Why is a roof crack meaningful?
Really not that important as long as the containment field is still holding. The room is like a weather cover and no fuel is exposed there.
“Why is a roof crack meaningful?”
—If this article is correct, then spent fuel of the Mark 1 design reactor No. 4 is located as follows:
“This diagram shows where spent fuel pools are located in the boiling water reactor system at Fukushima, a 1970s design by General Electric known as the Mark 1. As the graphic shows, while the spent fuel lies under a deep pool of water, it is outside the concrete-and-steel containment designed to trap radiation leaks.”
—This diagram shows what lies above the pool:
http://www.propublica.org/documents/item/ucs-spent-fuel-report#document/p6