An excursion event in a reactor isn't even close to the kiloton range (the one in reactor 4 at Chernobyl was only 33GW). The explosion you mentioned at Chernobyl was enough to toss a 2200 ton slab... but guess what... it was just steam radioactive as hell due to contamination from damaged rods (normally water in reactors when irradiated creates N-16 which is a short -lived (as in a few minutes) alpha emitter) ... but still just steam. The amount of force in the explosion at Chernobyl was only about 10 tons. Enough to totally jack up the core and it's fuel/control rods but not vaporize the plant which is what would happen if you got into the kiloton range.
Critical mass only means you've got enough neutrons to maintain a fission reaction, that's it, scale has nothing to do with it. The neutron flux between rods is moderated by steam, water, and other neutron absorbers, adding more fissile material into the equation doens't equal more energy. In fact it can STOP your reaction as enough material will absorb neutrons but not emit them with enough energy to really do anything.
Yes there is such a thing as passive cooling systems that require no moving parts. Most nuclear subs use molton sodium as a coolant and natural convection moves the sodium in a loop. Fukushima had such a thing but it the command lines to that value were cut and power to open the valve wasn't there anyway. That still didn't matter as the valve could be opened by hand except that the radiation in that part of the plant was too high. Some estimates but it at almost 30Sv MINUTE and no matter how fast you ran or with how much protection you'd still be dead before you could open the valve.
Dumping cold water into a reactor, especially sea water, is a big no-no as that'll immediately cause an excursion in 99% of situations, passive cooling or not. The water would stop the reaction immediately but the excess neutrons would have no place to go, as the water attenuated the neutron flux, and would MELT the fuel rods in a few minutes. If it's sea water you also get corrosion. When ordinary water is irradiated with strong alpha it becomes corrosive, reactor piping is designed to deal with the chemical reaction (peroxide) and so isn't in any danger as long as procedures are followed. Sea water on the other hand is corrosive against the same materials that are generally inert against irradiated water.
Fukushima had a N+2 failure system, you'd need two completely isolated systems to fail before things got ugly and the odds of that happening were extremely unlikely.
Um... what type of reactor are you an expert in as doing what you said in that last post would be a very stupid thing to do?