Every reactor design which had something mechanically or chemically complex going on inside the radioactive zone has had major problems. Pebble bed reactors jam. Sodium cooled reactors have fires. Helium cooled reactors have leaks. The interior environment is hostile, unmaintainable, and has to work for decades. That's why we're stuck with water-based reactors.
Well, yes and no. Basically LWRs are good enough for the time being. When/if there's a major expansion of nuclear power, we'll need breeders and reprocessing, but right now there's no pressing need.
Right now what nuclear power needs (from the technical POV) is to reduce the time and cost of construction. Which, most likely, won't happen by introducing new and unproven reactors and fuels.
