Part one of this article described the two main kinds of nuclear bomb – uranium or plutonium – and their trade-offs. Enriching enough uranium for a bomb is a long and onerous process, but the bomb is easy to design and will probably work without testing. Plutonium bombs are the opposite: it’s not so hard to extract plutonium from spent uranium, such as a civilian nuclear energy reactor produces, and accumulate enough for many bombs. But it’s very hard to design and build a plutonium bomb; it has to be tested. And such a test might trigger a response.
So which bomb would a nuclear-aspiring state choose now?
One other thing about the choice between pursuing uranium or plutonium bombs: for reasons related to critical mass and contact area, it’s unfeasible to miniaturise uranium bombs. You’re stuck with a size and weight about that of the Hiroshima bomb – as big as a car, and heavier. You’re unlikely to have a missile that could deliver it, only a lumbering bomber or slow and vulnerable surface routes. Plutonium bombs by contrast can, with re-engineering and testing, be shrunk enough to enable missile delivery (as well as to dial down the explosive yield, in case you envision contained uses).
A nation seeking to produce nuclear weapons therefore would almost always prefer the plutonium route. But it may not have that option: it may lack the physics and engineering expertise to design and build a plutonium bomb; or it might fear to test such a bomb, since modern seismic instruments will always detect such a test. Or it may decide its purposes would be served using only a single uranium bomb, without heed to what may follow, as in a suicide mission; or it might conclude that the threat of a single use serves its purpose, for example strict deterrence.
Threshold and breakout
Uranium enriched to 60% has no use except to be further enriched to 90% weapons-grade. Moreover, it’s easier and faster to go from 60% to 90% than the first leap from 0.7% to 60%, because in these final stages there is much less U-238 to separate out. Plus, it’s easier to conceal because there is now less material to centrifuge. A government that enriches to 60% is therefore said to be ‘on the threshold’. Going the final mile of enrichment and bomb-making is ‘breakout.’
Since 1970, the non-proliferation treaty (NPT) has committed its signatory states, now numbering 191, to allow intrusive inspections if necessary to verify their safeguarding of fissile material, and that their uranium enrichment sticks to the low levels sufficient for civilian use. Any movements towards the threshold can thus be detected early. Similarly, since plutonium can be extracted from the spent uranium fuel of legal civilian reactors, a major plank of the NPT is the monitoring of spent fuel. Under the NPT, the International Atomic Energy Agency (IAEA) monitors civilian nuclear energy systems (and others like research reactors) to ensure no plutonium Is being siphoned off for weapons.
Where does Iran stand in all this?
Iran has generally been in compliance with the NPT over the decades, though skirting the line with undeclared activities first reported by the IAEA in 2005. But the US has a sensitivity to Iranian nuclear development different from that it had towards Pakistan, for example, to say nothing of Israel.
Pakistan is believed to have both uranium and plutonium bombs: its early production centred on the simpler uranium bombs, then expanding into the more flexible and easier-to-fuel plutonium bombs. Its two known tests in 1998 may have been one of each. For the record, Israel is believed to have mainly plutonium bombs – some 90 of them – but also a uranium-enrichment capability that may have given them the option of making uranium bombs too.
During the Obama years, the US pressed Iran to concede more. In the 2015 Joint Comprehensive Plan of Action (JCPOA), Iran agreed to inspection and monitoring that went beyond its NPT obligations and extended to the research and engineering behind nuclear bomb-making. In return the US and allies would lift some sanctions. President Trump, for reasons never explained, abrogated the JCPOA in 2018, after which Iran predictably seems to have accelerated its enrichment.
What exactly are the US and Israel trying to prevent?
Observers agree that Iran has a few hundred pounds of 60%-enriched uranium, so it’s reasonable to argue that it has intentionally moved to threshold status, dangerous though it might be.
There seems no indication that Iran is going for plutonium bombs. What the US and Israel are trying to stop therefore is a one-off uranium bomb (or two or three).
Such a bomb or bombs would give Iran some defensive or deterrent ability (against a land invasion for example), plus the theoretical possibility of an irrational suicide strike – a massive one-off attack on an enemy whose own, far-greater, nuclear arsenal (or that of its allies) would then destroy all of Iran.
Going the route of plutonium bombs would have advantages for Iran, assuming it truly wants nuclear weapons, in that it’s easier to generate and refine the fuel, and to produce multiple bombs with more delivery options. But the test would be detected and would bring on a forceful reaction likely to set Iran’s options back. All that effort for a net loss.
Iran’s options will depend on its intentions
I’ll leave it to others to further analyse Iran’s intentions, whether its moves are offensive or defensive. The two-parts of this article should give an idea of what capabilities each kind of bomb would afford. But one wonders why Iran is bothering to go so far down the nuclear road. Neither kind of bomb helps it greatly. Maybe the regime is rational and calculating but myopic. Still, it might see its major foes as relentless, recklessly violent, and fanatical, much as it itself is depicted to the populations of those foes.
And as for the US and Israel, the lengths to which they should go to preclude the irrational possibility of an Iranian suicide strike is debatable. The Iranian regime is fanatical in many ways, but never reckless, and definitely self-preserving. And practically speaking, you can’t destroy Iran’s enriched uranium by bombing, since it’s an element; at most you could scatter it.
Maybe both sides’ nuclear priesthoods are suffering from myopia – which is why we laypeople need to educate ourselves.
