Was It Necessary to Drop Two Atomic Bombs on Japan?

A little primer on the science and history of the first nuclear weapons. You won’t need to remember much from high school except that atoms are made up of positively charged protons and neutral neutrons in the nucleus which itself is surrounded by a cloud of negatively charged electrons.

The World War II atomic bombs were fission-type. In other words, their explosive power were a result of splitting an atom’s nucleus and releasing the energy that previously was keeping the protons and neutrons of the nucleus together. In nature, there is a natural decay of the nucleus (a quantum mechanical process) where, occasionally, a very, very tiny piece (2 protons and 2 neutrons) is spit out and this process is exactly what causes radioactivity. In a bomb, rather than a very tiny piece occasionally leaving the nucleus, you instead split the nucleus into approximately two halves with a few extra stray neutrons (previously part of the large nucleus) coming out. These stray neutrons can then slam into other nuclei, splitting them in two pieces with, again, a few stray neutrons. The process repeats itself; this is the “chain reaction.” With all these nuclei splitting practically simultaneous, you release a lot of energy in a brief period of time and…. well…. BOOM!

Now uranium is a radioactive material. It is found in Earth ores and is relatively common and easy to produce from these ores. There are two types of uranium: U-238 and U-235. U-238 has 92 protons (and electrons) and 146 neutrons. Notice that 92+146=238. U-235 has 92 protons (and electrons) and 143 neutrons, 3 fewer than U-238. (92+143=235.)

Nearly all the uranium on the planet is U-238. However, a very, very small amount of the uranium on our planet, 3/4 of a percent (0.75%), is U-235. This type is uranium has three less neutrons than U-238 and it turns out (from quantum mechanical properties), you can make the nucleus of U-235 split into two large chunks with a few stray neutrons to create a chain reaction. So while you can’t make a bomb out of U-238 (which decays only by releasing a very, very tiny piece from the nucleus), you can make one out of U-235.

The key to the atomic (fission) bomb is to make sure your stray neutrons hit other nuclei to sustain the chain reaction. In other words, you have to make sure you have a critical density of atoms together to make sure all those neutrons flying off the split nuclei hit something. This is known as getting a critical mass to sustain the chain reaction.

Here’s where it gets really interesting from an engineering point of view:

It was known that if you slam two appropriately sized pieces of U-235 together hard, you would end up with a critical mass and, hence, a nuclear explosion. It was practically a guarantee. How much of a guarantee? The US was so confident that it never even tested this weapon idea — they just used it in battle without a test. The bomb that went off in Hiroshima was exactly this type of weapon and the first uranium atomic bomb ever. It consisted of one piece of uranium that was shot at another.

The problem is that the uranium needed to be U-235 and so you needed to separate this 0.75% of material from the 99.25% of the U-238. This is extremely difficult because the two types of uranium atoms behaved chemically identically as they both have the same number of electrons. The only way to separate these two materials was from the slight (3 neutrons!) difference in mass — you can do this, for example, in a centrifuge. However, this is a very laborious, time consuming process. It is estimated that 10-15% of all US electricity produced between 1943 and 1945 was used to separate the U-235 needed for the Hiroshima bomb. (Read that last sentence again. Yikes!)

So, while the engineering of the uranium bomb was relatively easy, getting the uranium for the bomb was extremely difficult. (In fact, it was the difficulty in coming up with enough U-235 that also prevented the testing of this type of weapon; hard enough to manufacture one bomb, let alone two.)

So, we are left with only a single atomic bomb. That’s not a practical weapon for military strategy. What to do? Well, there was another possible path. Plutonium nuclei can also be split into larger chunks with some stray neutrons to cause a chain reaction. Specifically, Pu-239 which has 94 protons (and electrons) and 145 neutrons. (Again, 94+145=239.) It turns out, it is pretty easy to come by Pu-239 as you can make it from the abundant U-238 by slamming some neutrons into a sample of U-238. The problem with Pu-239, however, is getting to a critical mass: you need to squeeze it down harder than you do for uranium to get the atoms dense enough to each other.

So, while getting the plutonium for the bomb was relatively easy, the engineering of the plutonium bomb was extremely difficult.

Since you can’t just shoot two pieces of plutonium together to get a critical mass (as you could with uranium), the scientists at Los Alamos, where the weapons were being developed, came up with a way to squeeze down a sphere of plutonium with absolute symmetry. This is one of the great “secrets” of the fission bomb, the symmetric squeezing, or implosion, of a plutonium sphere. Because this squeezing is an engineering problem (rather than a physics one), the US tested this plutonium atomic weapon before deploying it. The famous Trinity test in New Mexico was the detonation of a plutonium (Pu-239) bomb. It was this type of bomb that was dropped on Nagasaki.

The advantage of the Pu-239 bomb is that, once you understood the engineering, it was relatively easy to manufacture since, unlike uranium, you had plenty of plutonium available. With plutonium bombs, you could deploy them at will knowing more will be available.

As early as 1943, the US had made a decision to drop an atomic weapon on Japan. There was a concern for dropping one on Germany. The US feared that if the bomb didn’t go off, the Germans might be able to recover the wreckage and reverse engineer it. Inherent racism in the US created the assumption that the Japanese could not do a similar reverse engineering.

Ironically, the Japanese had initiated their own atomic weapons program but quickly decided to abandon it since it was too resource costly — especially during wartime — to separate enough uranium to create a bomb. (Remember how much electricity the US used in separating out the U-235.) In fact, the Japanese suspected (from local reports) that the bomb that detonated on Hiroshima was nuclear and they sent their physicists to confirm it.

Once confirmed, the Japanese assumed that single bomb was the entire US nuclear arsenal. That as terrible as the Hiroshima bomb was, the US was spent and couldn’t create another bomb for months (it would take a long while to separate enough U-235). The Japanese leadership assumed they had weathered the US’s best shot. Japanese leaders further assumed that while the US was gathering enough U-235 material for another bomb (if indeed it could do so), the islands of Japan would be defended fiercely from any invading force, thereby wearing down the Americans and gaining more favorable terms of surrender for Japan. Japanese leadership thought Truman’s announcement of the Hiroshima (uranium) atomic bomb detonation — including the statement that the Japanese, if they do not surrender, “may expect a rain of ruin from the air, the like of which has never been seen on this Earth” — was a bluff to frighten them into that position.

The second atomic bomb on Nagasaki, the plutonium one, three days later changed all that. It was this bomb that truly stunned the Japanese. Only then did they realize that Truman could make good on his threat. In fact, the US was already building a third bomb to be delivered to Japan. (Truman saw the intelligence of the massive destruction created by the Hiroshima bomb only after the Nagasaki bomb was dropped. It was at this point that he instituted the policy that only the US President could authorize nuclear weapons use during battle.)

In the end, it is both the difficulty of separating out enough U-235 and the engineering difficulty of symmetrically squeezing a sphere of Pu-239 perfectly that prevents other nations from creating their own nuclear weapons. Finally, the historical record shows that, without the US capability of rapidly creating a second nuclear weapon, the bomb at Hiroshima would not have brought about a quick Japanese surrender.