Bear in mind that these reactors were built in the 1970s. Modern designs are a lot better, and some shut down by themselves upon loss of power, with no need for active measures at all. Even some reactors built in the 1980s got through the events at Fukushima just fine.
I worked on airframe design in the 1970s. The ethos of design to withstand failure was well-entrenched then, and goes back a long way in the airframe industry.
The key idea is nobody has managed to design parts that won't fail. But they have figured out how to design highly reliable systems out of parts that fail.
Sure. The American nuclear industry kinda caught up with that after Three Mile Island, when they figured out that one nuclear accident put them all at risk. They developed an extreme culture of safety after that.
Current designs use lots of redundant safety systems. New designs depend more on simple physics...eg., fuel that expands enough when it heats to damp down the reaction, or large quantities of coolant with very high thermal capacity and conductivity. At Argonne's IFR project, they shut down coolant circulation and electric power, and the reactor quietly shut down with no damage.
The problem was not in shutting down the chain reaction - that worked just fine. The problem was in dealing with fuel rods that were still, due to decay processes that nothing in the universe can stop, producing massive amounts of heat that, without active cooling, would melt them and re-start the chain reaction.
Sure, but if the reactor can go a week without cooling and not sustain damage, you've got a lot more leeway. Some of the GenIII+ designs we have now can do that.
The ultimate solution is probably the molten salt reactor. With liquid fuel you continually remove fission products, and with most of those gone you don't have decay heat issues anymore.
