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Nuclear fission splits atoms in a dirty, intrinsically risky process.
By contrast, nuclear fusion releases energy by combining them in a physical process that is relatively clean and safe.
Fusion, although it has been proven to work in the lab, remains a pipe dream as it is a long way off becoming commercially viable. At present, more energy has to be put in to get the reaction going than can be extracted back out. Until that balance shifts, commercial fusion remains a hope, but not a reality.