I think your graphite point is valid.
However, we don't know what we don't know.
Say there was an earthquake; the pool was breached; and all the water was lost for 24 hours.
How bad could that get?
Most likely just a raised cancer risk for all but maybe 500 square miles.
But it's getting worse and we don't know what's still undiscovered or unreported.
From WSJ:
[...] Tepco said it doesnâ(TM)t think that water has flowed into the sea but canâ(TM)t say for sure. Some of the flooded reactor basements are similarly too hot to approach, and it is still not clear where the melted fuel cores are, or in what state.
âoeIn the future there might be even more heavily contaminated water coming through,â said Atsunao Marui, head of the groundwater research group at Japanâ(TM)s National Institute of Advanced Industrial Science and Technology and a member of a blue-ribbon panel set up in May to figure out ways of managing the radioactive water. âoeItâ(TM)s important to think of the worst-case scenario.â
Mr. Marui and others say the biggest reason for the scramble now is that Tepcoâ"and the government bodies that oversee itâ"werenâ(TM)t planning far enough ahead and waited too long to respond to problems they should have seen coming long ago.
Fukushima Daiichi was built some 40 years ago on the site of a river that was diverted in order to situate the plant, Mr. Marui says. It should have been clear that lots of groundwater would be rushing through the site, he says, and that any walls or barriers built on the seaward side would soon be overwhelmedâ"something that, indeed, has happened in recent weeks. [...]