Note: said society is preindustrial and has ample access to flowing water, but not fossil fuels or radioactive deposits. It also has access to mountain hot springs, which I thought could also be used as “batteries” during times when streams freeze. This last point is what set him off.
He also really hates wind turbines.
Naturally occurring nuclear reactors can happen, there was one that was active for a few hundred thousand years about 1.7 billion years ago in Gabon.