The recipe for making a nuke ...

... starts out the same as the recipe for "wabbit" stew.

First:  Catch your "wabbit"!!!

Hiroshima, the atomic-bombing of which occurred 66 years ago tomorrow, was the first of a one-two punch.  It is essential to understand that it took that second blow to finally convince the Japanese (at least, most of those who counted) that it was really all over.

You see, they had engineers and physicists that knew most of what was necessary to build an atomic bomb, and what they understood most was that the resources required were staggering, almost beyond imagination.

The August 6, 1945 detonation over Hiroshima didn't phase them all that much because previous fire raids involving hundreds of B-29s had already inflicted mind-numbing horror upon other cities.  Tokyo had already had the heart burned out of it, with a lot more casualties than Hiroshima suffered

That, this time, it was only one bomb didn't impress their top military people. They were positive that there was not going to be a rain of those bombs because that would simply be impossible for us.

In a way, they were right.  Almost all of the refined Uranium U-235 that the United States possessed was used in that one bomb.  Later T-shirts showing a mushroom cloud and emblazoned, "Built in the USA by stupid lazy Americans. Tested in Japan." were dead on;  we didn't have enough U-235 to test it anywhere other than over the target.

The problem with the Japanese High Command's assumptions was that there were other ways to skin that particular cat.

U-235 is desirable because it's relatively easy to detonate; usually making a cylinder of rings with a large enough hole to keep it below critical mass, making a cylindrical plug that would fill that hole, and putting said cylinder into a tube with a small explosive charge (a gun, in other words) and firing that cylinder onto the plug.

Yes, there truly is a bit more to it than that.  But it's not my intent to give detailed bomb-making instructions here, even if I did know enough of the details.  (Not that it would do a would-be terrorist any good -- remember the first part of the recipe above.)

The problem with catching the U-235 "wabbit" is that U-235 makes up less that 1% of natural Uranium (the other 99+% percent being the U-238 isotope).  Separating that by the gaseous-diffusion method takes enormous time.  Construction of the Oak Ridge, Tennessee facility began in Feb, 1943 and the fact that two years later there was only enough for one bomb would seem to confirm the Japanese skepticism in thinking that with Hiroshima we just may have shot our bolt.

But, there is another way.

Uranium U-238 can be used in a reactor to produce the isotope Plutonium P-239 which can also be made to fission.  Such a reactor was built in Hanford, Washington and it could crank out a lot of usable P-239, and did.

So, what's the catch?

The "gun" detonation technique worked just fine for U-235;  so well in fact that the engineers and physicists were confident that it would work and that they wouldn't be delivering a dud for the Japanese to study at their leisure.

With U-235, I believe a closure rate of around 3000 feet/second would insure a successful detonation.  That is easily achieved in a gun design as the explosives generated an expanding wave velocity of around 5000 feet/second.

With Plutonium P-239, the fission rate is so fast that the closure needs to be around 10000 feet/second or the energy from the fission would blow the pieces apart before the detonation commences, resulting in a fizzle that might release a small cloud of some of the most lethal toxins on Earth, but not the bang you were after.

So, instead of using the gun technique, they used the much more difficult implosion method by surrounding the P-239 with explosives and detonating them at the same time;  a process using klystron krytron switching and wires to each explosive element cut to the exact same length (to allow for the speed of the signal through the wire).

In short, calling it an exacting science doesn't even begin to cover it.  It took a lot of testing to perfect the technique before they used it.

Because the explosives completely surround the Plutonium core, this resulted in a problem.

When the Enola Gay took off, with the "Little Boy" Uranium bomb, it was intended to be already armed before it was loaded into the plane.  As the B-29 was to take off from an 8500 foot runway, laden with a five-ton bomb and all the fuel that could be crammed into it, there was always the possibility of a crash on take-off (the night before, four B-29's had met that fate; this was a dangerous business).  Such a crash, with an armed nuke aboard, could take out half the island.

So, the crew decided, "No F**king Way!", and had part of the bomb's detonation package removed, to be put back (arming it) while in flight.  With the "gun" design, that was possible.

The implosion design, necessary for the Plutonium bombs, was a whole 'nother story.  The "Fat Man" bomb for Nagasaki and its follow ons could only be armed prior to loading into the airplane.

According to the book "Day One: Before Hiroshima and after" (Peter Wyden - 1985), the officer in charge of that bomb was obsessing over the number 50.  That's how many of those bombs the top generals in the United States Army Air Forces thought it might take to compel the Japanese to surrender.  And, with Hanford cranking out P-239, there was no doubt that we could deliver that many if that's what it would take.  In fact, bomb # 3 was already on its way to Tinian.

To that officer, 50 more take-offs without a crash simply wasn't in the cards, and he would be cabling his superiors emphasizing the need for a quick redesign that would allow arming in flight.

Fortunately, that second bomb taking out Nagasaki on Aug 9, 1945 was convincing enough to bring about the surrender. (It's possible that Japanese scientists may have identified Plutonium at the Nagasaki site, telling them that the situation was far worse than any of them had assumed as to our ability to continue the attacks, but that's just a guess on my part.)

To nit-pickers:  You will almost certainly find mistakes and generalizations galore in this post.  I'm only trying to express the gist of things here.  Using any of this in a dissertation will probably get you flunked.  So, be warned. :-)
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