Fusion is a different process to fission. Two nuclei come together and form a bigger nucleus, giving off lots of . Far more energy is released by than fission. The nuclei concerned are deuterium and tritium, both of which are of hydrogen.

Extreme conditions are needed to force the nuclei . Temperatures of several degrees Celsius are needed.

Nuclear involving fusion have been set off. However it is harder to get fusion reactions to happen in a way. So far it has not been possible to make a commercial fusion .

Fusion occurs in .