i'm wondering about the main design principle of the reactor cores - by default they are "hot", ie. producing heat/energy, so one needs to make effort to keep them "cold", ie. non-producing heat/energy. This hot-by-default principle was the main reason of Chernobyl catastrophe and here as well.
Why not reverse the principle and design the cold-by-default core? For example, instead of inserting graphite rods into the core to calm it down, one would need to insert additional uranium rods (or pump molten uranium salt, or move the 6-8-12 pieces of core close enough, etc...) for the reactor to activate and become "hot".
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.
Control rod insertion is designed to be an entirely passive operation, effectively making the default state one where the fission reaction is not active. Your proposals are all different ways to stop the fission reaction but do not address residual heat.
What you're really asking for is a system wherein the cooling of the fuel after fission has stopped (this is where the problems occurred at the Fukushima plants) is entirely passive.
I'm sure somebody could get clever with siphons to make this happen but you'll need an absolutely massive amount of water to do so and/or a way to do the cooling such that the water itself is not exposed to radiation. This would allow you to, for example, safely take it from one side of the dam to the other.
An active solution instead allows recirculation of the same water (often borated) and can generally be a closed-loop (and radiation apathetic) system.
> What you're really asking for is a system wherein the cooling of the fuel after fission has stopped (this is where the problems occurred at the Fukushima plants) is entirely passive.
This can be achieved by designing the safe core temperatures to be much higher, which makes cooling easier. The very interesting small molten salt breeder design FUJI MSR http://en.wikipedia.org/wiki/Fuji_MSR is designed to survive complete external cooling loss by shutting down fission (either under control, or by the heat physically destroying the neutron mirrors required for it's operation), and then dealing with the decay heat by conductive transfer to the environment and simply letting the reactor heat up.
The fact that the core is molten in normal operation, under no pressure and entirely chemically inert even at elevated temperatures makes passive safety somewhat easier.
And that was a fun way to spend an evening: new reactor designs are quite cool :). It seems like a lot of the innovation is occurring in parallel with both fuel types and containment approaches.
I too look forward to having completely passive shutdown of reactors, but from what I can tell (outside of a few small-scale test reactors) we're just not there yet. Someday...
>Control rod insertion is designed to be an entirely passive operation, effectively making the default state one where the fission reaction is not active.
well, the rods stuck at 1/3 while being dropped into Chernobyl core as the things had already started going wrong.
The problem here isn't criticality but waste heat: heat generated by uncontrolled radioactive decay of waste products (fission products and products of neutron activation). There's no way to turn off this heat; it continues uncontrollably for a long time, decaying super-exponentially (here's a graph: [1]).
As far as the nuclear reaction goes, the Fukushima reactors were "off" within seconds of the initial earthquake, and remained so since then. (Though, tangentially, there's been speculation of some low-power criticality events since then, but the evidence is flimsy. It's not particularly important anyway). That's the polar opposite of the Chernobyl disaster, where it was an uncontrolled nuclear reaction which destroyed the reactor building and released vast amounts of nuclear waste in an explosion (and subsequent fire). In Fukushima (and Three Mile Island, and others) there was no uncontrolled reaction; but there was decay heat and its effects. The big explosions at Fukushima were an indirect effect of decay heat: explosions of hydrogen generated by when extremely hot zirconium "burned" in steam. And the large scale fuel failure ("meltdown") was also caused by decay heat -- too much decay heat, and not enough cooling.
Why not reverse the principle and design the cold-by-default core? For example, instead of inserting graphite rods into the core to calm it down, one would need to insert additional uranium rods (or pump molten uranium salt, or move the 6-8-12 pieces of core close enough, etc...) for the reactor to activate and become "hot".