Both the Chernobyl and Fukushima nuclear disasters were examples of engineers failure to anticipate all possible human or natural factors in maintaining a reactor.
Chernobyl was caused by the senior operator ignoring all reasonable safety rules of nuclear physics and continuing a test procedure that was unsafe because he would lose his job if he didn’t complete the test on time.
Fukushima was caused by a compound risk event that was never considered by the designers. The earthquake disabled the electrically driven cooling water pumps and the tsunami that followed disabled the backup diesel generators causing the core to meltdown and explode when hydrogen gas generated by the water exploded without a pressurized containment building to prevent it.
All during this time hundreds of other nuclear reactors have been safely operated around the world and in many cases improved by what was learned from these catastrophes. Two technologies, Small Modular Reactors (SMR) and Stable Molten Salt Reactors (SMSR), radically change the complexity and weapon potential of nuclear reactors by scaling down the core size and eliminating the need for active cooling systems to prevent meltdown. They simply make heat, boil water, and drive turbine generators in a self sustaining cycle. If they overheat they shutdown even without a human in the loop.
In the end we can not let an irrational fear of fire drive our engineering decisions or we will have to return to the stone age.
Chernobyl was caused by an experimental reactor with a high positive void coefficient (i.e. unsafe by design) being pushed too far. Nobody in the world runs reactors like that anymore (all existing RBMK-1000 reactors have been retrofitted to make them passively safe). Any newly built nuclear reactors are designed to be passively safe.
Fukushima was an example of an irresponsible company with a regulator asleep at the wheel. The cooling system design was known to have major design issues, but nobody made TEPCO fix it. They had all sorts of other regulatory non-compliance issues that nobody made them fix either. The culture in their nuclear regulatory space is extremely lax, from what I understand. They’re all pals hanging out together, rather than anyone providing real oversight.
I’m generally against a lot of regulation, but there are a few places where I see its value. Nuclear power can absolutely be done very safely and effectively, but there needs to be real motivation to do it correctly on the part of the plant designers and operators and there needs to be real ongoing oversight to ensure things are continuing down a safe path. If the design is good at the start, there’s not much you can do to screw it up down the road from a safety standpoint, but you can definitely have availability problems.