According to a western nuclear industry executive, quoted on condition of anonymity:
Broken pieces of fuel rods have been found outside of Reactor No. 2, and are now being covered with bulldozers, he said. The pieces may be from rods in the spent-fuel pools that were flung out by hydrogen explosions.
This is supported by the previously leaked NRC assessment, which stops short of identifying "very high dose rate material" as spent fuel pieces:
Fuel particulates may have been ejected from the pool (based on information of neutron emitters found up to 1 mile from the units, and very high dose rate material that had to be bulldozed over between Units 3 and 4. It is also possible the material could have come from Unit 3).
There is an inconsistency: the executive says the fuel pieces were found outside Unit 2, while NRC says it was between Units 3 and 4. The Reactor Safety Team speculates the source was the spent fuel pool of either Unit 3 or 4. Unit 4 was the one with the full-core offload, where the entire core was in the storage pool for maintenance. According to the document, those assemblies were 105 days old at the time; and according to JAIF, there were a total of 1,331 assemblies in that pool.
(It's unclear to me whether NRC's "particulates" refers to dust-size particles, or whether the executive's "pieces" refers to larger fragments).
High-resolution damage photos are hosted on Cryptome:
Here's one of them, of the area between Unit 4 (left) and Unit 3 (right) where the NRC says the "very high dose rate" material was found (click to see high-res version):