The author / artist clearly wants it to be a nuclear reactor, but I'm not sure it is a nuclear reactor. There are rods that seem like control rods, and weird symbols to warn people away... But if the ancient hero really knew what he was doing and made a nuclear reactor in another world, how did he screw up so badly that he put the
biohazard symbol (☣) instead of the
ionizing radiation symbol (☢)?
Did the ancient hero not get finish his nuclear engineering degree because he got isekai'd while still in college?
"That time I got hit by a truck and built a shoddy nuclear reactor in another world!"