The chain reaction shuts down in seconds.
but you get a lot of short lived isotopes from fission products and things that have been activated by neutrons.
They give off a lot of heat as they decay so you need to have some sort of cooling for a while until they calm down.
Most modern designs are designed to do it passively, so they work even if there's a loss of power. Older ones often need electricity to keep some pumps running for a few weeks, which was the problem at Fukushima.
If you start blowing holes in them you will be releasing highly radioactive crap whether they are currently turned on or not.
Fortunately it's starting to look as though the shelling didn't actually make any holes in the reactor itself.
(, Fri 4 Mar 2022, 9:07, Reply)