The blue light is known as Cherenkov radiation. It is similar to a sonic boom, but instead of an object travelling faster than the speed of sound, a charged particle is travelling faster than the speed of light in a medium. In this case, the speed of light in water is roughly 75% the speed of light in a vacuum.
It tests the time-dependent properties of the fuel. Engineers use that data to predict what would happen if there were an accident at a large reactor
Cherenkov radiation, also known as Vavilovndas
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