Answer: Blue

Pop culture depictions of glowing green radioactive materials like the “ooze” in Teenage Mutant Ninja Turtles or the green glow of radiation at Homer’s nuclear reactor job in The Simpsons have done a remarkable job of cementing the idea in peoples’ heads that radiation is green, green, and more green.

So you might be very surprised to find out that the interior of a nuclear reactor doesn’t glow green at all, but an incredibly serene blue color that belies the danger found therein. The blue glow found in water-submerged nuclear reactor cores is called “Cherenkov radiation”, named after Soviet researcher Pavel Cherenkov, and is a form of electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. In layman’s terms, the glow results from electrons freed during the nuclear reaction zipping along at very high speeds through the water.

Why blue instead of another color on the visible spectrum? Cherenkov radiation is very high frequency and the majority of it isn’t even visible to the human eye (but is in the ultraviolet range just outside the edge of the visible spectrum). The blue we see is the very lowest end of the frequency emitted by the reaction and, just over the frequency line, so to speak, from ultraviolet, and we see it as very bright blue.