Science

Economical energy production, sure, not any energy production. There is a reason we no longer burn wood to heat public baths.

I realize the science marketing of fusion over the past 60 years has been 'unlimited free energy', but that isn't quite accurate.

Fusion (well, at least protium/deuterium) would be 'unlimited' in the sense that the fuel needed is essentially inexhaustible. Tens of thousands of years of worldwide energy demand in the top few inches of the ocean.

However that 'free' part is the killer; fusion is very expensive per unit of energy output. For one, protium/deuterium fusion is incredibly 'innefficient', most of the energy is released as high-energy neutrons which generates radioactive waste, damages the containment vessel, and has a low conversion efficiency to electricity. More exotic forms of fusion ameliorate this downside to a degree, but require rarer fuels (hurting the 'unlimited' value proposition) and require more extreme conditions to sustain, further increasing the per-unit cost of energy.

Think of it this way, a fusion plant has an embodied cost of the energy required to make all the stuff that comprises the plant, let's call that C. It also has an operating cost, in both human effort and energy input, let's call that O. Lastly it has a lifetime, let's call that L. Finally, it has an average energy output, let's call that E.

For fusion to make economical sense, the following statement must be true:

(E-O)*L - C > 0.

In other words, it isn't sufficient that the reaction returns more energy than it requires to sustainT, it must also return enough excess energy that it 'pays' for the humans to maintain the plant, maintanence for the plant, and the initial building of the plant (at a minimum). If the above statement exactly equals zero, then the plant doesn't actually given any usable energy - it only pays for itself.

This is hardly the most sophisticated analysis, I encourage you to look more into the economics of fusion if you are interested, but it gets to the heart of the matter. Fusion can be free, unlimited, and economically worthless all at the same time.