Cherenkov radiation in an open nuclear reactor.
NRC picture of “Cerenkov radiation” surrounding the underwater core of the Reed Research Reactor, Reed College, Oregon, USA.
The file is copied from Wikimedia Commons {accessed 2021-01-01}: https://commons.wikimedia.org/wiki/File:Cerenkov_Effect.jpg
cerenkov_radiation_us_nuclear_regulatory_commission.jpg
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United States Nuclear Regulatory Commission, Public domain