Fusion is incredibly difficult just to start, let alone keep burning - unlike fission, which is only too happy to enter runaway conditions if not very carefully regulated. Fusion is like a little ember in your fireplace you have to carefully blow on to keep alight; fission is like keeping a fireplace lit by pouring gasoline into it.
Building bridges (and large structures in general) has always been about the tension between over-engineering (for safety) and under-engineering (for cost/aesthetics).
The Brooklyn Bridge is massive; they'd never built a bridge like that so they over-engineered it. But once they saw that it was more than strong enough to stand up, the next bridge was lighter. And the next one after that was even lighter. And so on, until a bridge collapses because some new factor came into play (e.g., harmonic resonance).
Source: To Engineer Is Human by Henry Petroski--one of my favorite engineering books.
Indeed there's no such thing as a free launch, and that is rocket science.
https://en.wikipedia.org/wiki/CIRUS_reactor
> "Canada stipulated, and the U.S. supply contract for the heavy water explicitly specified, that it only be used for peaceful purposes. Nonetheless, CIRUS produced some of India's initial weapons-grade plutonium stockpile,[6] as well as the plutonium for India's 1974 Pokhran-I (Codename Smiling Buddha) nuclear test, the country's first nuclear test.[7]"
https://library.sciencemadness.org/library/books/ignition.pd...