Question

why nuclear fission occur before nuclear fusion​

Answer 1

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When combining low mass elements, the product has less mass than the starting stuff and you also get energy. So, breaking large atoms gives energy (nuclear fission) and combining small atoms also gives energy (nuclear fusion). ... As of now, there are zero useful fusion reactors.

Answer 2

Fission is nothing but divididing

fusion means mixing .nuclear content must divide as per the stages of cell cycle to maintain chromosome number than later nuclear fission