NUCENG said:
Due to delays in the national commitment to accept spent fuel for geological storage US plants had to install dense pack fuel storage racks in spent fuel pools. That made the use of boral panels and inserts necessary to maintain the reactivity below criticality with the reqiuired margin. Later they had to begin installing dry fuel storage due to continued delays. If there is a criticism here it is that the lack of a go/no go decision on Yucca Mountain has led to the de facto risks of dense pack fuel pools and interim storage at over a hundred sites across the US. Nobody said, "STOP, we need to reevaluate this risk," instead of letting it force us into half measures to get through this short delay after that delay and so on. We are no closer to a solution today.
Back on topic, I don't know if Fukushima had dense pack storage or not. They have a common fuel storage pool on site and had fairly limited numbers of bundles in the units pools except for unit 4. Japan also sent their spent fuel to reprocessing. It is possible that the geometry was sufficient to prevent recriticality even in Unit 4, but I would have expected TEPCO to be shouting that from the rooftops if it were so. To be safe we have to assume that recriticality is a possibility and be alert to evidence that it is happening.
Because of the fresh fuel in Unit 4 the pool did boil in a little over a day after loss of fuel pool cooling. The steam would have introduced significant dilution and compressible vapor into the building. It is likely that helped mitigate the deflagration/detonation. It was also a significantly smaller production of hydrogen gas because there was little or no zirc-water reaction. As to the potential to move fuel racks, I am very doubtful that occurred. Incompressible Water level above the fuel would have actually spread the pressure pulse through the pool protecting the fuel and probably reflecting most of it back into the building. Fuel bundles are very fragile for side loading and I would expect to see evidence of broken fuel had there been much dislocation.
Wanted to mention here this interesting video:
Don't know this guy, but he seems to know what he is talking about concerning the high risk related to spent fuel pools in the US. As a french, with already much spend fuel on our territory because of the 58 reactors we have but with in addition a large amount stored at la Hague for treatment, I didn't know that the US had so many tons total (71 000 tons) and so many tons in average at each reactor in the SFP's (the guy says it's often 500 to 700 tons in the US reactors, so basically more than 5 times the average at Daichi plant, except gor N°4).
The reflexions about NRC's attitude on this subject (which acted to suppress the report made) are very much similar to the main conclusion that the chairperson of Japan's Nuclear Safety Commission, Haruki Madarame, has told NHK (see my post here:
https://www.physicsforums.com/showpost.php?p=3341445&postcount=284 ): a closed world with too much co-dependency between NRC and nuke industry, in the selection process it seems (with one recent exception), and also in the fact that NRC is more and more relying on inspection programs from the industry instead of independant inspections, due to budget cuts.
Again I don't know this guy and the US situation, but I' would be glad to hear US citizens what they think on this matter. the guy states that an accident or terrorist attack in the US in one of this plants could because of this SFP storage lead to an exclusion zone 5 times the one in Chernobyl (the Chernobyl zone is half size the New Jersey, based on its words).
Nuclear spent fuel and waste treatment is, in my mind, one of the main reasons why nuclear power cannot be a long term and widely used technology.
When i was a kid, they were talking about the fact that in 10 or 15 years in the future they would do "transmutation" (that was kind of a magic word as it was presented!) and that's why they were storing spent fuel, in order in the short/mid term to convert it to safe stuff with this technology. Where is "transmutation" to convert danger to safe stuff? We are at the same state where we were 25 years ago: still going around the problem and just storing, storing storing...
All this seems to me a "dead end".