Reports on how much highly enriched uranium (HEU) Iran may possess vary a bit, but a consensus figure is an estimate of about 440 kg, enriched to 60%+ content of fissile uranium 235 (U₂₃₅). The vast percentage of the remainder would be non-fissile uranium 238 (U₂₃₈). From the proliferation perspective, this is a lot. If further enriched it would be enough for, perhaps, nine weapons. But from the standpoint of detection and external visibility, the actual physical footprint of this much HEU is actually minimal.

Based on commentary on social media, it seems to be broadly the case that the general public and even some non-specialist military commentators seem operate under a belief that HEU is a radiation hazard and would be easily discovered remotely. The physics that makes U₂₃₅ fissionable and thus fuel for both reactors and bombs is only partially related to the characteristics that make things into unhealthy radiation hazards. The banal truth is that many other radioactive substances, like caesium 137, iodine 131, radium 226, and cobalt 60 have far stronger radioactive emissions, more detectable than U₂₃₅. Let us look at the details.

First, let us consider size and volume. Uranium is a dense metal at 19.1 g/cm³. That may not mean anything to most people, but it is 70% denser than lead. 440 kg of uranium works out to only about 23 litres of material. This is less than a small suitcase, slightly more than five standard rugby balls if the uranium is in pure metallic form. It is likely that some or most of the HEU is in the form of uranium hexafluoride (UF₆). This form is gaseous at high temperatures, but solid below 56.5°C. This form is less dense, but 440 kg of HEU in UF₆ form is only the size of a few suitcases. There are safety reasons why it would not be in one spot but divided up.

But surely, that much HEU is noticeable by radiation detection? Only if you get close to it. Radiation physics has a lot of units of measurement that are arcane to the layperson, but they are necessary to understanding the situation. Radioactive materials are radioactive because their atoms are built in a way that is fundamentally unstable. These materials ‘decay’ by shedding mass and energy, which we call ‘radiation’. This can happen quickly with some materials and very slowly with other materials. A term of measurement for this is the ‘half-life’ – the length of time for half an amount of material to decay into something else. As a rule of thumb, the fast-decaying things – so those with short half-lives – give out more radiation than things with a long half-life. Cobalt 60 is dangerously radioactive with a half-life of just over 5 years. U₂₃₅ has a half-life of over 700 million years, which is quite the difference.

The other thing is that the different ways in which atoms shed energy and mass to decay from instable radioactive forms in gradual steps differs from element to element, but is well known. This ‘radiation’ takes the form of charged particles (alpha particles or beta particles) or energy (gamma rays); often a mixed cocktail is emitted. If we know the rate at which uranium atoms decay, how many atoms are there in a particular bit of uranium, and the types of radiation that will be admitted, it is actually a simple calculation to see how much radiation energy will be emitted. A big issue with detecting uranium is that much of the radiation emitted consists of alpha particles. They travel only 2 to 4 cm in air, so remote detection is impossible.

But HEU does emit gamma activity, which is theoretically detectable at distance. Let’s apply this to a notional 440 kg lump of HEU purified to 60% U₂₃₅. How radioactive would that be? Again, not as much as you think. For the physics nerds in the readership, this works out to 0.57 Curies of U₂₃₅ and a paltry 0.06 Curies of U₂₃₈. I have done some manual calculations (your correspondent did a course on this in 2002) but feel free to do similar calculations online (Note: if you actually do this yourself, you’ll need to remember to add the U₂₃₅ and U₂₃₈ figures together).

This amount of HEU gives us, maximum, something like 781 microRem per hour at a distance of 10 m, totally unshielded. This is enough to detect, but still not a lot in the overall scheme of things, seeing how natural background from cosmic and geological sources ranges from 5-20 microRem per hour. But if you get further away, say 50 m, that chunk of HEU is only going to be 15-16 microRem per hour, so only around twice the background. Possibly noticeable. If you fly over at 100 m distance, it’s 3.3 microRem per hour. Lost in the variability of background noise. And this assumes no shielding, and all of it in one solid lump. This much HEU, in practical terms, is totally not detectable remotely. Iran is a big country, and the US is not going to fly over the length and breadth of it at 50 m or less.

If the US is going to find this HEU, which is likely in separate batches and in containers, radiation detection is not going to find it. Other forms of intelligence will be needed, such as communications intercepts and human informants.