I wrote a short paper explaining in equations how the following solution for hot spots in Ground Zero of all three collapsed WTC buildings could work.
The idea is that the lack of a suitable candidate for a crater makes the big nuke theory less probable, though not completely impossible, but I prefer to have a thermite-based theory for these hot spots.
It is possible that there was one very hot spot of some 3000 Celsius, a small one of 4.5 m radius. It slowly cooled mainly through concrete or stone, being underground, in the basement of a building. But there were steel structures, which conduct heat ten times as well as concrete or stone, and these steel structures got hot. They conducted the heat far away and the far away end was still something like 300 Celsius and could ignite office material immediately when oxygen was replaced by diffusion from air. Therefore, when a fire burned out in lack of oxygen, it would again start because of these hot metal objects.
Here is my short paper as a pdf: