—”The possibility of recriticality is not zero” [ Fukushima ]

URGENT: Spraying boracic acid eyed to prevent recriticality at No. 4 reactor | Kyodo News.

[via New Scientist, Kyodo News]
I missed this news item from Wednesday until this morning.
“Tokyo Electric Power Co. said Wednesday it is considering spraying boracic acid by helicopter to prevent spent nuclear fuel rods from reaching criticality again, restarting a chain reaction, at the troubled No. 4 reactor….” [my emphasis]
I thought one of the principal and most comforting safety factors in modern nuclear power systems was that criticality was supposed to have been made impossible. In fact, I was under the impression that the nuclear reactions used to generate power were carefully controlled to keep them sub-critical. Now we are told that under certain circumstances, the geometry of the spent fuel rods in storage at Fukushima could not only allow a criticality incident [although hopefully not a fission-bomb-like explosion] , but that criticality in the storage pool at reactor 4 might happen AGAIN?!
I supposed we should add this to the growing list of design deficiencies in the Fukushima nuclear plant. Are there any other surprises we need to know about?


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