His assertion was that global warming was caused by heat from radioactivity generated by the nuclear materials used in and produced by nukes, not by the fission itself.

The response of those here who disagreed with him was that compared to the energy produced by the plants, the heat produced by radiation from the fuel and wastes was insignificant. And of course, the percentage of total energy from nukes was only a fraction of that from fossil fuel. We've burned a lot of coal and oil, and split a lot of uranium nuclei, but all that heat is completely insignificant compared to the total amount of energy we get just from sunlight hitting the earth's surface.

Regardless of where the heat comes from, nukes, solar or fossil, as the earth gets hotter, it radiates that excess heat back into space. Greenhouse gases allegedly interfere with this process by making it harder for the earth to radiate this excess back into space, so the the earth heats up as a result. Heat comes in, heat goes out, but the earth is in equilibrium with the cold darkness of space, Sooner or later the earth will get hot enough to radiate enough heat back so that its average surface temperature will stabilize at some new, higher, equilibrium.

The hotter something is, the more it radiates. Sooner or later, an equilibrium temperature will be reached that allows heat output by radiation to exactly equal heat input from fossil fuels, sunlight, nukes, volcanoes, and everything else. The role of greenhouse gases is simply to raise that equilibrium temperature. If a planet's temperature has stabilized, then whatever heat flows in from sunlight (and everything else) must exactly equal the amount radiated. Venus, for example, has an extremely high equilibrium temperature. Mars has an extremely low one. The two planets have different surface temperatures determined by different amounts of greenhouse gases in their atmospheres.