If enough uranium / plutonium
gets together in the same place, you get critical mass, and
BOOOOOOOOMMMMM!

No, you dont.

You get a red or white hot stinking highly radioactive mess, but you do
NOT EVER get a BOOM!

And atomic bomb is enormously difficult to make, even with the right
fuel purity - which a reactr simpy does not have, and requires some form
of conventional detonator- usually TNT - to force enough mass in one
place to get a BOOM.