That's due to the heat that remains in the nuclear core, both from the recently-disabled but still-hot fuel rods and from the various byproducts of the fission process.

Nuclear power is produced by harnessing the heat produced by the splitting of atoms inside uranium - a process known as fission.

And then of course as part of the absorption we have a fission cross section which, as I said earlier, is what we want to focus on so we can make some power and some heat.