The debris was found in the North side of the top, where used to be the operation floor.
When they loaded it onto the remote controlling truck, they measured 540 mSv/h. [...]
From Tepco’s April 22, 2013 release: To prepare for the second phase of protection installation on the spent fuel pool, the steel beam truss debris, etc. which interfere with the work will be removed.
See also: US nuke industry report: Explosions at Fukushima Units 1, 3, 4 may have caused inventory (nuclear rods) to be lost from spent fuel pools -- "Debris" on ground near Unit 3 was extremely radioactive after blast
Published: May 4th, 2013 at 12:00 pm ET