What if the water could not have been drained from the Chernobyl tanks?
(...and if for some reasons computer science had stagnated after)
In short, the disaster takes on an apocalyptic dimension with a large part of Europe rendered uninhabitable for centuries and numerous consequences due to mass irradiation and tens of millions of refugees.
This explosion put an end to the cold war by destroying a large part of the Soviet hearthland and ironically by creating this large strip of irradiated land, the explosion created the clear western border that the Russian powers have always dreamed of.
Most of the Central and Eastern European states collapsed and were massively depopulated withn most cases the state presence limited to a light control of the cities while the countrysides remained lawless zones.
Soviet power managed to survive at the cost of a violent takeover of power by the army and orthodox factions of the party.
Western Europe found in the European Economic Community a good scale to face the crisis and its sanitary and security consequences. A large wall has been built on the eastern border of the Community to limit the flow of refugees from the East, who have settled in the still relatively stable states of Austria and East Germany.
Switzerland became a radicalized version of itself, increasingly paranoid and xenophobic and its border with Austria is one of the most militarized in the world.
The UN quickly became the preferred organization to deal with the international consequences of the disaster, but its action is limited to a mostly theoretical control of the Continental Exclusion Zone.