Fusion uses hydrogen, which is free, because there is basically an unlimited supply of it in water.
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The benefits of fusion are many, but it’s important to remember and take note that water is not an unlimited resource. In many places of the world it is an incredibly scarce yet essential resource. If fusion is the way of the future, we will need to confront the water scarcity problem, that is assuming the water used in fusion cannot be reused and that “new” supplies of water cannot be made.