What is the name of this sci-fi-ish effect that occurs in some conditions, especially in nuclear reactors?

The effect is named after this chap:

Cracked by: varuns88 , akhi , udupendra , alahk_niranjan , Goyal , yaksha , priyananda and Prasad.

Answer: Cherenkov Radiation.

Happens when a particle travels through a medium faster than the speed of light in that medium (it’s only the speed of light in a vacuum that’s an unbeatable upper limit)