So, do these operate on the same design principles as the various Gen 2 reactor designs that have been involved in nuclear accidents, e.g. Three Mile Island, or Fukushima? I raise this question not out of an anti-nuclear hysteria, but a wondering if the safety issues that seem inherent in those designs are addressed in these micro-reactor designs.

I'm not particularly worried about shutdown mechanisms, but more so about containment of decay heat after an emergency shutdown event. By my understanding, our current reactor designs still produce decay heat for a time after shut down, and that heat still needs to be dealt with. And it was this decay heat and the failure to control it that lead to the Three Mile Island and Fukushima incidents.

Decay heat isn't a concern for naval reactors because 1) they're much smaller than a commercial power reactor, with order(s) of magnitude less output, and 2) they have the ocean to use as a heat sink to prevent overheating. Micro-reactors probably also have the first advantage, with outputs ranging from 1-20 MW. However, 60-1200 kW is still not as easily shrugged off when you don't have an ocean to dump heat into. IME, 60-1200kW is enough to heat steel, if not to melting, to certainly a loss of strength, and that does give me some worry about damage to containment.

So I guess my main concern is thus: Are these things being designed to be able to safely shed their worst-case heat completely passively in an emergency?

If so, awesome, and I look forward to the possibilities these designs open up.