The conclusion, that a “thorium bomb” is not very effective as a “dirty bomb”, is correct. Quoting,
This is more tentative than necessary, the normal reaction towards information of an unfamiliar nature. The word “doubt” can be replaced with “certain to fail.”
If the planned bomb had gone off in a busy public space, the area would be safe for transit as soon as a normal cleanup were completed, the same kind of cleanup required by any terror bombing. There is more thorium in the granite of many public buildings.
Thorium shares a property of some other elements: a fine powder burns spontaneously in air. A dust cloud can explode like flour. So the addition of thorium to the mix could increase the “bang”, with some of the properties of a very small thermobaric weapon. But TATP is uniquely unsuitable for this combination. There are easier ways. Let’s skip the details.
This misdirection of terrorists, apparently from the association of thorium with atomic energy, keeps them from more serious mischief. The dirty bomb threat is real.