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.
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?