(Conventional explosives could also cause a billion or more dollars worth of damage to a major wafer fabrication plant, of course, but the manufacturing capacity could be shifted to other plants in a matter of months. Some major short sale opportunities, but not nearly what a nuke could do to a _region_, in terms of direct blast effects, fallout in surrounding city blocks (tens of square city blocks, at the least, esp. give OSHA standards, etc.), and the sheer panic effect.)
On of my favorite analyses of a similar scenario is contained in "The Curve of Binding Energy" by John McPhee (available at your local Borders or Barnes and Noble). He basically interviews a high energy physicist and works out the back of the envelope calculations on yields, where to get the plutonium, where and how to place the device, etc. A key point was that a high efficiency device is not required. A dirty 1.5 kiloton gadget placed on the 40th floor of the World Trade Center takes out one tower and kills a shit load of folks in the adjacent tower. Includes other rules of thumb such as "one kiloton of explosives vaporizes one kiloton of matter". YMMV, don't try this at home kids, etc, etc.