There are two real problems with basic high pressure nuclear reactors.
The major one is steam explosion.
This will meltdown any water cooled reactor independent of safeguards.
This is what ultimately destroyed Chernobyl.
Cause was a design flaw in the control rods and operator error.
The second is the High Power in most big plants.
On a SCRAM, new energy generation slows but there is still residual energy in the system.
The higher the power output at SCRAM, the longer the cooling pumps need to run to prevent a meltdown.
Usually 2-5 days depending on the power output of the plant.
This is what killed Fukushima.
The external power to the plant failed.
A modern plant requires about 20Mw just to function.
They lost lights, pumps, controls...everything
Another plant 20 miles down the coast had the same SCRAM but never lost external power.
What ultimately destroyed Chernobyl was a reactor design (RBMK) with a huge positive void coefficient and insane communists in control of it. The RBMK design could never have seen licensing in any western country.