`one of the most "advanced" -- has about 72 hours of heat-absorption capacity.` That is still a pretty decent "oh shit" buffer. I imagine Fukushima or Chernobyl would have benifited from 72 hours extra to get their act together.

Well, I'm not sure. Chernobyl isn't relevant because its issue was one of supercriticality (milliseconds, explosions), not decay heat (hours, gradual melting). In Fukushima, there actually were passive systems, not as sophisticated as ESBWR, and some failed instantly (a steam injector in reactor #1). And IIRC off-site power too a lot longer than 72 hours to restore, but I don't know if the explosions contributed to this. I'm not a nuclear engineer so I can't meaningfully assess this. But there is data here.

As a curiosity, ESBWR is actually a direct descendant of the BWR/3 at Fukushima, from the same designer (GE Nuclear). Not a scare tactic: I'd be pretty happy living next to an ESBWR.

So 72 hours = 3 days would be not enough alone, but I also don't know how the other actions had made the situation better or worse.

The first seconds/minutes are the most critical and after 72 hours the residual heat is down to a level which can be much more easily handled.