If the earth were a closed system, it would long ago have become a cold rock at least. However, the earth gets most of its energy from the sun, we’re not a closed system.
That certainly begs the questions of: How much heat energy is present from the atmosphere on down to the center of the core? What fraction of that is input from the sun per year? What fraction of that is lost to Space per year?
Actually, all the heat in the interior is due to radioactivity, right? There is no way solar radiation gets past the first few feet of the earth, and nor past the water envelope of the crust.