You might say: “so what?” and you might be justified in posing the question if reactors always functioned as expected. While a reactor is working as it should, heating is taken care of by cooling circuits usually filled with water. However, if there is a sudden loss of coolant a large component of the fission reactions will continue for some time and this could contribute to overheating and core meltdown. Of course, many of the effects of gamma-ray decay can be traced by making calorimetric measurements on model systems. Some of these tricky and dangerous measurements have indeed been made, but none have been conclusive.