When that white-hot, melting reactor core touched the water, it would trigger a massive, radiation-contaminated steam explosion. The fallout created by it would contaminate much of Europe. The death toll would make the first Chernobyl blast look almost insignificant.
This would trigger a nuclear explosion that, so Soviet physicists calculated, would vaporize the fuel in the three other reactors, level 200 square kilometers [77 square miles], destroy Kiev, contaminate the water supply used by 30 million people, and render northern Ukraine uninhabitable for more than a century”.
The 2009 School of Russian and Asian Studies had an even bleaker assessment: If the melting core reached the water, the resulting explosion “would have wiped out half of Europe and made Europe, Ukraine, and parts of Russia uninhabitable for approximately 500,000 years.”