If that water touched the corium nuclear fuel, cladding, concrete mix touched the water pools below there is a good chance most of us here would be dead or not born now. Outside of the initial blast every gram of u235 would have been ejected. There was 200 tonnes in reactor 4, 8 of which was ejected on the initial explosion and fire. It is safe to assume the other 3 reactors stored the same amount. 800 tonnes of it then ejected into the atmosphere.
The Pripyat river would be toasted which of course leads into the dneiper river. A river that serves some 50m people in an immediate fashion. The UK may have been ok-ish but certainly Nordic countries would be totally screwed. Sweden especially considering the prevailing winds at the time. For all intents and purposes Europe would be useless for habbitation. Along with a large chunk of Asia. Amazing how close to total oblivion we really were.
