Without any hard data, I really doubt it went critical. Unless I'm faced with incontrovertible evidence, I'm going to conclude that if there was a fire, it was due to failure to remove decay heat, not due to a criticality excursion.

It's possible, of course. But I just don't think anyone would be dumb enough to put the fuel in such an arrangement that lack of water in the pond would cause a criticality excursion.