What are the issues in highly enriched nuclear reactors? [closed]

The kinds of reactors used in submarines and rockets, are known to have highly enriched uranium ~80% and CO2 as the moderator in subs, and carbon(graphite) I guess in rockets. What are the extra problems that arise compared to regular reactors? How are the control mechanisms more or less complicated?

• I don't know about issues that are directly linked to the level of fuel enrichment, but in both of the applications that you mentioned, refueling is not an option, and long service life is desirable. I have heard that in naval reactors, they start with some kind of neutron poison in the fuel that slowly burns off as it is irradiated, while at the same time, other neutron poisons (fission by-products) slowly accumulate. The end result is that the reactor's behavior is nearly constant over most of its lifetime. – Solomon Slow Aug 17 '18 at 16:58
• very many details are available-see-world-nuclear.org/information-library/… – drvrm Aug 17 '18 at 17:52
• in short: size matters. – ZeroTheHero Aug 17 '18 at 22:04

The other problem is that highly enriched Uranium isn't necessary for most reactor applications. Nuclear reaction rates are equal to $\rho$ * $\phi$ * $\sigma$, where $\rho$ is atomic density (atoms/cm^3), $\phi$ is neutron flux (total path length of all neutrons per cm^3) and $\sigma$ is the fission cross section (dependent on neutron energy and isotope). With enriched Uranium, you raise the cross section by a large factor- U-235 has a fissile cross section of around 500 barns at thermal neutron energies, whereas U-238 has a fissile cross section of around 1 barn at thermal energies. But if you want to "fully" utilize the power of the U-235 (use a neutron flux that doesn't threaten the structural integrity of the vessel), you have a meltdown. Typical reactor cores operate at around 1000 degrees C. At this point, you are close to melting point for certain reactor materials and are most likely dealing with problems related to materials softening/weakening under heat. If you increase the reaction rate by 500x, you have a meltdown - the limit for our strongest materials is too low.