09-25-2021-1522 - The first nuclear fission process discovered was fission induced by neutrons. Because cosmic rays produce some neutrons, it was difficult to distinguish between induced and spontaneous events. Cosmic rays can be reliably shielded by a thick layer of rock or water. Spontaneous fission was identified in 1940 by Sovietphysicists Georgy Flyorov and Konstantin Petrzhak[3][4] by their observations of uranium in the Moscow MetroDinamo station, 60 metres (200 ft) underground.[5]

The first nuclear fission process discovered was fission induced by neutrons. Because cosmic rays produce some neutrons, it was difficult to distinguish between induced and spontaneous events. Cosmic rays can be reliably shielded by a thick layer of rock or water. Spontaneous fission was identified in 1940 by Sovietphysicists Georgy Flyorov and Konstantin Petrzhak^[3][4] by their observations of uranium in the Moscow MetroDinamo station, 60 metres (200 ft) underground.^[5]

https://en.wikipedia.org/wiki/Spontaneous_fission