Fusion
If hydrogen fusion (including deuterium and tritium) was easy, we wouldn't be here. Very small masses of hydrogen would have "burned" to iron very early in the formation of our galaxy; not enough time for complex life to form.
"Cold Fusion" aka "Low Energy Nuclear Reactions" assumes that deuterium fusion is easy - that is, it can happen with electron-chemistry energy levels (electron-volts) and solid-matter atomic spacings (nanometers) rather than the 300 KeV, 300 femtometer deuteron spacings encountered (for about one microsecond) in 100 million Kelvin temperature fission-fusion weapons, or the vastly slower (gigayear rates) proton-proton reactions that occur in the dense 15 million Kelvin temperature cores of gigameter-diameter stars like our Sun.