I suspect that the cesium detected may have came from the fuel pit at the top of the outer containment building — from the explosions. Why do you say it wouldn’t be an issue? What’s the heat load? When were the reactors last fueled or unloaded? Do you know?
In general the pools sit unsealed in plain water or boric acid. Our small research reactor where I went to school had a couple rods in a pool and we went in once to stare into the pool. There could be many more rods in an operational reactor, but the principles would not be different, they will be much farther from producing critical amounts of neutrons than the freshly processed fuel. But I don’t know how old the spent rods are (older=better) nor the heat load (but it’s not going to boil off in minutes IMO). I suspect that info will come out now that people are proposing the spent fuel scare story.
I saw on another thread that Unit 1 was slated for shutdown (permanent) at the end of this month. From that, one could surmize based on typical refueling cycles, that their last refueling was anywhere between 1 and 2 years ago -- i.e., whatever number of assemblies they offloaded at the last refueling have been cooling for at least one year.