Ft. St. Vrain was a sad example. That was a nice gas-cooled reactor. But they had some leakage in the helium circulator, which was supposed to keep water and helium separate. This resulted in corrosion and eventual reactor shutdown and decommissioning.
The Three Mile Island meltdown was due to a valve problem. Nobody was hurt, but cleaning up the mess cost billions.
That's part of the problem with all these new reactor designs. A good design with 20 years of operating experience is preferable to a new design, where you don't know what's going to give trouble.
Hyman Rickover on reactor design: "An academic reactor or reactor plant almost always has the following basic characteristics: (1) It is simple. (2) It is small. (3) It is cheap. (4) It is light. (5) It can be built very quickly. (6) It is very flexible in purpose. (7) Very little development will be required. It will use off-the-shelf components. (8) The reactor is in the study phase. It is not being built now.
On the other hand a practical reactor can be distinguished by the following characteristics: (1) It is being built now. (2) It is behind schedule. (3) It requires an immense amount of development on apparently trivial items. (4) It is very expensive. (5) It takes a long time to build because of its engineering development problems. (6) It is large. (7) It is heavy. (8) It is complicated."
I worked a few summers in a small, wood-burning power plant. A couple times a year, they would shut down for one or two weeks, and do some heavy maintenance on different systems. One of the times I was there, they replaced all of the tubes for making steam in the boiler. Every single weld joining together the panels of tubes had to be inspected, and xrayed to ensure that there were no gaps and sufficient penetration. Finding people that can weld to that standard is a more difficult task, because it's not easy, and takes skill, and training, and experience. Moreover, those people who have those skills can command higher rates.