to produce neutrons.If you have a cheap source of neutrons, you could also consider Calcium-48. If turned into Calcium-49, I think the decay chain Calcium -> Scandium -> Titanium gives a total of 12 MeV, via beta minus decay (I looked it up years ago, this is from memory). Not so good for an engine, but good as a powerplant.
I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.So, where exactly does this "clean" come in?
Quote from: Pete on 11/01/2019 11:19 amI'm a little boggled at how a system that expels masses of Tritium is considered "clean"?Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.So, where exactly does this "clean" come in?https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket#LimitationsIt's "clean" only in that it's cleaner than Zubrin's NSWR, which exhausts actively fissioning uranium/plutonium salts.