The US Navy has probably around 100 nuclear powered vessels, both submarines and Nimitz class carriers. Each of those have miniature nuclear plants on them.

I know their use cases are different but small and portable is small and portable. Virginia class subs typically stayed within cost budgets, but newer V blocks saw cost overruns, as well as the Gerald Ford carrier, which was about 3 billion over budget if I remember.

Not sure if overruns were due to being nuclear or because of other reasons. They are high tech military items that aren’t exactly mass produced, so lots of ways to overrun. However they are more mass produced than nuclear power stations in the civilian sector. Maybe some lessons can be learned.

Edit: Also forgot an important point that modularization was a key design point of the Virginia sub.