Remember that Unit 3 was in a maintenance outage when this happened. For some reason they had to do a complete core offload. When the outage was over there was the intent to reload that same fuel back into the core. So it made very practical sense to have that fuel where it was, right at the discharge point from the reactor vessel head. That is why those pools are where they are, so make the transfer from pressure vessel to storage as simple as possible to avoid mistakes.
It was the worst misfortune that the seismic event happen in this particular outage configuration. If the material were in the core it would have been better, because even if it was damaged it would be contained by the pressure vessel and the containment. But it was in the storage pool. Very, very unfortunate that the timing was as it was. But how can you tell when you're going to have a 300-year earthquake, one that exceeds all historical records for the region and thus exceeds the design basis earthquake for the plant design?
I thought that it was Unit 4?
Wow. So a couple of the reactors had their non-spent rods stored in the pools when the disaster occurred ? Thought they were replacing them as spent, but they were removed for routine maintenance ?
I presume you knew but made a typo. It was Unit 4 that had completely offloaded its core into the reactor pool. Unit 3 was operating when the quake hit. Unit 3's unusual fact was it was using MOX fuel.
Looking at various things here I believe I've found one small silver lining for the US in these events! There are only 6 BWR-3 reactors in the US, comparable in design and vintage to Daiichi #1. Two of them are near me, just north of the Quad Cities in Cordova, IL. Twenty-two miles NNE of them is the Thomson Prison which Illinois was trying to sell to Obama as a Gitmo replacement. The new US House wasn't willing to spend that money, but the broke IL government was still pursuing the sale. I don't see much chance Obama would transfer precious jihadists from Gitmo that close to nuclear plants 'just like' Fukishima Daichi #1. I doubt any of Obama's buddies have thought of that angle yet, but once it is brought up I'm confident they'll SCRAM the proposal. The liberal reaction is predictable!
Living about the same distance in the opposite direction I fervently hope and believe the reactors are safe. We have trivial quake risks, nil tsunami risks, and record Mississippi floods have caused no problems there. I do hope they review their contingency plans for a possible T5 tornado after these events, but they've long claimed they can survive that too. Both are licensed through 2032, when they'll be 60 years old.