Official news release speaks of a 20 by 20 feet section of soil that’s sunk.
There are some tweets from this reporter at “OregonLive/Portland Oregonian”.
https://twitter.com/SMatsumoto55
https://twitter.com/hashtag/HanfordEmergency?src=hash
Thanks for that link. she links to the above link.
Seems that this is an abandoned tunnel, and rail cars of contaminated stuff were parked inside the tunnel and left there as a form of “disposal”. The practice was stopped in the 90’s. The reports indicate that there is no evidence of airborne contamination. I would imagine that the railcars were boxcars, with contaminated stuff stored in drums as well.
They say the shelter in place is standard protocol, and was probably triggered as soon as the collapse occurred - or was noticed. It wouldn’t surprise me if the sinkhole wasn’t discovered right away.