Question

what is nuclear fusion and what is nuclear fission

Answer 1

$hi$
$here \: is \: your \: answer$
$nuclear \: fusion -$
a nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy.
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$nuclear \: fission -$
a nuclear reaction in which a heavy nucleus splits spontaneously or on impact with another particle, with the release of energy.

$hope \: helps:D$

Answer 2

Nuclear Fission : The process in which the heavy nucleus of a radioactive atom splits up into smaller nuclei when bombarded with low energy neutrons, is called neuclear fission.

Nuclear Fusion : The process in which two nuclei of light elements combine to form a heavy nucleus, is called Nuclear Fusion.