There's a difference between sub/super critical (producing energy via fission) and "hot" (producing energy _at all_). Chernobyl became more reactive (= more fission) as the water boiled away, leading to a runaway criticality, and explosive disassembly of the core.
Fukushima, however, has a negative void coefficient. Moreover, the fission reaction was stopped immediately after the earthquake successfully by the automatic insertion of control rods. The heat that is being generated is from short-lived fission products (ie, nuclear waste) undergoing spontaneous decay events, thus producing heat (but only a small fraction of the heat of an active reactor).
It is physically impossible to design a reactor in which such spontaneous decay can be stopped. You can only remove the energy actively until these short-lived products decay to the point where air cooling is sufficient to keep them at a safe temperature. You can, however, design a reactor in which the passive containment structures can withstand the temperatures of fresh waste material with no active cooling; however such technology was not available at the time the Fukushima plants were built.
No. The new reactors under construction are mostly very conservative and traditional Gen III+ PWR designs, with some added safety features. The reactors that can survive complete loss of cooling are all Gen IV designs, that are currently under discussion, not construction.
>You can only remove the energy actively until these short-lived products decay to the point where air cooling is sufficient to keep them at a safe temperature.
this is my point - necessity for active removal of energy.
The closest thing to what i was talking about seems to be molten salt reactors where energy producing reaction goes only when salt is pumped through the core/moderator. If something goes wrong - the valves open and the salt is dumped [gravitationally] into the tanks which can be large enough to allow for passive air cooling of the secondary decay energy.
Fukushima, however, has a negative void coefficient. Moreover, the fission reaction was stopped immediately after the earthquake successfully by the automatic insertion of control rods. The heat that is being generated is from short-lived fission products (ie, nuclear waste) undergoing spontaneous decay events, thus producing heat (but only a small fraction of the heat of an active reactor).
It is physically impossible to design a reactor in which such spontaneous decay can be stopped. You can only remove the energy actively until these short-lived products decay to the point where air cooling is sufficient to keep them at a safe temperature. You can, however, design a reactor in which the passive containment structures can withstand the temperatures of fresh waste material with no active cooling; however such technology was not available at the time the Fukushima plants were built.