We've effectively forgotten how to build (specifically engineer) nuclear reactors. This is very similar to problems faced by the space program somewhat recently. During the 3 decade pause people retired and memories faded, there were no apprentices to carry the wisdom forward.

> How can costs be reduced?

Build more reactors, and re-learn how to build them. Note that this doesn't touch on economies of scale, which will likely never really apply to nuclear power. Nuclear is likely to always have immense up-front costs, but it shouldn't cost this much.