Falsifiable, Politics, Science

Nuclear Weapons: 3.0 Proliferation

There are currently nine countries with acknowledged or suspected nuclear arsenals. Five of them are signatories of the Non-Proliferation Treaty (NPT), the main international treaty aimed at minimizing the number of nuclear armed states. Ideally, no country or group would have nuclear weapons. Unfortunately, we don’t live in an ideal world; the NPT is maybe the next best thing.

The NPT acknowledges the right of the permeant UN Security Council members (UK, USA, France, China, and Russia) to possess nuclear weapons even as it bans anyone else from getting (or trying to get) them. The remainder of the nuclear armed states (Israel, Pakistan, India, and North Korea) haven’t signed on to NPT or signed and later withdrew from it. South Sudan also isn’t a signatory of the NPT – I think they just haven’t gotten around to it – but no one is particularly worried about that (for reasons that will soon become apparent).

In the nuclear sense, proliferation – the thing this treaty is trying to prevent – is the spread of nuclear secrets, nuclear capability, or nuclear weapons to state or non-state actors that do not already possess them. Proliferation is fairly universally regarded as a bad thing. Luckily, attaining nuclear weapons is very difficult.

Earlier, I talked about how much information is classified. Interestingly, it tends to be only the specifics of nuclear weapons that are classified. Schematics and detailed procedures are under lock and key, but general principles are all over the internet. The next section of this post series covers historical and modern nuclear weapon design. This is totally legal. There’s no law in Canada or the US against disseminating any of this information (really mom and dad, I promise, this is okay).

There are a couple reasons for this laissez-faire attitude.

First, the general design of nuclear weapons is fairly easy for any competent physicist to derive from first principles, once they know what they’re looking for. The Teller-Ulam design was probably re-created 2-4 times by separate groups of physicists. Some of these re-creations used only basic knowledge of what byproducts the bomb produced and its approximate yield. Since computers will only get more powerful and physicists will only understand physics better, it stands to reason that this feat is becoming easier and easier to repeat. In any given university physics department, there are probably physicists who’ve guessed at the parts of the Teller-Ulam device that are still classified.

Second, this doesn’t really matter. Nuclear weapons are incredibly hard to actually build. Understanding in abstract how something works doesn’t mean you can go out and build it. Think about cars; even though you might understand the principles behind an internal combustion engine in the abstract, you’re probably incapable of building one. I know I certainly couldn’t. I don’t have the right materials. Even if I had them, I don’t have a blueprint to go by. And even if I were to come up with a design and assemble it, I’d most likely end up creating an underpowered and unreliable engine, not up to the standards of Ford, let alone Ferrari.

This maps well to the problems a state or non-state actor would face if they tried to build a nuclear weapon. First, they’d have difficulty acquiring materials. Then they’ve have more serious difficult coming up with a blueprint that makes good use of those materials. If they persevere and successfully create nuclear weapons, they’ll at first only have weak weapons, not up to the standards of the rest of the world.

In terms of materials, all nuclear weapons require inherently unstable isotopes – otherwise there would be no neutrons with which chain reactions could occur. These unstable isotopes (mainly plutonium-239 and uranium-235) are rare (or in the case of plutonium-239, basically non-existent) in nature. The half-lives of these isotopes see to that. Pu-239’s half-life is many orders of magnitude less than the age of the world. All of the Pu-239 the Earth original had is long since decayed. U-235 is longer lived than Pu-239 (it’s half-life is about 1/5 the age of the Earth), but it’s still rare; uranium deposits are mostly of the more stable (and therefore useless) isotopes.

Any moron can cause a nuclear detonation if given a critical mass of pure plutonium-239 (for the record, this is 12kg). Luckily (and despite what Doc Brown thought about the future), it is almost impossible for anyone (let alone morons) to get their hands on any amount of pure Pu-239. Furthermore, there is a huge gap between setting off an explosion in your lab and building a weapons system that can reliably deliver a nuclear payload to a hostile target.

For the actors that wish to try this, there are two paths to a bomb. The first requires uranium, the second, plutonium.

3.1 Uranium

There are a few advantages of using uranium in a nuclear weapons program. You don’t require research reactors just to get fuel for your bombs and the actual bomb design is much simpler. That said (and fortunately for the world), it is much more difficult to enrich uranium than it is to enrich plutonium.

Enriching the uranium – separating out different isotopes so that you’re left with only the most unstable ones – is a massive technological undertaking. First you have to mine it or import it. Either way every other country with a functioning intelligence service is going to find out about it and take note. But that’s just the first and easiest step.

Once you have your uranium ore, you have to dissolve it in nitric acid. Then you add ammonia. Then hydrogen gas. Then you mix it with hydrofluoric acid (which is incredibly nasty to work with). Then you fluoridate it some more with fluorine gas (this is even worse; it’s poisonous and it turns into hydrofluoric acid in your lungs if you happen to breath it in). This laborious and dangerous process gives you uranium hexafluoride, which is a real joy to work with (in the same way that being on the beach in the middle of a category 5 hurricane is a relaxing tropical vacation). Uranium Hexafluoride (hex for short) is incredibly toxic, explodes on contact with water, and corrodes most metals.

You’re going to need to find a metal it doesn’t corrode though (I recommend aluminum) because the next step is putting this terrible chemical into a giant centrifuge, adding some heat to turn it into a gas, and swinging it around as fast as you can. Since all of the isotopes have different masses, this will eventually create a distribution, with heavier isotopes (those with more neutrons) near the bottom. It’s the same principle as sand settling to the bottom of water in gravity, except that here gravity alone isn’t enough.

Even the immense force of the centrifuge really isn’t enough to get an appreciable amount of the necessary isotopes. You need to repeat the process with a thousand other centrifuges, all feeding forward, all enriching the uranium just a bit more, until you get a few kilograms of highly enriched uranium (you’ll have started with several dozen tons of uranium ore). The amount of energy, engineering, and time this takes is staggering.

Actually, this laborious and energy intensive process is the “easy” way of enriching uranium. Prior to the invention of centrifuge enrichment, a process known as gaseous diffusion was used. Compared to it, centrifuges require very little energy. This has been great for the energy return on investment of civilian nuclear plants, but less good for proliferation risk.

This process may eventually become even easier, due to technological advances like laser enrichment. But for now, you have to put in this work if you want uranium for your bomb.

Again, all of this is necessary for uranium bombs because naturally occurring uranium is depleted. To make nuclear weapons, you need highly unstable uranium. But all the uranium on earth was created billions of years ago, in the hearts of now long-dead stars. Over time, more and more of the unstable uranium has broken down, leaving mostly the more-stable isotopes.

2 billion years ago, fission could occur in uranium deposits. That’s now impossible.

When people discuss enriched uranium, they frequently talk about enrichment percentage. This is the relative mass of the fissile isotopes compared to the total mass of the material. For most power plants, an enrichment of 3-5% is used. For certain experimental reactors, including those that produce the radioisotopes necessary for medicine, 15-20% is more common. For nuclear weapons, the uranium must be enriched to about 80%, although higher enrichment is better. Low amounts of contaminants are necessary for nuclear weapons to function, whereas contaminants are well tolerated in nuclear reactors.

“Simple” enrichment of uranium to weapons grade costs untold millions of dollars. Of all the countries that do not currently possess nuclear weapons, only Iran, Germany, and Japan could enrich uranium up to weapons grade in a reasonable time frame. Non-governmental actors (like Al-Qaeda or Daesh) would find it essentially impossible to create their own fissile materials. The only way that they could gain access to the fuel for a nuclear weapon is if given it by another party.

3.2 Plutonium

It is much easier to acquire enough weapons-grade plutonium for a bomb than it is to acquire weapons-grade uranium. There are two reasons for this: separation is easier and the mass required to produce a plutonium bomb is lower than the mass required to produce a uranium bomb. Plutonium has a critical mass of 11kg, about five times less than that of uranium. This means that a viable plutonium bomb requires about one fifth the fissile material as a viable uranium bomb.

Weapons-grade plutonium starts with uranium. Like I said before, the plutonium-239 used in bombs no longer exists in nature.

To get plutonium from uranium, you need a nuclear reactor that’s running on uranium. This means that even when a country builds a bomb using plutonium, they must enrich some uranium. This presents all of the challenges outlined above, partially allayed by the fact that the enrichment percentage required is not very high, merely a few percent.

Inside any uranium fuelled nuclear reactor, some U-238 atoms will absorb neutrons, becoming U-239. This is a very unstable isotope (half-life: 23 minutes), which tends to briefly moonlight as Np-239 (half-life: ~2 days) before fulfilling its destiny and becoming Pu-239 (half-life: 24,110 years). Thus in many nuclear reactor designs, this Pu-239 will hang around in the fuel rods, occasionally decaying, fissioning, or absorbing neutrons (to form Pu-240), but largely just sitting there, waiting to be recovered.

If you want to get plutonium for a bomb, recover it you must. There are a variety of ways to do this, but all of them are much easier than centrifugation. The most common one is PUREX, which I’ll use as a representative example. In PUREX, you take the uranium fuel pellets from the reactor and dissolve them in very concentrated nitric acid, discarding anything that doesn’t dissolve. You then run a mixture of tributyl phosphate and kerosene over the acid. Any uranium and plutonium will move into the kerosene phase, while other fission products will remain in the acid. To separate the uranium from the plutonium, you run the kerosene mixture over water with ferrous sulphamate dissolved in it. The plutonium will react with the ferrous sulphamate, pick up a charge, and move into the water. The uranium will stay in the kerosene.

It’s easy to separate uranium from plutonium because they have a different number of protons. This means that they react differently with many chemicals. Extraction schemes take advantage of these different reactions to set up a scenario where the plutonium ends up in one solvent (like water in the PUREX example) and the uranium ends up in another (like kerosene). Complicated centrifuge arrangements are only necessary when dealing with different isotopes of the same element, where you can’t use tricks like this.

Weapons-grade plutonium (largely Pu-239) does have some problems with other plutonium isotopes. Remember how the critical mass for a plutonium bomb is much lower than that of a uranium bomb? This is possible because plutonium-239 is on a hair trigger where fission is concerned. This is doubly true for plutonium-240, in a way that is very problematic for weapons designers. Plutonium-240 is so prone to detonation that it often detonates early, blowing apart the bomb prematurely and leading to what’s termed a “fizzle” (this will be covered more in the next section; for now, you’ll just have to trust me that fizzles lead to uselessly weak bombs).

No one wants to separate Pu-239 from Pu-240; one of the major advantages of plutonium bombs are that you don’t needs to set up a huge centrifuge plant. To avoid having to separate the two isotopes, groups that are preparing plutonium for bombs aim to avoid the production of Pu-240 altogether. This means that any reactor optimized for producing plutonium can only run for about 90 days at a time. After uranium has been in a reactor for 90 days, the Pu-240 concentration is too high to easily build a working bomb. Despite this, the International Atomic Energy Agency runs audits on spent fuel from reactors, ensuring that no plutonium can be diverted from civilian reactors to weapons.

To economically start and stop a reactor every 90 days, you need a special reactor design. Civilian light-water nuclear reactors won’t do the trick. They have a huge pressure vessel that needs to be laboriously disassembled by specially trained divers every time they needs to get at the fuel. This is so inconvenient that civilian reactors are normally only cracked open once a year – by which point the plutonium has far too much Pu-240 to be viable in most weapons.

Reactors optimized for plutonium production allow for the rapid cycling of U-238 pellets through the reactor core. This requires quite a bit of engineering work – which has to be done from scratch unless you can find a country willing to share their schematics with you. It’s also basically impossible to hide the purpose of a reactor like this. Any IAEA inspector who sees your reactor will understand right away what you’re doing with it.

Once you have your reactor and your fuel, you have to decide how you want to run it. The shorter your cycles, the closer to your plutonium to pure Pu-239, but the more you’re going to pay per gram (in reagents, labour, wasted fuel rods, and wasted time as you cycle the reactor). The US uses >97% Pu-239 in the nuclear weapons on its submarines (Pu-240 produces a lot of gamma rays, which would be dangerous for the crews in close quarters), while its silo based weapons only use ~93% pure Pu-239.

To give an idea of the cost difference for increasing purities, the US Government helpfully lists 87% Pu-239 as $5,840 per gram, while 94% Pu-239 is $10,990 per gram. At these prices, the 3-4kg of plutonium in an atomic bomb would cost about $44 million if weapons-grade (and untold millions more if super-grade).

3.3 Next Steps

I asserted above that it is much more difficult to build a bomb out of plutonium than uranium. This is because the simplest type of nuclear weapon, the “gun” based design, does not work with plutonium. I’ll cover this in depth in the next post in the series. Here, I’m going to discuss problems common to all nuclear weapons designs because there’s significant overlap. Even with the simplest gun design, building a reliable, deliverable nuclear weapon is a significant engineering challenge. There’s no off the shelf design to copy, so you’re going to have to come up with your own solutions to problems like:

  • How can I get this to detonate every single time?
  • How can I make this package small enough that it is easy to deploy?
  • How can I ensure this thing detonates at the correct altitude?
  • How can I package this such that it doesn’t get messed up by wild swings in temperature and pressure?
  • How can I mount this on a missile?
  • How can I keep that missile from burning up on reentry? (This is one of the last things preventing North Korea from having a weapon system capable of targeting the USA)

Eventually, you’re going to have to test out your design. This means you’re going to have to set up specialized test chambers underground. You won’t want to detonate a bomb in the atmosphere because a) this is super-duper illegal under international law and b) It’s super-duper obvious to all of the UN Security council members (and any country with a functioning espionage program) that it was you who just test detonated a bomb. A combination of a) and b) means that atmospheric detonations are a one-way ticket to becoming an international pariah and all the sanctions that entails.

Underground tests are only slightly better. Seismometers will detect all but the smallest nuclear test detonations when they’re conducted underground, allowing several countries (most notably the US, with its excellent seismometer arrays) to detect and precisely locate the test detonation and estimate the yield. On the bright side, if you have the good sense to conduct your tests underground, you’ll probably avoid crippling sanctions (just don’t be surprised when you can no longer buy anything that might be used in a missile from any other country).

While it’s probably possible for a sufficiently advanced country to build a very simple uranium-based nuclear weapon and have it function with some degree of reliability without testing, any design that uses plutonium, as well as all of the largest, most complicated weapons systems (like the Teller-Ulam design) require either testing, or advanced simulations packages seeded with top-secret data from past tests. Without these tests, your weapon has a very low chance of actually working in the field.

This doesn’t even get into the colossal amount of effort required to develop and test all of the technology necessary to successfully deliver a warhead to a target. Stealth bombers, advanced missiles with MIRV technology, and silently running nuclear armed submarines are all necessary for a country to have full nuclear capability and all of these are impossible to develop in secret.

In summary, there are 6 things a country or non-state actor most do to develop a full, modern nuclear weapons program:

  1. Enrich or acquire a suitable supply of fuel
  2. Design a weapon to make use of that fuel
  3. Miniaturize the weapon
  4. Design robust (capable of surviving an initial nuclear strike) delivery systems for the weapons
  5. Design and test boosted weapons and fusion weapons
  6. Tests these systems, individually and as a whole

There are currently 4 or 5 countries that have completed these steps: China, Russia, America, and India (Israel keeps so much of its nuclear program secret that no one knows if it’s completed all these steps or not). Pakistan, the UK, France, and North Korea have checked off some, but not all boxes on this list, leaving them vulnerable to pre-emptive nuclear attack.

Even if checking off all the boxes is hard, there is a significant risk whenever a country or non-state group checks off any of them. Enriched materials can be used to make dirty bombs, conventional explosive devices that disperse highly radioactive materials (as an area denial or terror tactic) even if a true nuclear device cannot be made. Potentially city destroying nukes require relatively little fissile material or technical expertise. Untested weapons can still work some of the time. The UK, France, and Pakistan all possess a lot of warheads and their inability to protect themselves against a pre-emptive nuclear strike from Russia or the US doesn’t mean they can’t annihilate less powerful countries if they so choose.

All of these represent significant threats to individuals. A small nuke or a large dirty bomb can do a lot of damage if used on the correct target. But so can a conventional weapon. A MOAB or FOAB used on a sporting event would be at least as devastating as a dirty bomb. It is only large arsenals of modern thermonuclear weapons, possessed by countries in numbers large enough that some can reasonably penetrate any countermeasure that represent a true existential threat to humanity. Furthermore, all groups or countries that haven’t completed every single item on my list are vulnerable to having their whole nuclear program destroyed by an adversary before they can use it at all; this severely limits their ability to make threats.

Because of the difficulties inherent in proliferation, the greatest gains to be made in protecting humanity from nuclear weapons must necessarily come from convincing existing nuclear powers to decrease their stocks of weapons or delivery vehicles. This is especially true of Russia and the US, which together possess more than 93% of the world’s nuclear weapons. The challenge will be to do this while maintaining a credible deterrent against any rogue nation that wishes to develop a nuclear weapons program of its own.

Additional Reading: NPT (Full Text), Enriched Uranium, “Weapons Grade” Nuclear Material, Plutonium-239, Nuclear Reprocessing, Proliferation/Breakout Capability, Dirty Bomb, and Nuclear Weapons Testing

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