Iran’s nuclear programme has got plenty of press attention lately, with US and Israeli air attacks in June, and talks now possibly resuming with European powers. But it’s hard to find a succinct presentation on the technicalities of the nuclear weapons a nation like Iran might be trying to develop. Even the Oppenheimer movie didn’t make clear that there are two main types of bomb; and current news coverage hasn’t analysed which type Iran has (allegedly) been going for, and what that implies about their motives, their likelihood of success, and what it would take to stop them.
The Cold War engendered a sort of nuclear priesthood, a cadre of specialists in political science (like Kissinger), physics, or military affairs, who nurtured the perception that only they could grasp the technicalities of nuclear competition so as to keep us safe. But even though some aspects of nuclear weaponry are intricate, it’s nothing that only a priesthood can understand. De-mystifying it can help us double-check our leaders’ moves, to make sure we’re not being played with the threat of a “mushroom cloud.”
Disclaimer: I don’t like Iran’s theocratic government and I don’t think they should have nuclear weapons. In fact, I think most countries shouldn’t. However the technology can’t be un-discovered, so it may not be the worst thing for a half-dozen or so reasonably stable states to have some, in case a rogue government gets close to acquiring its first.
What I hope this article will shed light on is how close is close, what kinds of bombs can be used (or threatened) in what ways, what are the trade-offs a nuclear-ambitious government faces in deciding which to pursue, and what that says about their motives.
Uranium bombs: the basic atom bomb
Nuclear bombs and power plants alike rely on fission – splitting atoms, which releases enormous energy and sustains itself through a chain reaction. A slow, controlled chain reaction, like revving your car engine, is what nuclear power plants use: it produces enough heat to boil water, which turns steam turbines to generate electricity. (It’s been called the worst way ever invented to boil water.) A fast, uncontrolled reaction is like throwing a match into your petrol tank – an atomic blast.
The basic atom bomb is a uranium bomb. Uranium atoms are potentially ‘fissile’ (susceptible to fission) because of, basically, their weight – the high number of protons and neutrons in their nuclei. Whereas normal uranium has 238 neutrons (hence ‘U-238’), the U-235 isotope, with only 235 neutrons, is even more prone to fission because of the imbalance that the missing neutrons cause.
The effort of making U-235
You can only make a uranium bomb with U-235, not with normal uranium. It occurs in nature – it can’t be manufactured – but in tiny quantities, amid normal uranium. To get it, you have to purify raw uranium ore, transform the pure uranium into uranium hexafluoride gas, and then centrifuge the gas in metal tubes, for weeks, such that the normal, heavier U-238 sinks to the bottom and a whiff of U-235 remains at the top. Extract that whiff carefully, centrifuge it again, repeat a zillion times. That’s uranium enrichment.
It’s like boiling down maple sap for syrup, only worse. Pain in the ass doesn’t begin to describe it. U-235 composes about 0.7% of average uranium, whereas a concentration fissile enough for a practical, transportable bomb has to be about 90% U-235. (In theory one could make a bomb with much less-enriched uranium, say 60%, but it would be so large and heavy as to be impractical for weaponry.)
You need thousands of tons of uranium ore to extract the 100lbs or so (approx. 45kg) of U-235 necessary for a bomb. It takes hundreds or thousands of centrifuges, working around the clock for months or even years, to do it. It’s not extremely high-tech, but it is a massive industrial undertaking. How the Manhattan Project managed to do it in secret was perhaps its most impressive feat.
Next, the easy part
But once you have your U-235, it’s technically simple to make it into a bomb, because it’s so fissile. You only need to slam two blocks together, with conventional explosives as propellant (the ‘gun-type’ bomb design). To maximize contact, the two parts might have a mortise-tenon or ‘male-female’ shape. (The design of the Hiroshima bomb is still classified, but educated guesses converge on the male-female shape.)
Why do you need as much as 100 lbs of U-235? Because of the famous ‘critical mass’. Knocking a bunch of uranium atoms together hard enough to dislodge some of their neutrons will send those neutrons in random directions; by the law of averages some will collide with other uranium nuclei, dislodging their neutrons.
If enough such collisions happen, they set off a chain reaction of fission – each split atom in turn splits a few more. But to get enough collisions to sustain the chain reaction, you need a certain bulk of the fissile material, at least if you’re using the basic gun-type design. Otherwise, too many freed neutrons will fly away without hitting any other uranium atoms, and no chain reaction.
Hard to hide, hard to scale up and tricky to deliver
This underlies the most basic form of preventing nuclear proliferation. The process of refining uranium into enough U-235 to make a bomb is so onerous that it’s conspicuous and offers many opportunities to disrupt – destroying or damaging the centrifuges, cutting off the supply of uranium ore. And the bomb itself will be unwieldy: it can’t be miniaturised, because of critical mass, so it’s difficult to deliver by missile. Generally, it could only be dropped from a bomber or moved by land, or undersea.
By the same token, such a bomb would usually be a one-off, because it’s such an effort to amass enough weapons-grade uranium. A nation with a huge enrichment effort might manage to produce enough U-235for two or three bombs instead of just one; but if it’s trying to be covert or to protect the enrichment effort from possible attack, that gets more difficult with scale. And a nation with only one or two bombs has very constrained choices about how to use them, especially if everyone knows that’s all you have.
Plutonium bombs: differently difficult
A plutonium bomb is a different animal. Plutonium is a by-product of uranium decay or fission; it doesn’t occur in nature (except in wisps from natural uranium decay). It’s available in spent uranium, which is moderately enriched uranium that’s undergone controlled fission, as in a power plant, until there’s not enough fissile isotope left to sustain a chain reaction.
The plutonium must be separated out by an agonizing chemical process. That process too is a fairly large industrial undertaking, but it’s easier than enriching uranium with centrifuges to a weapons-grade concentration. So, a nation that goes the plutonium route to a bomb has an easier time producing fissile material: it only has to enrich uranium enough for a slow controlled chain reaction (3-5% U-235, vs. 90% for a uranium bomb), let that reaction play out over some weeks or months, then separate out the plutonium.
A feat of weaponeering
But then it gets tricky. Whereas U-235 is so fissile that the bomb design can be crude, plutonium is much harder to excite into fission: the high-speed contact area has to be much larger. That’s why the Manhattan Project had to choose the elaborate concentric-sphere design, in which a plutonium core was surrounded by, first, a buffer substance, then a hollow sphere of plutonium, then finally an enwrapping shell of high explosive. The shell would implode, forcing the outer plutonium sphere to compress itself into the waiting plutonium core.
As you can imagine, even apart from producing the nuclear material, that bomb design is extremely hard to engineer, machine, assemble, render safe for transport, and electronically detonate. Plus, if you accidentally let two of the plutonium pieces touch each other during assembly, they let off a flash of deadly radiation. A plutonium bomb is a feat of weaponeering, and it needs to be tested.
The Manhattan Project’s dilemma
That’s why the Manhattan Project’s one and only test, the Alamogordo test, was of a plutonium bomb. The project had been running twin tracks to produce fissile material: enriching uranium with centrifuges for a simple bomb, and extracting plutonium from spent low-enriched uranium for plutonium bombs. (That’s why the Oppenheimer movie showed two jars gradually filling with marbles – one for highly enriched uranium, one for plutonium.)
After years of enrichment, they had enough weapons-grade uranium for only one bomb. So, they couldn’t test it. But on the other hand, they were pretty sure it would go off. They had somewhat more plutonium, but that entailed the tricky design that needed testing.
Even after the successful Alamogordo test, the machining and assembly were so complicated they couldn’t be sure the next one wouldn’t be a dud. They didn’t want their first atomic attack on Japan to fail. So, they dropped the sure-thing uranium bomb first, even though it left the US without enough weapons-grade uranium for another. They dropped the second one, a plutonium bomb, probably in case the Japanese government had physicists well-enough informed to advise, after Hiroshima, “That was a uranium bomb. They probably don’t have enough enriched uranium for another one anytime soon. So we can hold out. Unless they’ve tested a plutonium bomb.”
Next in part 2: So which kind of bomb would a nuclear-aspiring state choose now?
