Nuclear fusion reactions release high energy and required tremendous temperature of nuclear reactors is the main reason for not used to generate electricity.
Nuclear reactions are those reactions which are taking place in the nucleus of any unstable atom and produces energy.
As in the question it is given that nuclear fusion reactions releases more energy and less wastage, but this energy are releases in the tremendous amount which can be proved harmful for the environment and the humans. To carry out this reaction we have to maintain the very high temperature of nuclear reactors of about 6 times greater than sun's temperature, which is not possible manually.
Hence, high temperature of nuclear reactors and high amount of energy released is the main reason.
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