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The biopic Fat Man and Little Boy depicts a fictional criticality accident based on the event which killed physicist Louis Slotin. This was one of two real-life lethal incidents involving a plutonium mass that would become known as the 'Demon Core
'.
Here's the relevant clip:
https://www.youtube.com/watch?v=AQ0P7R9CfCY
This seems to match pretty closely to what you saw.
Edited by Aspectifer

I only saw a clip of it on an article somewhere about nuclear weapons. So the critical mass sphere is accidentally completed with a blue flash by some carelessness, and, without thinking, the scientist removes it by hand to stop it from reacting. He mutters out some technobable and concludes that everyone else will be fine except him.
Edited by dvorak