This is a video of a General Atomics TRIGA (Training, Research, Isotopes, General Atomics -- the most common non-power nuclear reactor in the world) starting up. Some more info while I watch it again and flinch again like it was just about to melt my face off, which it totally wanted to, I could tell:
The blue light is from electrons moving through the water causing Cherenkov [electromagnetic] radiation.
"The reactor is brought to criticality and then the transient control rod is forced out. This causes a power pulse of about 500-1000 MW."
Per some insightful Youtube comments: "And that's how I found out I am sexually attracted to nuclear reactors," and "The fountain of youth has been discovered I see, might just dip the ol'pecker in and see if it grows bigger, I mean it can't get any smaller right?" Ahahahhaha, admittedly, I did forget writing those.
Keep going for the worthwhile video (the audio really makes it 10x better).
Thanks to Terrance B, who agrees they should add a few rubber duckies floating on top of the water to lighten the mood a bit.