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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.You are assuming that Russians actually did things (securing the mine, protecting etc) up to a standard that protects the public.
From articles I've read, the concern is sludge contaminated along with uranium salts leeching into river which is primary water source.
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.