Explanation:
A nuclear fusion is defined as the reaction in which two small nuclei will combine together and result in the formation of a heavier nuclei.
As the nucleus of both the combining atoms are involved in a nuclear reactions. Hence, there will be electron clouds repulsion between these combining nuclei.
Therefore, these atoms will not be able to combine then. As a result, very high heat or temperature must be provided so that these electronic repulsion can overcome and tend to form a heavier nuclei.
