We’re don’t know but betting the cask had transported damaged NRU reactor fuel from from Chalk River Laboratories in Canada to SRS. Shhh….DOE doesn’t like to talk about that damaged fuel being dumped on SRS….
DEFENSE NUCLEAR FACILITIES SAFETY BOARD weekly report, June 1, 2018 – linked here
“L-Area: As it was exiting the transfer bay, a trailer containing an empty spent fuel shipping cask became disengaged from the tractor after it was pulled about 50 feet. The trailer came to rest and neither the trailer nor the shipping cask were damaged. The trailer was coupled to the tractor inside a contamination area. Although workers made visual and audible checks of engagement, workers did not perform a visual inspection underneath the trailer to confirm the trailer was fully coupled with the tractor. Furthermore, the pull test was not as hard as normal due to a desire to avoid tearing the contamination control paper underneath the trailer landing gear. The trailer may have been sitting a bit too high when the tractor pulled underneath it, thus causing the trailer to sit on the fifth wheel rather than fully engaging it.”