You're doing a bit of assuming. I didn't assume anything. Here in America a Uranium mine hole in the ground is generally secured in that they put up a chain link fence with barbed wire at the top to keep people and animals out of the hole.
Hell, the Russians might do more to secure a mine, I don't know.
Sludge sounds like something they might find at a Uranium mill site. Sludge is created by rock crushing at the mill, they don't do that in a mine. They do crush Uranium ore very fine. Fine as talcum powder.
That was what my dad did, run a rock crusher at a Uranium mill.