Thread by @nuclearkatie

Iran is blowing past their stockpile limit, making them out of compliance with the Iran nuclear deal aka JCPOA. Have you ever wondered what that *actually* means from a scientific standpoint? Thread 1/

This thread is for everyone so lets start with the very basic chemistry and nuclear physics. You all remember the periodic table right? All the elements we’ve ever discovered or created, organized by their number of protons 2/

The number of protons (atomic number) governs the element; an oxygen atom always has 8 protons. The atomic number along with the number of electrons govern the chemical properties of an element 3/

But wait, there’s more! In addition to the atomic number, there’s the atomic mass. But when you look at the periodic table, you see that many masses (like Cobalt here, at the bottom) aren’t nice round numbers. Why? 4/

Well, partially because protons, neutrons, and electrons have weights that are close to 1 or 0 (electron) atomic mass units (amu) but not quite there.

Note that amu is 1.67377 * 10^-27 kg. Atoms are incredibly SMALL, and their constituent units are even SMALLER, its wild 5/

But also because of isotopes. What tf are isotopes? Well, an isotope is when an element can have different numbers of neutrons while the number of protons remains fixed. For example, here are three isotopes of hydrogen. 6/

We typically think of hydrogen as one proton, zero neutrons (hydrogen-1 aka protium), but adding neutrons results in deuterium (one neutron) and tritium (two neutrons). When water is comprised of deuterium and not protium, we call it “heavy water" 7/

So the atomic mass has to take into account these isotopes. Each of the individual atomic masses is weighted by it’s natural abundance to get the atomic weight 8/

Quick aside: don’t get the masses confused! Atomic mass or mass number is equal to the number of protons plus neutrons and it IS a round number. The atomic weight of an *isotope* is defined as the mass of an atom relative to the mass of a neutral carbon-12 atom. 9/

Atomic weight of *element* takes into account the atomic weight of all naturally occurring isotopes.

We identify isotopes by their mass number, i.e. cobalt-60 or cobalt-58

Clear as mud? Great, let’s move on 10/

It’s been a long way already, but we’re ready to start talking about uranium (that’s what you came here for right?). Uranium has two naturally occurring isotopes, U-235 and U-238

Answering a common question: yes you can own uranium ore 11/

https://twitter.com/nuclearkatie/status/1052586382949007361

The number of neutrons in an atom governs its nuclear properties, and these two isotopes are totally different from a nuclear standpoint! Uranium-235 is fissile, which means it can keep a nuclear chain reaction going (with slow neutrons but that’s not important right now). 12/

We use U-235 to drive our nuclear power plants, and it’s also the isotope that most countries use in their first nuclear weapons. 13/

On the other hand, U-238 can’t keep a chain reaction going, so it’s not what you want for a uranium bomb. It can however be used to create plutonium (which can be used in a nuclear weapon), but transmutation aka fuel breeding is slightly outside the scope of this thread 14/

Plutonium bombs are a lot harder to make, so if you’re a country with a fledgling (or clandestine) nuclear weapons program, you’re almost certainly gonna start with uranium bombs 15/

Another aside: please ask questions if you have them! I can talk about things like fuel breeding or fissile/fissionable/fertile in comments or in later threads if desired 16/

So here’s the bad part for anyone interested in nuclear chain reactions with U-235— only one out of every 139 atoms of natural uranium is U-235. Pretty much the rest of it is U-238 17/

You can’t really make a bomb with natural uranium, 0.7% U-235. In fact, here in the US we don’t even make nuclear power plants that use that low of a fraction of U-235. Though Canada does, but they use heavy water in their reactors while we don’t, happy belated Canada Day! 18/

We say X% enriched to refer to the U-235 content in a sample of uranium when it has been raised above 0.7%. Correspondingly, the “waste” or tails with a U-235 fraction below 0.2% is called depleted 19/

What you really need for a US nuclear power plant is like 3-5% enrichment, maybe up to 20% for some fancy advanced reactor concepts. But even that isn’t anywhere near weapons grade. You need >90% U-235 to be considered weapons grade 20/

Now let’s bring back the chemistry we just learned! I promise it wasn’t all for naught 21/

Remember how I just said that protons and electrons govern chemistry? That statement remains true even for atoms with different numbers of neutrons.

In simple terms, U-235 and U-238 are identical from a chemistry standpoint 22/

This means you can’t use any fancy chemistry tricks to separate U-235 and U-238. The main difference between the isotopes from a non-nuclear standpoint is their mass difference 23/

U-238 has three more neutrons than U-235. That’s not nothing, but it’s also a tiny tiny difference. 5*10^-27 kilograms, in fact, or about 1% of the mass of the uranium atoms. It’s small, but it’s enough 24/

So to separate the isotopes, you make it a gas (UF6, in fact) and put it in a centrifuge. Literally a big fancy metal cylinder that spins really fast until the slightly-heavier U-238 moves towards the outside while the U-235 moves inward 25/

But like I said, the mass difference is tiny. So a single centrifuge just isn’t going to get the job done. You need to connect a bunch of centrifuges together, called a cascade. And if you want to produce more than a small amount, you need many cascades 26/

To produce lots of enriched uranium you need big halls of centrifuges spinning at supersonic speeds using tons of energy 27/

Enrichment is one of the areas limited by the Joint Comprehensive Plan of Action, commonly known as the Iran nuclear deal. There are limits on
✔️number of centrifuges (5060)
✔️the number of cascades (30)
✔️enrichment they can produce (3.67%)
✔️how much they can stockpile (300 kg)

One year after Donald Trump began violating the terms of the JCPOA, Iran exceeded its stockpile limit. It now has more than 300 kg of 3.67% enriched uranium, although it has not broken other clauses yet. 29/

So what this really means is that Iran hasn’t done anything to change their facilities, they just continued to produce material until they had more than they are allowed to have under the JCPOA. It's one of the more minor infractions they could've chosen 30/

I don’t want to wade into politics too far because this is a science thread, but this process suggests Iran is not yet “breaking out” or moving towards a weapon. Rather they appear to be putting political pressure on Europe and the US in a bid to get sanctions relief 31/

However, Iran has also threatened to start producing material at an enrichment higher than allowed (3.67%), which likely requires them to modify the operation or the design of their centrifuge cascades (my group does some research in this area) 32/

Anyway, I hope this thread has given some context behind how uranium enrichment works, why its necessary for pursuing nuclear weapons, and therefore why we care that Iran is (also) breaking the JCPOA.

Pls don't ask me what's going to happen next in politics cuz ¯\_(ツ)_/¯ 33/

Happy to answer questions like “is it necessary to have a domestic uranium enrichment program for countries who want nuclear power?” (No) or “can you even publish open science in enrichment?” (Yes, but carefully) 34/34