Also we're not only talking about the main reactor housing. Much more radioactive material resides in spent fuel pools which might not be as well protected as

Also we're not only talking about the main reactor housing. Much more radioactive material resides in spent fuel pools which might not be as well protected as the reactor, see also https://en.wikipedia.org/wiki/Spent_fuel_pool#Risks https://en.wikipedia.org/wiki/Spent_fuel_pool#Risks