They look like aggregates in concrete. How did they get inside the channel box?
Inspection was carried out in the Spent Fuel Common Pool building.
From TEPCO's Photos and Videos Library, August 28, 2012 "Soundness Investigation of the Unused (Unirradiated) Fuel Removed from Unit 4 Spent Fuel Pool at Fukushima Daiichi Nuclear Power Station":
(Click to enlarge.)