Nuclear energy can be harnessed by two ways, either we can break larger atoms into smaller atoms or we can rely on the formation of larger nucleus from smaller nuclei. The first process is known as the nuclear fission, while the second is known as nuclear fusion. In this process, smaller nuclei such as hydrogen combine together to from helium. This is the process that powers our Sun and other stars. To initiate this process, extremely high temperatures are required. It is simply not possible to generate such high temperatures for extended duration to maintain the process of nuclear fusion for longer time scales. This is the reason why nuclear fusion is not yet the answer to our energy crisis. Even in extremely controlled conditions of laboratory, nuclear fusion cannot be maintained for more than a small fraction of a second or so.
Till we can sustain the nuclear fusion for longer duration, nuclear fission is the only possible and practical solution of harnessing nuclear power. But yes, once we can sustain fusion, it can provide us clean energy for a really long time.
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