What is (roughly) the half-life of the materials used/generated in a nuclear plant, if that plant goes into meltdown? Could the world bounce back from global warming in a shorter time period than that half-life? Even in a worst-case scenario, could full-scale global warming be a better outcome than a cascading series of nuclear power plant failures (due initially to a natural event such as an earthquake, or to deliberate attack, war, etc.)?
Maybe you can describe how many degrees of global warming are you trying to compare that massive meltdown.
There are several nuclear waste, like the following: Xe133, I131, Cs134, Cs137, Te132, Sr89, Sr90, Ba140, Zr95, Mo99, Ru103, Ru106, Ce141, Ce144, Np239, Pu238, Pu239, Pu240, Pu241, Cm242
Some isotopes have a short life, so they don't matter, while others might have a half life of millions of years. In fact, uranium is so stable that you can still find it in the Earth crust.
But usually, the most dangerous for health are the ones that are between some days and some years of half life, like I131 (8 days of half life), Sr90 (28.78 years) and Cs137 (30 years).
Maybe you can give a look to Chernobyl case in order to get more info.