Rosatom SO6 “Sunshard”

Rosatom SO6 thorium thermal nuclear fuel rods (commonly called “sunshards”) are a standard fuel for early interplanetary NTR designs. Produced since early 2100, SO6 utilize liquid thorium-based core encased in a solid shell. Composition of the shell is a closely guarded secret. Reverse engineering shows mainly tungsten, vanadium carbide and graphene. Shell is known to be constantly under large residual compressive stress with positive feedback from expanding thorium core. The stress and composition yield extremely high operating temperatures. Rosatom lists 3500K as standard operating temperature and 4500K as failure point, but there are unconfirmed reports of sunshards being able to withstand over 5000K and recover safely. Unfortunately, due to enormous residual internal stress failure mode is usually explosive.