The ultimate source of nuclear energy is not the sun, but rather other stars. Stars undergo fusion reactions that turn lighter elements like hydrogen into heavier elements like helium, carbon, and eventually uranium. When massive stars reach the end of their life cycle, they explode in supernovas, spreading these heavy elements across space. The uranium used in nuclear power plants was created in ancient supernovas and became part of Earth when it formed. Therefore, nuclear energy comes from other ancient stars, not directly from the sun like other energy sources.

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What is the ultimate source of nuclear energy?
We were asked this by a student after we made a generalization that all
forms of energy, including nuclear, derive eventually from the sun. That
is not exactly accurate. Nuclear energy does derive from a star, but not
the sun.
Stars use fusion reactions to turn matter into energy. Inside a star,
hydrogen, atomic number 1, is squeezed and heated to make helium and
heavier elements, step by step down the periodic table. This process
stops around iron, atomic number 26. Iron is so stable that once a star
has made enough of it, the iron starts to quench the fusion furnace, and
the star starts to die.
As a star dies, it collapses, and this collapse sometimes causes the core
of the star to become so hot, it explodes in a nova or supernova. During
this explosion, nuclear reactions are powerful enough to populate the
rest of the periodic table down to around uranium. The explosion also
blasts the star-stuff into space, where some of it was once lucky enough
to coalesce into planet Earth when our solar system formed.
The uranium on Earth, with its 92 protons and
146 neutrons, was formed in the explosion of a
star that was somewhere near here before the
formation of the sun. Nuclear power is not
solar in the same way that wind, hydroelectric,
photovoltaic, or even fossil fuels, which are
derived ultimately from plant matter, are.
Nuclear energy is perhaps more correctly
called a form of stellar energy.
7/2017