I live not that far away from a (now decommissioned) nuclear power station, and from what I keep hearing from old people who lived through the time of its construction, there was a lot of lenience and looking the other way involved. For instance, (A very persistent) Rumor has it, that many houses built in the nearest town around that time, had their foundation, cellar walls, and floors poured with the heavy anti-radiation concrete intended for the power station, because quite a number of the concrete trucks destined for the containment building (which is made of an awful lot of concrete) did conveniently take a wrong turn and ended up at the wrong construction site for some odd reason. (Maybe the drivers getting a case of beer for every detour they took might have been a factor)

Makes you think how any of the old plants were ever built.

Experience. The first one you build will have massive teething problems, but then the second one will just be a copy of the first.

The problem nowadays is that we in the west go "Oh no, learning stuff was expensive! Lets not use any of that and instead never do this again!" then, ten years later we start over again.

Nuclear reactors, despite being one of the safest forms of energy acquisition, have some of the strictest safety standards which drives up cost significantly.