The "hot start" model of heating suggests Enceladus began as ice and rock that contained rapidly decaying short-lived radioactive isotopes of aluminium, iron and manganese. Enormous amounts of heat were then produced as these isotopes decayed for about 7 million years, resulting in the consolidation of rocky material at the core surrounded by a shell of ice. Although the heat from radioactivity would decrease over time, the combination of radioactivity and tidal forces from Saturn's gravitational tug could prevent the subsurface ocean from freezing. The present-day radiogenic heating rate is 3.2 ergs/s (or 0.32 gigawatts), assuming Enceladus has a composition of ice, iron and silicate materials. Heating from long-lived radioactive isotopes uranium-238, uranium-235, thorium-232 and potassium-40 inside Enceladus would add 0.3 gigawatts to the observed heat flux. The presence of Enceladus's regionally thick subsurface ocean suggests a heat flux ~10 times higher than that from radiogenic heating in the silicate core.