jlduh
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To Dmytry:
This extract is very informative of the scenario that we can see appearing at any of the Fukushima pool if the situation deteriorates on any of the reactor and the level of water goes to zero in the spent pool. And this risk is going to last for a very long time as i don't see know how to secure this scenario to happen, based on the more than fragile (but in fact we may say lucky after all!) situation in which the Fukushima plant is now:
If the pool were to be drained of water, the discharged fuel from the previous two refuelings would still be "fresh" enough to melt under decay heat. However, the zircaloy cladding of this fuel could be ignited during the heatup.543 The resulting fire, in a pool equipped with high density storage racks, would probably spread to most or all of the fuel in the pool. The heat of combustion, in combination with decay heat, would certainly release considerable gap activity from the fuel and would probably drive "borderline aged" fuel into a molten condition. Moreover, if the fire becomes oxygen-starved (quite probable for a fire located in the bottom of a pit such as this), the hot zirconium would rob oxygen from the uranium dioxide fuel, forming a liquid mixture of metallic uranium, zirconium, oxidized zirconium, and dissolved uranium dioxide. This would cause a release of fission products from the fuel matrix quite comparable to that of molten fuel.545 In addition, although confined, spent fuel pools are almost always located outside of the primary containment. Thus, release to the atmosphere is more likely than for comparable accidents involving the reactor core.
Clearly it is recognized that the fact that there is no real strategy in place other than storing spent fuel has created the conditions for reevaluating the risks with SFP, because of increased volume of spent fuel and BECAUSE OF RERACKING... They reevaluated it but to do nothing because the probability was considered insignificant, after a very brilliant calculation. Obviously, big explosions as consequences of reactor damages that could deteriorate integrity of the pool, of even explode it completely and send in the air all its content were not part of the probabilities which end up terribly low -who would fear this so small risk? (really, didn't we go very close to this on reactor 3 when you see the force of the explosions ans the amount of destructions, same thing at reactor 4...).
I also like this sentence:
Ultimately, makeup to the pool could be supplied by bringing in a fire hose (60 gpm would suffice). Although one would expect that the failure probability associated with bringing in a hose (over a period of four or more days) would be very low, it must also be remembered that working next to 385,000 gallons of potentially contaminated boiling water on top of a 10-story building is not a trivial problem. We will assume, based purely on judgment, that the conditional failure probability for this method of makeup is on the order of 5%. When these probabilities are combined, the result is a frequency of 1.4 x 10-6/RY for an accident initiated by loss of spent fuel pool cooling.
Well at least they envisionned the difficulty if it could happen. But finally considered the probability to happen insignificant. Period.
I'm very surprised to see how risks assessment can be done without really taking into account domino effects. The only serious explosion that could destroy the pool is the one of a tornado missile... Well, well.
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