Why nuclear fission is relatively easy compared to nuclear fusion

We’ve been reaping the fruits of nuclear fission for decades, and the process is relatively simple– shoot neutrons at radioactive elements, bust them apart and start a reaction that produces energy. But nuclear fission, for all of its promises of massive energy gains with only helium and hydrogen as byproducts, fission is a real pain in the ass. There are fusion reactors out there, but so far, there are none that are actually efficient. One massive billion dollar machine, squashing atoms together for a small output of energy.
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