Subject line is it. A nuke uses conventional high explosive to compress the nuclear material and render it supercritical. How much conventional explosive is needed to get the job done? 10 pounds? 1000 pounds?
I’m guessing that’s probably classified.
We can do a sort of back-of-the-napkin upper-bound calculation for Fat Man though.
We know that the diameter of Little Boy was 60 inches/5 feet. So the sphere of explosive can’t be any larger than that. That’s a sphere with a 30 inch or 76.2 cm radius, giving us a volume of 1,853,333 cubic centimeters.
We know that the most dense conventional explosive used in Little Boy was Baratol, which has a density somewhere around 2.5 g/cc.
We also know that the actual pit was surrounded by an aluminum pusher sphere 18.5"/47cm in diameter, and that there was no explosive within that.
So a good upper bound would be the difference between that 1,853,333 cc outer sphere and the 54,361.6 cc inner sphere (the aluminum pusher & rest of the pit), giving us 1,798,971.4 ccs of explosive.
Multiplying that by the 2.5 g/cc density and then dividing by 1000 gives us 4497.43 kilograms of explosive. That’s at the very most, assuming 100% baratol, which we know isn’t the case. 100% Comp. B (1.65 g/cc) comes out to 2968.30 kg, which is our lower bound. The big question then becomes how much baratol and how much comp B the bomb used?
Fat Man, not Little Boy.
Apparently, in 1958 we accidentally dropped a nuke with no warhead onto a playhouse in Florence, North Carolina:
So, enough to make a crater. A big one.
We’ve accidentally dropped nukes with warheads, too.
There is a lot of variation in what constitutes a “nuclear bomb.”
From Wikipedia on what I would call a real nuclear warhead, the the W88:
“The value of a prolate primary lies apparently in the fact that a MIRV warhead is limited by the diameter of the primary — if a prolate primary can be made to work properly, then the MIRV warhead can be made considerably smaller yet still deliver a high-yield explosion — a W88 warhead manages to yield up 475 kt with a physics package 68.9 in (1.75 m) long, with a maximum diameter of 21.8 in (0.55 m), and weighing probably less than 800 lb (360 kg).[6] Smaller warheads can allow a nation to fit more of them onto a single missile, as well as improve in more basic flight properties such as speed and range.”
So there is another upper bound, 800 lbs, but I would guess the explosives portion if that is much smaller. Total WAG is no more than a couple hundred pounds.
From the Wikipedia page on “Suitcase Nuclear Device”:
“The lightest nuclear warhead ever acknowledged to have been manufactured by the U.S. is the W54, which was used in both the Davy Crockett 120 mm recoilless rifle–launched warhead, and the backpack-carried version called the Mk-54 SADM (Special Atomic Demolition Munition). The bare warhead package was an 11 in by 16 in (28 cm by 41 cm) cylinder that weighed 51 lbs (23 kg). It was, however, small enough to fit in a footlocker-sized container”