I continue to follow the events at the nuclear power plants hit by the 2011 Sendai earthquake and tsunami.
The heat currently being generated in the subcritical reactors is from decay heat. It’s not from the primary nuclear reaction that is used to generate heat and power. But rather it is because of the products of the nuclear power generation.
The fission of the uranium (and in some cases plutonium) is the process of the large unstable atomic nucleus splitting into two lighter nuclei and releasing neutrons (which then can continue the reaction by causing other uranium nuclei to split) and energy (following the famous E=mc^2 mass-energy equivalence equation).
The smaller nuclei produced by fission are often also radioactive. Meaning they also decay by fission over time. The description of how frequently the smaller fission products themselves decay is measured by the half-life of the isotope. The description of the chain of radioactive decays is called the decay chain. For nuclear fuel, this decay chain ends with the stable lead nucleus.
The heat being generated in the subcritical core is simply the natural process of the fission products decaying down to a stable nuclei. This natural fission also produces heat. Over time, the heat produced will decrease, at a rate defined by the half-lives of all of the products and sub-products.
The d’oh moment was when I realized that that is why the used fuel rods are always kept in a cooling pool next to the reactor. Otherwise, if there was not ongoing heat from the spent fuel, the used fuel rods could just be left in a secure warehouse. I knew about the cooling pools, but never connected it to the heat from the decay chain.