Since it is anticipated that North Korea will restart a reactor for plutonium production, this is an executive summary, a mere capsule, for those who may wish to understand a little of the difference in the title. As with the explanation of Teller-Ulam, there are huge omissions. But rather than be accurate, let’s aim for digestible.
The scope of what follows is specific to the current endeavors of North Korea to deploy, and the presumably halted endeavor of Iran, to build, the A-bomb.
- Bomb-grade plutonium is ” relatively easy” to make. In a relatively simple nuclear reactor, not very different from the first one constructed by Enrico Fermi, it shows up as a decay product. To obtain the plutonium, it is “only necessary” to remove fuel from the reactor and chemically isolate it.
- Bomb-grade uranium is very difficult to make. It is one of the most abundant elements in the earth’s crust, but there is no way to chemically isolate it. Hence Iran’s thousands of centrifuges.
Ease of use
- Plutonium is hard.
- Uranium is easy.
Plutonium is a horror in the machine shop. It has six forms (allotropes) which it seems to pick at random, each with different densities. Expanding and contracting under the slightest provocation, it is very crumbly. It also corrodes in common gases that do not affect other metals. When exposed to damp air, it flakes off, and the flakes spontaneously combust in air. This is not something you should store in your closet. In a nuclear weapon, the metal with all the crumbles is in close proximity to high explosives.
Plutonium requires implosion. If a hunk of the metal, called a “pit”, is squashed in just the right manner, which is very complicated and difficult to achieve, there is fission, and, BOOM! Otherwise, it fizzles. The explosives used to implode the pit require precise combinations, shaping, and uniformity.
Uranium is relatively easy to use. A warhead can have a gun barrel aimed at a hunk of uranium. The “bullet” is another hunk of uranium. When fired, the two hunks assemble a critical mass and, BOOM! This method cannot be used with plutonium. Uranium is not terribly stable, with changes called “phases”, but they are much more moderate than the allotropes of plutonium.
Advantages of plutonium
- Plutonium is the high performance option.
- Both metals are very heavy, but less plutonium is required than uranium. An enhancement called boosting, which uses tritium gas, greatly increases performance while reducing weight, but limits storage lifetime to a few years.
- Plutonium is much cheaper to make.
Advantages of uranium
- It holds shape at any reasonable temperature.
- The gun-type weapon is low-tech.
- Since it can also be used for implosion, it is the flexible choice for the aspiring superpower.
- A low-tech uranium gun-type bomb made by an aspiring nuclear power is likely to remain functional for a longer period than a plutonium device. An article published by Los Alamos National Labs is not directly related, but gives a general idea of the problems.
If for some reason the shape of the warhead is important, it is not difficult to build a uranium gun barrel weapon of very narrow diameter, resulting in a skinnier weapon than a typical plutonium design. But plutonium bombs using “linear implosion” can fit inside a 155mm artillery shell. Photo of a full scale model shell. Even narrower plutonium bombs are postulated, but would be entirely new designs.
If this has held your interest so far, you may be curious as to which technology has the greater chance of use by non-state actors. In At The Center of the Storm – My years at the CIA, George Tenet wrote,
Tenet was specifically concerned with the Russian “suitcase nukes”, approximations of which have been made by both sides. See Alexander Yablokov’s PBS Frontline interview. They are likely too bulky to fit in suitcases, but portable in bulky backpacks, which has caused some to discount Yablokov. Since weight, not bulk, was the determining factor in the designs, it follows that all of them were plutonium weapons boosted with tritium, which has decayed.
The denials of risk by Russian officials, who assert that all of the suitcase nukes are accounted for, are unconvincing. The Russia of the Yeltsin period was the Wild, Wild West. There is even an anecdote of one Russian whose debt was paid by a hydrogen bomb, a big thing delivered on a truck, which he kept in his garage for a few days. It was subsequently reclaimed. Have you ever been tempted to lie if your dog pooped on someone’s lawn? This is bigger.
But current threat assessments have gravitated to crude unminiaturized gun-type weapons, the main requirement of which is stolen bomb grade uranium and a white van for the delivery vehicle. These opinions are likely based on the decay of the tritium in the Russian nukes.
Each type of bomb has an argument against it:
- Uranium is traceable to the producer, so it isn’t likely to be sold by a country that understands they could be blamed.
The Russian suitcase nukes have aged to the point that they don’t work anymore. They need to be taken to a garage shop for replenishment of tritium, battery replacement, checkout of the explosive lenses, and adaptation to suicide operation.
The decay of tritium calms the nerves much more than moral arguments along the lines of “who would do this?” Until recently, it was impossible to conceive of a state with morals so demented, it might miscalculate. North Korea doubtless understand that their plutonium is traceable. But tritium is not, and they are making it.
In answer to Tenet’s question, we shield ourselves from the unthinkable with the assumption that the adversary has a moral standard. During the Cold War, this was true. Current events suggest otherwise.