What does that have to do with whether the technology exists to improve nuclear?
I hope you realize energy is regulated and taxed such that it is controlled by cartels. Getting rid of that doesn't imply a Dark Age, rather it frees us to realize innovations which were suppressed because they are paradigm shifts which compete with the cartels.
You fail to understand anarchy. Anarchy had never ever been something different than self-sufficiency. Whoever is (had been) self-sufficient, doesn't have a desire to trade with aliens and building (maintaining) complex structures. Anarcho capitalism is an oxymoron promoted by pseudo-anarchists. They are collectivists.