Trump wants to nuke hurricanes!

[Anomalocaris]

Fallout isn't a product of the design of a bomb, it's material that has been irradiated in the blast.

It is the product of bomb design.  Most of the fallout material derives from the material in the bomb, where the radiation is by far the most intense at the moment of the fission or fusion reaction.   Different atomic nuclei under the radiation converts to different isotopes, some more radioactive, some less so.  Their presence can also alter the total yield of the nuclear explosion itself.  Military weapons specifically select different temper material to put in the bomb to adjust or boost the the yield within a compact size, and also to directly increase radioactive fall out.    In fact there are military weapons specifically designed to achieve the maximize fall out for a given explosive yield not only of just any radioactive material, but the most dangerous radioactive material whose half life is the ideal compromise between being very dangerously radioactive and being radioactive long enough to keep effected area contaminated for years. 

The so called colbalt bomb is one such example.   Cobalt bomb is a two stage hydrogen bomb with temper material added to produce copious amounts of radioactive cobalt during the explosion which will then come down as fall out.   Cobalt is particularly nasty because it is highly radioactive, readily absorbed by the human body, and stays radioactive for a relatively long time.    The Russians claim to be building a 100 MT cobalt bomb.  They say they are doing this to get around america’s Missile defence.  They plan to deliver the warhead using giant nuclear powered robotic torpedos that would creep silently over weeks or months past any ASW defense to detonate off enemy coastal cities.   Calculations show if such a bomb detonates off New York in the right season it would drop enough fallout down to Miami to make the entire eastern seaboard uninhabitable for several decades.

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Okay, so looking at the Tsar Bomba, the only 50 MT nuke ever made, some salient facts:

• It was so big that the deploying plane needed to have its bomb bay doors and fuselage fuel tanks removed.
  
• The fireball (with a diameter of 5 miles) never reached the ground, due to the shockwave, and reached the same altitude as the deploying plane
  
• To give the pilots a 50% chance of survival, the bomb was given a small parachute to buy them time before the bomb went off, and the creation of this parachute disrupted the Soviet nylon industry. 
  
• Everything in a 35 kilometer radius of ground zero was destroyed.
  
• Admittedly, the fallout was limited, due to a highly efficient detonation (the effects of hurricane winds might compensate for any particular safety measures that ensured this the first time.)
  
• The 50 MT yield wasn't even the full projected yield, with a potential 100 MT yield possible. If detonated, it was calculated that "The effect of this bomb at full yield on global fallout would have been tremendous. It would have increased the world's total fission fallout since the invention of the atomic bomb by 25%."
  
• There is a general consensus that a 50 MT nuke is too big to be useful.
  
• The detonation of the Tsar Bomba was one of the major catalysts of the 1963 Partial Test Ban.
  
• The biggest nuke ever detonated by the US, the Castle Bravo test, only had a yield of about 30% that of the Tsar Bomba.
  
• The biggest nuke we have at the moment, the B83, is about 2.4% the yield.
  

I'm still kind of dubious that a bigger nuke could potentially be cleaner.

Large hydrogen devices achieve their yield by having multiple stages.   Only the very first stage necessarily produce substantial amount of material that are radioactive and long lived and this dangerous in fall out.   Subsequent stages don’t have to produce much if any additional long lived radioactive fall out.

The fact that actually nuclear weapons have more fall out than the minimum is by design, not necessity.

The czar bomba is too large to be useful as a military weapon to be delivered by bombers or ballistic missiles.  But if the goal is to heat air to stop a hurricane, the measurement of usefulness would be different.   Then the limit would probably be how big of a device can you physically deliver into high altitude in The eye of a hurricane far off shore and detonate without killing anyone.

Also, there are military uses for even larger bombs.  See cobalt bomb described above.

Nuclear weapon technology right now imposes no limit on how powerful a fusion explosive device can be.   One could arbitrarily add stages to a hydrogen bomb to achieve any yield desired. The practical limit is really how Unweirdly would the resulting device be.