vanesch
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mheslep said:So is it physically impossible for a cooling storage pool w/ freshly spent fuel to explode or not?
It certainly won't explode. I don't know what it WILL do, but I'm sure that this has been studied, as the document that was linked in the thread.
The possibilities are these:
-slow evaporation of the water (5 days seems short to me, but ok).
- some equilibrium temperature of the rods will appear ; if this temperature is above the self combustion temperature of zircalloy, then a fire might indeed result, but not a big one: after all, once the metal is burned up, there's nothing else to burn.
- if the equilibrium temperature of the fuel is above 2850 degrees, then it will melt. That will be about it.
- I think you won't reach a critical situation, as there is no water (moderator) anymore, and the fuel is already depleted. In any case, this can be studied.
One can design a pool that can hold these elements indefinitely, but I think that current pools are not designed that way, simply because it was not one of the criteria. Normally, one specifies a certain "time of autonomy" a system has to be able to cope with. I don't know what is this specification.
The way to solve this is simply by increasing the volume of water per element stored, and to provide passive cooling (convection, cooling wings,...). So it has a price. People won't over-design things. Given a certain power density, one will be able, using usual engineering techniques, to estimate the temperature evolution and equilibrium temperature of a system (the hotter the system gets, the more heat is given to the environment, so at a certain temperature, there will be as many heat lost, as there is generated, and equilibrium is reached). It is hence sufficient to design the system such that the equilibrium temperature is acceptable (that no self-destruction occurs).
I don't know what is the current design of pools - I don't know what are the requirements. The only thing I want to say is that this is not a problem of principle. It is a matter of specifications. If people consider it ridiculous to expect 5 days of autonomy, then this is not specified. If this is specified, then it can be handled. Even current pools can continue to exist, one simply has to limit the amount of allowed elements inside.