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I have a spaceship that's powered by an efficient hydrogen fusion drive, but the drive outputs a large amount of dangerous neutron radiation as a side effect, and those neutrons are not captured and directed away from the ship by its magnetic nozzle. Fortunately, my research indicates that hydrogen-based compounds can efficiently block neutrons, and liquid hydrogen is certainly hydrogen-based, so I plan to have the ship use its own reserve fuel as radiation shielding.

Question: How deep would a container of liquid hydrogen have to be to reliably stop neutron radiation?

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  • $\begingroup$ Aneutronic fusion is a thing. Doesn't work with ordinary "light" hydrogen, though. $\endgroup$ – AlexP Mar 29 '18 at 17:25
  • $\begingroup$ @AlexP , I considered that, but given the circumstances of the ship's construction, having a "pure" reactor that works off of normal hydrogen fits the story better. If I don't like the answer, then I might go with a helium-3 to helium-3 reaction instead. $\endgroup$ – TPK Mar 29 '18 at 17:35
  • $\begingroup$ Have you considered what happens to the crew when they have to use the fuel reserve? The reserve is typically meant to get you to safety, not take that safety away by exposing you to radiation. $\endgroup$ – Unassuming Guy Mar 29 '18 at 17:51
  • $\begingroup$ @Christian That would be the last part of the fuel to be used, after the ship has offloaded its human cargo (via colonization landers) and been re-purposed into a robot ship setting up autonomous mining in the system's asteroid belt. $\endgroup$ – TPK Mar 29 '18 at 17:58
  • $\begingroup$ @TPK Great idea. $\endgroup$ – Unassuming Guy Mar 29 '18 at 18:02
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There are three parts to neutron shielding:

1) fast neutrons - hydrogen atoms in water or paraffin serve to slow down neutrons. The neutrons share their energy as they elastically scatter off the protons. The neutrons ending up equally sharing energy with the protons, “thermalized” to the room temperature energy. I haven’t seen an exact calculation for liquid hydrogen, but it has more H atoms per cubic meter than water, so the water shielding tables should be an approximation. The amount needed depends on the incident flux and acceptable fast-neutron leakage, but 3-8m should cover a lot of dynamic range.

2) Thermal neutrons are still dangerous, so you need to absorb them. Hydrogen has a small chance of absorbing them, but for water shields it’s best to mix in boron because of its large absorption probability. Boron also has interesting fusion aspects, so maybe it can plausibly mix with your fuel. But if not, put a few cm as a shield wall.

3) But you also need to stop the 2.2MeV gammas from neutron absorption on H. They’re unpleasant. If you have other need for gamma shielding, you can combine those. Otherwise, you’ll need either some high-Z material or a bunch of water. (It’s important to keep the fast neutrons out of that water, or you have to start all over with the thermal neutron and gamma shielding)

I can add numbers to this if you’ve got an estimate of the neutrons per second.

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  • $\begingroup$ So I'd also want something like some Boron carbide or a similar substance between the liquid hydrogen tanks and the crew compartments, or even just plating the crew compartment all over. That sounds like it'd work. $\endgroup$ – TPK Mar 30 '18 at 12:10
  • $\begingroup$ Boron carbide is good for neutrons, but it's not so good for gammas. For that, you need something with heavier nuclei. Steel and titanium turn out to be pretty good, aluminum is OK, so if you can put your crafts structural elements on the outside of the crew space that'll cover some of the need. $\endgroup$ – Bob Jacobsen Mar 30 '18 at 15:59
  • $\begingroup$ sciencedemonstrations.fas.harvard.edu/presentations/… This link is a study where 1cm of lead stopped half the gamma from some Cobolt-60, which emits 1.3MeV gamma. So somewhere in the order of a quarter of a meter sounds somewhat reasonable for lead? $\endgroup$ – TPK Mar 30 '18 at 16:36
  • $\begingroup$ 25cm of Pb will probably drop the flux enough (pending how much power your fusion reactor needs to be putting out). It's massive, but Pb is pretty mass-efficient for this. (If you want to research materials, look up the "Radiation length" in units of g/cm2. That's the number of grams of material you need to shield 1/cm2 enough to drop gammas by 1/e; for lead, that's 6.37 gm: pdg.lbl.gov/2015/AtomicNuclearProperties/HTML/lead_Pb.html ) But material that's already there is always more efficient than shielding material you have to add. $\endgroup$ – Bob Jacobsen Mar 30 '18 at 17:01

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