It seems not possible to put equation formulas to a comment, so I wrote them to a media file. It is word, no viruses in it.
Calculation of WTC nuclear bomb case
The calculation of the cooling time shows that even a mini nuke has enough energy to keep hot spots hot for a long time. The result does not mean that a large nuclear explosive would take years to cool. It only means that if the only cooling way were conduction through granite, this might be so, but naturally there are other cooling ways. To the needed energy should be added all that is required to melt steel, heat the cloud and whatever the scenario requires, I did not include them.