[FreedomPoster]

It is not only residual heat. There is some natural radioactive decay of the materials going on. I am told this is on the order of 5-8% of the plant output, at least immediately after a shut-down. It probably reduces a little once you get past a few half-lives of the shorter-lived isotopes.

[sefarkas]

... residual heat. There is some natural radioactive decay ...

Residual heat comes from the radioactive decay of fission products (Iodine, Cesium, Strontium, Xenon). The rate at which residual heat forms gets exponentially smaller each passing hour meaning it takes less and less water to keep it cool. A residual heat rate of 0.05% of full power in a commercial reactor is still a significant amount of heat.