Big Bomb

What if a nuclear bomb the size of a 747 crashed into England? I didn't ask the question (saw it on Yahoo Answers). This is so going to get me tagged by homeland security. But here goes:

Let's assume the most powerful kind (fission-fusion-fission) available to date. Biggest detonated ever Tsar Bomba had a theoretical yield of about 100 megatons (though actually detonated at 50 megatons) at 8 meters long and two meters diameter. A 747 is about 70 meters long so let's say roughly 10 times the available material. At that scale the resulting bomb would be, using Tsar Bomba as our unit, 270 tons.

Assuming a linear relation between amount of explosive material (fission/fusionable) and output, that would put our bomb at about 1000 megatons. Approximating the relationship between fireball size and power megaton as a square relationship, we would estimate a fireball diameter of around 10 miles. Doubling it for fun we get a 20 mile radius fireball. Total area of fireball coverage would be about 314 square miles.

England is 80,823 square miles, so about 80K square miles. 300/80000 = ~0.4%.

So even if the bomb were WILDLY successful, it would still blow up far less than 1% of Great Britain.

Problem 1: I'm not sure that a fission-fusion-fission design could scale up that far. Perhaps a multi-stage fission - fusion - multi stage fission...

FYI 2: For the brief moment the bomb would be busy exploding, the power generated would rival a significant percentage of the Sun's power output. I'd say less than 5% but I have no idea how to calculate that. Tsar Bomba got to about 1%.

Anybody have a better estimate?