Why did it take an army of physicists, working on a huge complex, so long to produce a working nuclear device, while it seems so easy in theory?

Question

I think most of us know about the construction of the first atomic bomb at Los Alamos, with Robert Oppenheimer (who said he became "The destroyer of worlds", which goes to show he regretted his participation; nevertheless he did participate) in charge of an enormous complex where many, many brilliant physicists (including Feynman) were offered (ordered for participating in?) a well-paid job, housing, food and drink, etc. The project was initiated by Einstein after sending a letter to Roosevelt (correct me if I'm wrong), which seems to contradict his pacifist attitude. But that's not that relevant to my question. Which is:

Why did it take such a long time (2-3 years) for all these men (and some women), working on that enormous complex, to construct an actual working device [the first test (called the Trinity nuclear test) hit the jackpot], while in principle you "just" have to smash together two masses of Plutonium below the critical mass, which after the smash have a mass above that mass? That was known (i.e. in theory) at the time. Was it because it was the beginning of the atomic era, and there was still much to learn? Was it to prevent failure? I've read many times the Nazis were on the verge of constructing one too, and I suppose the Americans knew that too. So why not hurry a bit more? "Luckily", the Americans were first, though there were two dropped on Japan since Germany had already surrendered. Even a third was planned to be thrown because there could be used three different elements in the bomb, and the Americans wanted to see how all three exploded. The second one, dropped on Nagasaki, was i.m.o. totally superfluous. By the way, the only relevant tag I could find was "nuclear physics".

Answer 1

Some of the most critical numbers needed are the reaction cross sections. There was basically no serious way of calculating those beyond orders of magnitude. The cross-sections had to be measured, and this is a tedious process, even with good sources and pure samples.

Next, understanding (in an age where all computers were human) the hydrodynamics of the shock wave propagation in plasma bounded by a medium that is about to be blown to bits was a big headache, never mind the reflection of the shock waves to produce spherical compression. The precise timings of the various physical phenomena, and how one step in the sequence affected later steps had to be understood. The thermodynamics alone is not exactly equilibrium stuff. Indeed, the actual compression design was rather controversial as it required machining precision that was, at that time, exquisite. Working out the actual geometry of the lenses, and getting rid of the imperfections, were also necessary breakthroughs.

As an aside, you can get a good survey of the math involved in thermonuclear (i.e. fusion) devices from the book The physical principles of themonuclear explosive devices by F. Winterberg. This is not what you’re asking for but it will give you a sense that the ideas might not be so complicated, but the math remains challenging and the engineering to make it work is even more sophisticated.

Answer 2

One of the big issues was isotope separation. Extracting uranium 235 from uranite was a huge problem that took quite an effort to overcome. In fact, just before the war started Niels Bohr believed that extracting enough uranium 235 to build a bomb would be an impossible task (see Margaret Gowing's Niels Bohr and Nuclear Weapons, in Niels Bohr: A Centenary Volume (Harvard University Press, 1987)).

I suggest that you read Richard Rhodes' The Making of the Atomic Bomb.

Answer 3

(1) Weapon-grade uranium is hard to make - it took almost a year to separate 64 kg of weapon-grade uranium for the bomb in the Y-12 Oak Ridge facility, so "Little boy" bomb was not even tested (they had no spare). And because of the "Little boy" bomb design, most of that expensive uranium was NOT involved in the nuclear explosion (bomb exploded into sub-critical mass before most of the uranium was involved in the chain reaction).

How hard is to make the weapon-grade uranium: in Y-12 complex in Oak Ridge electromagnetic isotope separation, they used 12 300 tonnes (1 tonne = 1000 kg = 2204 lb, $300 million worth) of silver (copper as strategic material was not available in necessary quantities) converted to wires for the coils. After manufacturing the wires, facility was burned down to extract spilled silver from the ashes. Less than 0.036% was lost. As you can see, Y-12 is extremely complex process, resulting in building 1 bomb per year.

(2) But to have more feasible nuclear bomb ("Fat Man"), you need plutonium, which has to be first isolated (in 1940) and researched, then, industrial amounts of sufficiently pure specific isotope of plutonium (Pu-239, because not all isotopes are equally good for chain reaction - one of the problems they need to figure out and to overcome) has to be manufactured in a breeder nuclear reactor, which is also an easy to bomb target (and extremely expensive to build).

(3) Shaped explosion: Scientist were well aware of the inefficiency of the "Little boy" design, therefore they used shaped explosion to create superctitical mass of 11 kg of Plutonium - which has to be created from industry-grade (not weapon-grade) uranium in a breeder reactors, so it is much cheaper and faster process (it took about 1 month to breed enough Plutonium for 1 bomb). But you need to build also breeder reactors to manufacture the plutonium.

Because scientists were not sure if plutonium bomb would even work (decision could not be made based on the minute amount of the plutonium isotopes available), Manhattan Projects decided to invest in both approaches to save time and halve the risk.

(4) Calculating explosion-shaped charge used in Fat Man design is substantially more complex design (as compared to just two pieces of metal hitting each other as in Little boy): requires precise synchronization of multiple explosions (to shape explosion inward: hundred nanoseconds difference can make it or break it), so mathematical models of such explosions had to be developed, and calculated (by hand), therefore it was tested - Trinity - and they could not use computer simulation for explosion because computers were not invented yet, all calculation were made by hand. And wiring to deliver signals to start shaped explosions needed to be invented and tested.

(5) Bomb delivery is another big problem: Developing B-29 "strategic" bomber was even more expensive than the Manhattan project. It was about twice heavier than "standard" heavy bomber, B-17 and so substantially more complex. Getting enough power from just 4 engines, and fully pressurized cabin (to be able of long flights in high altitude, where enemy fighters cannot reach it), was especially hard.

Unexpected part that it at the end, nuclear bombing not only saved lives (it prevented expected 500K US casualties and tens of millions expected Japanese casualties during the planned invasion Operation Downfall - bigger than D-Day), but also saved money: because it shortened war by several months, war cost were about 1 Billion per week, and cost of B-29 and Manhattan Project were in total only about $4B.

Answer 4

You should do some preliminary research before asking such a question. It was not so easy, even in theory. They did not know anything about Plutonium when they started, it simply did not exist. They had to make it, and study its properties etc. This was a slow process. The first bomb was based on Urainum rather then Plutonium, and again there was a huge problem to obtain the correct isotope of Uranium. There were many other difficult problems, or example how exactly you compress a piece of Plutonium to achieve critical mass. This is done with ordinary explosives, and the technology which does this (called implosion) had to be developed both in theory and in practice. These are only some of the most difficult problems they had. And it is actually amazing that they were able to do all of this so quickly.

Answer 5

I can only recommend R. Rhodes, The Making of the Atomic Bomb once more and very strongly. It is also instructive to note that although Leo Szilard [sp] was inspired to understand and fight for the chain reaction concept and also held the patent it was years before he or anyone else could figure out what material could be used to demonstrate the concept. Now, of course, there is a whole declension of what elements turn into other elements as a sun burn through its life.