With this GoPro footage of the Penn State research reactor, you can see the intense blue glow of Cherenkov radiation which occurs when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium.
The phase velocity is slower than in a vacuum and thus electrons can outrun the phase velocity of light in water. A common analogy is the sonic boom of a supersonic aircraft or bullet.
The sound waves generated by the supersonic body propagate at the speed of sound itself; as such, the waves travel slower than the speeding object and cannot propagate forward from the body, instead forming a shock front.
In a similar way, a charged particle can generate a light shock wave as it travels through an insulator. The hum in the first video example is from the nitrogen-16 pump which hangs from the tower of the Beazeale Nuclear Reactor.
The second example is one of the Annular Core Research Reactor (ACRR) which generates a whopping 35,000 MegaWatts of power for seven milliseconds.
For more in-depth guide into the world of nuclear energy, check out the third post for a fun animation discussing the impact of this technology in today's modern world.