Ka-freaking-boom. The piercing blue flash reverberates through the water and looks cooler than my imagination. If watching movies has taught me anything, it's that when you see that particular shade of glowing blue, something superhuman is happening. And, yes, a nuclear reactor starting up is as powerful as it gets.
Why does it glow blue? It's the Cherenkov radiation:
Cherenkov radiation is electromagnetic radiation emitted 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.
It's going faster than the speed of light (in water). The blue glow is basically a sonic boom, but for light.