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I was reading about thorium reactors here. It says 'Th-232 is fissionable with fast neutrons of over 1 MeV energy'. Most cosmic rays have lots more energy than that. I haven't found a reference for uranium but I suppose it must be similar.

And I was reading about current NTR designs here.

The NTR uses a compact fission reactor core containing 93 percent “enriched” Uranium (U)-235 fuel to generate 100’s of megawatts of thermal power (MWt) required to heat the LH2 propellant to high exhaust temperatures for rocket thrust.

It mentions elements that sound like maybe they exist to address this issue, but they don't make sense to me:

Multiple control drums, located in the reflector region surrounding the reactor core, regulate the neutron population and reactor power level over the NTR’s operational lifetime. The internal neutron and gamma radiation shield, located within the engine’s pressure vessel, contains its own interior coolant channels. It is placed between the reactor core and key engine components to prevent excessive radiation heating and material damage.

nuclear thermal reactor engine cutaway schematic

The parts marked shields really don't seem adequate to deal with cosmic rays. So are GCRs not enough to matter? Or maybe they are of little enough significance that minor shielding is all that is needed? Are GCRs considered in such designs?

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    $\begingroup$ You need a certain neutron flux density, measured in neutrons per gram second. I'm not sure what that is for Th-232, but it should be pretty high, it's not very fissionable. That's why there's neutron reflectors. $\endgroup$ – TildalWave Oct 30 '15 at 4:13
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Reactors are, in general, tuned so that the proportion of delayed neutrons (on the order of seconds to minutes after each fissioning) out of all neutrons is enough to make the reactor more or less dynamically stable. (Specifically, the prompt neutrons released within nanoseconds, which are upwards of 95% of all neutrons, aren't enough to match the neutrons consumed, so the delayed neutrons are essential to sustain the reaction for any length of time.) It takes some time for the delayed-critical chain reaction to speed up or slow down, enough time for the control elements to react automatically. So if there's a larger flux of cosmic rays that knock off more neutrons, nothing special happens: the control drums shift position as normal to compensate and the reaction continues at the same pace. Given the very large flux of neutrons in the core normally, it would take an enormous surge of cosmic rays to cause enough of an increase that the reactor became prompt-critical, i.e. explosive.

What's more, one of the defining features of most cosmic ray particles is that they are individually very high-energy. That makes them quite dangerous to living creatures, but because the total energy is spread across fewer particles, less dangerous to reactors. A given GCR can trigger a cascade of multiple particles, spreading their energy between them, but the probability of doing so in a way that creates the maximum number of neutrons with enough energy to trigger fissioning is low.

(There are no neutron GCRs; free neutrons have a half-life of about 15 minutes.)

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  • $\begingroup$ A thing to keep in mind is one needs a neutron source to jumpstart the engine anyway. $\endgroup$ – Deer Hunter Oct 30 '15 at 19:04
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A couple of things:

The reference OP provided for cosmic rays states:

Cosmic Ray Composition: Cosmic rays include essentially all of the elements in the periodic table; about 89% of the nuclei are hydrogen (protons), 10% helium, and about 1% heavier elements.

This implies that neutrons are not a significant constituent of cosmic rays. Since fission is driven by neutron absorption, reactor control is unlikely to be affected by non-neutron radiation.

Also, discussing fission from fast neutrons OP states:

I haven't found a reference for uranium but I suppose it must be similar.

It's not.

A search for "neutron absorption cross section," will yield more detailed explanations, but basically the likelihood of neutron absorption is highly dependent on the energy of the neutron, and for uranium, LOW energy neutrons are much more likely to cause fission. These neutrons are called Thermal Neutrons, or Slow Neutrons.

Speaking of Slow Neutrons:

Multiple control drums, located in the reflector region surrounding the reactor core, regulate the neutron population and reactor power level

These reflectors keep neutrons IN rather than provide protection from external neutrons. Neutrons born from fission are born Fast. If they slow down before the leave the core, they are likely to cause fission. If they don't slow down, they leak out of the core and do not cause fission. Neutron reflectors allow the system to control that leakage rate: if the leakage rate is too too high, reflect more neutrons back into the core so that they have a second chance to cause fission. If leakage rate is too low, reflect fewer neutrons back into the core.

Generally, controlling this leakage rate is done to control core temperature: the reactor remains critical (steady state) by design, because a change power causes a change in temperature causes a physical response like thermal expansion/contraction to change the leakage rate, which means the system returns itself to steady state. But the new steady state may not be at the optimum temperature, so an operator adjusts the reflectors / absorbers in the system to return the system to steady state at the temperature she desires.

As Nathan points out, this means the system is dynamically stable. If cosmic radiation does produce some Thermal Neutrons, the system will remain in steady state so long as the neutron production rate is low / constant compared to other sources of Thermal Neutrons.

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Neutron flux created by cosmic rays would be small and would actually make control of the reactor slightly easier in terms of neutron economy. The more non-prompt neutrons are involved in a reactor, the farther you can get from prompt criticality while maintaining the reaction, which makes safety easier. This slight benefit would be outweighed by the risk of electronics failing due to cosmic rays.

Spallation by high energy protons is commonly used to produce neutrons. This paper (http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/23/015/23015552.pdf) has a handy graph of proton energy versus the number of neutrons produced on a tungsten target. It's almost linear, with about 20 neutrons per GeV. This will vary a bit depending on the target.

Most cosmic rays are protons around 1GeV.(https://en.wikipedia.org/wiki/Cosmic_ray). Higher energy cosmic rays exist, but above 1GeV the flux falls off very rapidly, so higher energy cosmic rays contribute a negligible amount of the total energy. So let's approximate the cosmic ray flux as 10^4 1GeV protons per square meter per second. Let's suppose your space reactor is large enough to get hit by 10^5 cosmic rays per second that produce an average of 20 neutrons each.

Each neutron causes an extra ~0.5 fissions every 10^-4 seconds (because your reactor is thermal and delayed-critical). Each fission is around 200MeV, so each neutron is worth 100MeV every 10^-4 seconds or 10^6 MeV/s.

Final calculation: 10^5 protons/s * 20 neutrons/proton * 10^6MeV/s/neutron = 2*10^12 MeV/s^2 = 0.32 watts/s. So because of cosmic rays the reactor power level would increase by approximately 0.32 watts/s unless you lowered the control rods slightly to compensate (and thus got a bit farther away from prompt-criticality, which is good for safety)

If cosmic ray flux increased by a billionfold due to flying too close to a supernova, then it might be enough to cause a meltdown, but in that scenario the meltdown is probably the least of your problems.

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    $\begingroup$ This sounds like it might have some truth in it, but for a good stackexchange answer you should cite some independent reference(s) to help validate it for others. $\endgroup$ – uhoh Feb 24 '17 at 6:27

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