Nothing terrorist originated will have a yield over ~40 kilotons (Kt), and that would be a long shot in hell in and of itself...
Megatonnage (Mt) is indicative of thermonuclear weapons, the biggest purely fission weapon was a few hundred Kt yield.
Any nuclear bomb exploded in a US city can cause tremendous damage in property and lives.
"Nothing terrorist originated will have a yield over ~40 kilotons (Kt)"
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Gee, so it won't be worse than Hirshima!
Yield
The first parameter to establish is the explosive yield of the device. Based on the various media reports and articles I have examined, the alleged nuclear "backpacks" or "suitcases" would appear to be in the one to ten kiloton range.
As a benchmark, the uranium fission bomb dropped on Hiroshima was just over ten kilotons. However, the majority of the articles tend towards the lowest figure of one kiloton and that is the number I will assume.
http://www.lewrockwell.com/watson/watson27.html