WTC nuclear demolition, D. Khalezov, 2013 (full)

Dimitri Khalezov was an intelligence officer in a Soviet agency collecting information about any production, storage and distribution of nuclear material in the surrounding world. He knew about a treaty from 1976 between USA and USSR* limiting the maximal charge of nuclear devices for peaceful purposes to 150 Kt (equivalent to 150,000 tons TNT). Due to the problems with taking modern sky scraper steel constructions down in a controlled manner without harming the surroundings, this rather well known technique for demolishing such buildings with directed nuclear charges installed below the foundation was chosen for some suitable cases. Facilities to rail the nuclear charges into position just before use were installed below Building 1, 2 and 7 (possibly a few others), when the World Trade Center was built 1966-73. The blueprints showing this facility were even unintentionally published by a newspaper. The technique secures a perfect vertical fall into the footprint when such buildings have to come down due to age - or maybe because of a asbestos problem. 99 percent of the radiation is trapped under the surface but will escape more or less controlled when ground zero months later is cleaned up. Under controlled circumstances such a demolition on Manhattan would include an evacuation of the close neighborhood. After removal of remaining materials (80 percent of the total mass will typically disappear) - and the final cleaning, the area would be habitable again. Unfortunately the 3 charges were set off for reasons that excluded normal protocol. For good reasons the public has still not been informed about the real motive.
Due to the bilateral treaty, Soviet authorities were cleared about any instance of such non military use of nuclear devices in US and vice versa, and for the same reason Dimitri Khalezov (living in Thailand today) is ready to testify under oath in any American court about the fact, that facilities for this nuclear demolition technique had been implemented in WTC. The typical signature of the "collapses" indicates to him, that nuclear charges were used - just not quite as planned.
*) http://www.nti.org/learn/treaties-and-regimes/treaty-between-united-states-america-and-union-soviet-socialist-republics-underground-nuclear-explosions-peaceful-purposes-pne-treaty/