A nuclear reaction is a process from physics. In it, two or more particles collide. The result are new particles which are different from the first ones. In the case of radioactivity this reaction is spontaneous. It can also be done on purpose, to release energy. This energy can then be used for different things, for example to make electricity (as in a nuclear power plant). It can also be used as energy for a bomb.
In the example figure 6Li reacts with deuterium. This makes Beryllium which then decays into two alpha particles.
Nuclear reactions occur in the sun, in nuclear reactors, in particle accelerators, and in outer space. Other than radioactive decay, there are very few nuclear reactions that occur outside of these special places on earth. Nuclear reactors use nuclear reactions to make heat and electricity. Accelerators sometimes cause nuclear reactions to make radioactive materials. Particles from outer space cause nuclear reactions in earth's atmosphere that make air slightly radioactive.
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