Just to recap, in part one, I discussed the premise for this series of posts: that most Americans have an unmerited fear of nuclear weapons, in particular the type being developed by North Korea. I also stated that I felt that fear was grounded in two things, the first being the unknown — most Americans have no idea what a nuclear weapon actually does, or how it works, and the second being most American’s only experience with nuclear weapons — the Cold War.
In part two, I discussed The Manhattan Project and The Trinity Experiment, as well as the two different types of bombs that we dropped on Japan during World War II, including how they were made, how they worked, their blast yield, and the effects they had, both during the actual explosion and afterwards.
Here in part three, I’m going to go over the types of weapons that were built after World War II, and how they led to the types of nuclear devices we have today. I’m going to explain how they work, what they do, and how they are delivered to their targets. I’m also going to discuss how fission and fusion actually work in nuclear devices, and how different elements and designs can be used to increase the yield of the bombs.
It didn’t take long after World War II for America to continue testing nuclear weapons, this time at Bikini Atoll in the western Pacific. They used the same type of bombs there as Fat Man and The Gadget; plutonium implosion bombs. Let’s talk a bit about just what exactly plutonium is. This is important because, since the early 1960s, plutonium, not uranium, has been the standard fission material used in nuclear weapons.
Plutonium is a radioactive element that’s capable of decaying into other elements. The most stable version of plutonium that’s found in nature is the one on the periodic table, plutonium-244, and it’s only found in trace quantities – thought to be remnants of the formation of galaxy. This form of plutonium is completely irrelevant to nuclear weapons.
The plutonium isotopes we’re concerned with are plutonium-239, plutonium-240, and plutonium-241. These are the ones that capable of fission and they’re created in nuclear reactors, not in nature. Nuclear reactors produce plutonium-239 as the uranium-238 that powers the reactors decays and the neutrons it gives off are captured. Capturing a neutron changes uranium-238 to uranium-239. Uranium-239 rapidly decays into neptunium-239, which then decays once more to plutonium-239. This isotope is the one we want to capture to build a nuclear weapon.
Interestingly, plutonium-239 eventually decays into uranium-235, which is enriched uranium and what we try to process out of the mostly uranium-238 we mine from the ground so that we can make a nuclear bomb, which gives all of these isotopes an incestual relationship reminiscent of the Targaryens and the Lannisters.
All of this decaying of atoms and capturing of the isotopes is a complicated and technical process, and it’s not that important. What is important is that when the plutonium-239 is captured, it always contains some percentage of plutonium-240. Plutonium-240 is spontaneously fissile, so it’s unstable and unusable in nuclear weapons if the percentage present in the plutonium-239 is too high. If the plutonium-239 isn’t captured quickly, the 240 isotope builds up and it becomes only suitable as a reactor fuel. It isn’t feasible to separate the plutonium-240 from the plutonium-239, so the mixed plutonium product must be removed frequently from the reactors, which means that it requires very specialized nuclear reactors to make.
Why is this important? It’s one of the ways we’re able to monitor non-nuclear countries for adherence to the nuclear non-proliferation treaty. These countries who want to develop nuclear power, can create power reactors that are incapable of producing pure enough plutonium-239 to make a bomb. 98% of the world’s nuclear reactors are not capable of making weapons-grade plutonium.
Although plutonium radiation is extremely dangerous to humans, it’s very difficult to actually get infected. That’s because plutonium-239 decays with Alpha radiation. Alpha radiation is weak and can’t penetrate the skin; in fact, it can only barely penetrate the air, and only then just a few inches. That means you can actually pick-up a lump of plutonium-239 with your bare hands without negative effects from the radiation.
The largest danger from plutonium (other than being present when it goes super-critical of course), is from inhaling it. This can happen if it catches fire, such as when a plane carrying nuclear weapons crashes and burns, or in lab accidents, or potentially from breathing radioactive fallout in a ground burst. Although inhalation is thought to be the most likely way for plutonium to kill or cause cancer, there were about 25 workers at Los Alamos in the 1940s who inhaled a large amount of plutonium dust, which was thought would give them each a 99.5% chance of contracting lung cancer. None did.
Interestingly, Wikipedia has a footnote that states that plutonium has a metallic taste. First, duh, it’s a metal. Second, who tried it??
Apparently, although inhaling it is likely to have long-term and serious health effects, the Los Alamos workers notwithstanding, you actually can safely eat a small amount of plutonium, as the body tends to simply expel it without actually processing it.
Back to the Bikini Atoll tests, known as Operation Crossroads. The first test was an air drop that was unremarkable and quite similar to the previous two implosion devices that had been detonated. The main lesson learned from it was that the initial beta and gamma radiation from an airburst, while strong enough to kill any human or animal exposed to it, was short-lived and the long-term effects of the radiation on the ships in the harbor was negligible. While the first test was considered unimpressive, the second test was quite remarkable, and it showed the dangers of radioactive fallout in a ground burst.
The purpose of the second test was to determine the effect a submerged nuclear weapon would have on naval ships, and for this test, the navy assembled a fleet of 95 U.S. and Japanese ships in the bay at Bikini Atoll. Then, they detonated a plutonium implosion device under the water at a depth of about 90 feet. This was the result.
What you’re looking at there, is hundreds of millions of gallons of instantly radioactive seawater. If you look closely at the bottom right of the water column, you see a vertical battleship. A 27,000 ton battleship. Amazing amount of force.
The ships that survived the explosion were painted with radioactive seawater when the wave passed over them. Despite numerous efforts to scrub many of them clean, the Navy was unable to make them non-radioactive. The seawater had settled into the microscopic cracks and seams in the metal of every ship it touched, contaminating them permanently. Of the total of 95 ships in the harbor that day, only 9 were able to be salvaged. The remainder either sank from the bomb and the water column, or were scuttled by the Navy because of their radioactivity.
What we learned from that bomb was that radioactive fallout is pretty dangerous when a nuclear weapon explodes on the ground, (or under the sea as in this case) and that such an effect can have a devastating impact for a long time. How long? Well, let’s discuss radioactivity and exactly what it means.
When we speak about how radioactive a substance is, we usually talk about its half-life. In nuclear physics, a half-life is the amount of time it takes for a substance to lose half of its atoms through radioactive decay. Some substances have a half-life in the billions, or even trillions of years, and others a half-life of only fractions of a second. Many of the radioactive isotopes that are present in a nuclear explosion have these very short half-lives, which is why they are only deadly during the explosion itself, and not afterwards.
What about the elements in a nuclear weapon? Well, in a nuclear reactor, uranium-238 is bombarded with neutrons. When it captures one, it becomes uranium-239. Uranium-239 has a half-life of 23.5 minutes. It decays into neptunium-239 which has a half-life of just over two days. Neptunium-239 decays into plutonium-239 which has a half-life of 24,100 years. Plutonium-239 decays into uranium-235 which has a half-life of just over 700 million years.
The longer the half-life, the less radioactive and dangerous the substance is. Radioactivity is inversely proportional to half-life, which is why the two main fissionable fuels of a nuclear bomb are so safe to handle. Both Plutonium-239, with it’s half-life of 24,100 years, and uranium-235 with its half-life of 700 million years can be safely picked up with your bare hands. (The mostly alpha-decay of these metals is the other reason they’re safe – alpha particles are incapable of penetrating even the dead layer of the outside of your skin.)
A lot of radioactive elements are produced in a nuclear explosion, and some of them have half-lives that extend beyond the initial effects of the explosion, but are short enough to be dangerous. Strontium-90, for example, has a half-life of 28 years, Iodine-131 has a half-life of 8 days, and cesium-137 has a half-life of 30 years. All of these substances can cause cancer in the human body, all live long enough to survive the nuclear explosion, and all have short enough half-lifes to be dangerous with regard to the inverse proportion rule.
Now, half-life doesn’t mean that the element is safe at the conclusion of that time. Far from it. Take iodine-131. It has a half-life of 8.1 days, but that doesn’t mean that after 8.1 days it’s safe to be around. What it means is that after 8.1 days, half of its molecules have decayed, which means that half of them remain. After another 8.1 days, it still contains one quarter of its original molecules, and so on. Typically with this halving, a radioactive element is considered safe after 10 half-lives, which is when it becomes undetectable to most instruments. So, for iodine-131, that would be about 81 days. For cesium-137, that would be about 300 years. Detectable amounts of plutonium-239 then, would stick around for the next 241,000 years.
While that sounds horrible, it really isn’t. Remember, radioactivity is inversely proportional to half-life, which means that the longer an element sticks around, the less radioactive danger it typically poses. (With a few exceptions)
Shortly after the underwater explosion at Bikini Atoll, nuclear scientists began experimenting with trying to make the reaction in the explosion a lot more effective. Remember, in the first detonations, less than 2% of the uranium in Little Boy, and only about 15% of the plutonium in Fat Man went critical and contributed to the blast yield. Scientists realized that there was something missing in the equation, and there had to be a way to harness more of the power of all that uranium and plutonium that went to waste.
The problem was that when a mass of plutonium goes critical and begins the fission chain reaction, it expels neutrons. Neutrons, along with protons, are what gives an atom its mass. You can see the number of neutrons in an element by looking at the periodic table and subtracting the atomic number from the weight. For example, plutonium-239 has an atomic number of 94, which means that it contains 94 protons and therefore, 145 neutrons. (239-94)
So, as the fission process begins (with the explosive compression and a small neutron generator in a plutonium bomb) some of those neutrons are thrown out of the reaction. They become what’s known as free neutrons, and some of them get absorbed by the other plutonium atoms. Free neutrons are the key to a good nuclear chain reaction. When they get captured by other plutonium atoms, it creates another fission event, which releases more neutrons, which get absorbed by other atoms, and so forth. This is the nuclear chain reaction, and in a nuclear power plant, it’s carefully controlled so that only small amounts of energy are produced and the mass of fuel can never reach a super-critical state.
In a nuclear bomb though, that chain reaction isn’t dampened, and it quickly reaches the super-critical state where it gives off so much energy so quickly, that we get the nuclear explosion. It happens so quickly though, that all that other plutonium gets wasted. The idea the scientists came up with, was to harness some of the free neutrons that were escaping, and redirect them back into the critical mass so that more fissions would occur before it went super-critical.
What they found was that covering the entire thing with a thin sheet of beryllium would reflect the free neutrons and cause them to re-engage the chain reaction, causing more fissions to start before the mass goes super-critical, and increasing the efficiency of the weapon, which also increases the blast yield.
(Later we’ll discuss how neutron bombs do the exact opposite, encouraging the free neutrons to actually escape the critical mass rather than joining it, and why that would be desirable with regards to killing humans.)
Beryllium also acts nicely as a tamper, helping to compress the plutonium when the explosive spheres surrounding it are detonated. Not only that, but it also produces some of its own free neutrons during the process, all of which help make the entire detonation more efficient and more powerful.
The other discovery nuclear scientists made shortly after the end of the war would prove to be even more significant, as it would lead to the ability to begin the miniaturization process of nuclear weapons, which would eventually allow them to get small enough to be mounted on missiles as warheads. This discovery involved hollowing out the center of the mass of plutonium and filling it with a gas. The gas used is a mixture of both tritium and deuterium.
Tritium and deuterium are both isotopes of hydrogen (think, hydrogen bomb), which simply means they are hydrogen atoms with additional neutrons. Everybody is familiar with hydrogen – it’s the most common element in the universe by a long shot. It’s also the simplest, with the nucleus of the atom containing just one single proton. When a single neutron is added, this hydrogen atom becomes deuterium (also known as hydrogen-2) and it’s a non-radioactive, stable isotope that can be found in small amounts in nature. When you add the second neutron though, that’s when it becomes radioactive, and we call that tritium (or more rarely, hydrogen-3).
Most people are familiar with tritium, at least having seen it if not knowing what it was. Tritium is used for night sites on guns, luminescent numbers on watches, altimeters, and clocks. The radioactive decay of tritium is what causes it to glow with a greenish hue. Tritium has a half-life of just over 12.3 years, which means it’s quite unstable. What makes it safe around humans is that it decays to a helium gas, helium-3, and though it decays through beta emissions which are typically less safe than alpha emissions, the beta particles are slow moving and can’t penetrate more than an inch of air, nor can they penetrate the human skin.
Because of tritium’s short-ish half-life, it is a bit of a problem. In the civilian world, for example, your gun sites and watch faces lose half of their brightness every 12.3 years, which means a 25-year-old watch that uses tritium to glow in the dark will be at one-fourth of its original strength. In nuclear weapons, this is a much worse problem as the helium-3 it decays into can cause problems during the nuclear reaction. Helium-3 has a large cross-section and the greatest ability of any isotope to grab the free neutrons in a nuclear explosion. We don’t want the free neutrons being grabbed by all the helium as that would be counter-productive toward the effort to create them in the first place. Therefore, not only does the tritium need to be replaced fairly frequently in the entire nuclear arsenal because of its short half-life, the helium-3 needs to be drained from the pit even more frequently, which is one of the reasons regular maintenance of the arsenal is so extensive.
So, what happens when deuterium and tritium are added to a hollowed-out pit in the plutonium core? Fusion occurs during the compression process, and fusion reacts with fission to vastly improve the efficiency of the weapon. This is known as a boosted fission weapon, and it was the first step toward the weapons of today, the two-stage thermonuclear warheads. It turns out that fusion is even more efficient than fission when it comes to nuclear weapon yields, though these early boosted fission weapons did not make use of the fusion as a yield increaser, but rather they used it to harvest more free neutrons to add to the critical chain reaction and increase its efficiency.
When I first learned of fission and fusion, way back in grammar school, it was explained to me quite simply. Fission is the splitting of the atom, and fusion is the joining together of two atoms. Very simple. I had an image of a scientist with a tiny knife cutting an atom in half and creating a huge explosion in a lab. Not quite accurate, obviously.
Fission doesn’t occur by means of a knife, or a laser, or any other type of weapon, but rather by bombardment. Throwing heavy atoms (like uranium and plutonium) into each other causes the neutrons in the nucleus to split off, which is what fission actually is. And, throwing lighter atoms (like hydrogen and helium) into each other causes the nucleus to bind together, creating heavier atoms and dispersing enormous amounts of energy. Fusion is the energy that powers most stars, including our sun, where it’s known as stellar nucleosynthesis.
When deuterium and tritium are compressed during the initial conventional explosion and compression of the plutonium core, they fuse together, and that causes those hydrogen isotopes to form helium and free neutrons instead of the energy that might be released were hydrogen-1 used instead of its heavier isotopes. The helium is simply consumed in the fireball which adds a small, but mostly negligible increase to its yield. The free neutrons however, contribute greatly to the chain reaction, vastly increasing the yield.
By hollowing out the pit of the plutonium to add the fusion gases, it not only increases the yield, it also decreases the weight of the bomb, not just through the lost weight of the plutonium, but through the smaller explosives needed to compress the core. It’s significantly easier to compress a hollow sphere than a solid one, and far less explosives are needed. There were almost 3000 pounds of conventional explosives in Fat Man, for example, which shows just how much explosive power was needed to compress the heavy plutonium. (Just to be perfectly clear, there was a small hollow pit in the Fat Man plutonium design – about 2.5 centimeters, into which was inserted a neutron generator which was found to be insignificant to the chain reaction and unnecessary to the design.)
Once boosted fission devices were perfected and tested, nuclear scientists had reached the limit of blast yield for a single stage weapon. They were eventually able to harvest some of the energy of the fusion reaction in the core, but the maximum amount theoretically possible was about 20% of the total yield. So, 80% of the nuclear blast yield still had to come from the fission of the plutonium. By playing around with highly enriched uranium-235, and surrounding it with lithium-deuterium fusion atoms, and other various configurations, they were able to achieve some impressive blast yields.
The United States produced a bomb known as the Mark 18 or the Super Oralloy Bomb (Oralloy is super-enriched uranium). The Mark 18 was tested one time at Enewetak Atoll in the Pacific. It had a blast yield of 500 kilotons, a 25x increase in yield over Fat Man.
Great Britain created a boosted fission weapon known as Orange Herald, which had a blast yield of 750 kilotons. Fifty times the blast yield of Little Boy over Hiroshima, and an incredibly destructive fireball.
This was nothing however, compared to the yield achieved when two-stage thermonuclear weapons were invented in the early 1950s.
In the next part, I’ll discuss the concepts of multi-stage weapons and the blast yields achieved, including talking about the largest ever nuclear detonation, and the advances in miniaturization and delivery vehicles that were achieved in the heart of the cold war. This will lead us to a discussion of the types of nuclear weapons that North Korea has, and to my thesis of why we shouldn’t fear them or their weapons, and why we shouldn’t fear nuclear weapons in general today.