nuclear industry is finished, page-61

binbin,

Agree with your interpretation. The heat source now is not fission (which stopped as soon as the plants were shut down) but decay heat from radioactive decay of the daughter isotopes.

This heat is normally removed by cooling systems in a shutdown situation but of course these have failed and the buildup of decay heat is quite capable of severely damaging the reactor core, popularly known as a meltdown, as well as triggering explosions which can release these radioactive daughter isotopes (iodine, cesium etc) into the atmosphere.

Three Mile Island was this type of incident (though what's happening now in Japan is much worse). Chernobyl was different because the reactor core was still fissioning when the explosion occurred.