Thank you, SteveElbows, for this very helpful data.
Does not the relatively low radioactivity in SPF 4 invalidate the hypothesis that the explosions in reactor 4 arose out of a fuel cladding failure after the SPF ran dry?
Surely SPF4 would be much more contaminated if unclad, partially used fuel pellets were in the water.
Separately, I'm puzzled by the reference to Iodine 131 levels on P13 of your previous linked document here:
That shows I 131 levels at about 10**4 Bq/cc pretty steadily from June to late Oct. As this is about 20 half lives, something does not jibe.