If the processing plant keeps operating then tailings keep...

If the processing plant keeps operating then tailings keep getting pumped into the broken tailings dam which then overflows into the cell next to it.

Presumably the adjacent cell is already at its maximum capacity so any overflow into it is not permitted. Some operations deposit into two cells which gives plant operating flexibility if there were a wall failure.

I described tailings dams to someone a few years ago as like the toilet of a mine site. If the toilet plumbing is broken then you can't keep sh*ting into it.