US warplanes have bombed three nuclear sites in Iran, the same places Israeli planes had targeted in its ongoing war with Iran. One of the targets was Fordo, a uranium enrichment plant hidden in a remote mountainside that is vital to Iran's nuclear ambitions. Key nuclear facilities at Natanz and Isfahan were also hit. It is not yet clear how much damage was done to the facilities, though the UN's nuclear watchdog says there had been no increase in off-site radiation levels reported. But what do we know about the sites, and what are the risks of attacking nuclear facilities? The IAEA described Israel's attacks on Iran's nuclear facilities as deeply concerning. Last week, its Director General Rafael Grossi said that military escalation increases the chance of a radiological release with serious consequences for people and the environment. Uranium enrichment sites are used to build up supplies of a particular type - or isotope - of uranium. When you dig uranium out of the ground, it comes in two forms: 99.3% is uranium-238 - and 0.7%, or about one atom in 150, is uranium-235, and this is what you need to work in your nuclear reactor, explains Professor Paddy Regan from the University of Surrey and the UK National Physical Laboratory. Burst of energy The process of nuclear enrichment basically means increasing the amount of uranium-235. This is done by taking uranium in its gas form and spinning it in machines called centrifuges, Prof Regan said. And because uranium-238 is heavier than the required uranium-235, the two separate out as they spin. This is repeated again and again to boost the enrichment. Nuclear power stations typically need about 3-5% of this enriched uranium to generate a controlled nuclear reaction that releases energy. But when the aim is to make a nuclear weapon, a much higher proportion of uranium-235 is needed - about 90%. Essentially, the more enriched the uranium, the bigger the burst of energy when all those atoms split. The IAEA said that Iran's uranium had reached about 60% enrichment - so well on its way to being concentrated enough for a nuclear weapon. But firing a rocket into properly stored stockpiles of enriched uranium would not pose a nuclear incident on the same scale as disasters that occured at nuclear power plants in Fukushima or Chernobyl. Highly enriched uranium is about three times more radioactive than non-enriched uranium. But in fact, on the scale of things, neither of them are particularly densely radioactive. It wouldn't cause a major environmental contamination problem, explains Prof Jim Smith, from the University of Portsmouth, who has studied the aftermath of the Chernobyl disaster. We're more concerned about what are called the fission products - the things that uranium splits up to when it's in a reactor or in a bomb - things like radioactive caesium, radioactive strontium, radioactive iodine. They are more of an environmental contamination issue. But because no nuclear reaction is taking place at the enrichment sites - and a blast from a bomb would not trigger one - these dangerous radioactive fission products would not be present, he said. Instead the uranium could be dispersed locally by a blast. Localised threat The IAEA said there had been no increase in off-site radiation levels reported after the US attacks. Last week, when Isreal hit the Natanz facility, the IAEA found radioactive contamination at the site, but they said the levels of radioactivity outside remained unchanged and at normal levels. With uranium... the radiation doesn't really travel very far, says Prof Claire Corkhill, Chair in Mineralogy and Radioactive Waste Management at the University of Bristol. But for people close to the site, there could be health risks, she said. In terms of toxicity to the human body, you certainly don't want to breathe in uranium particles and you don't want to ingest them either, she said. That's because the uranium particles could become lodged in the cells, inside either your lungs or your stomach, and slowly, radioactively decay, and that will cause damage. As well as radioactivity, chemical exposure could also be a problem for anyone nearby. If there was an incident and the centrifuges were to release the uranium hexafluoride, the gas contained within the centrifuges, then it would be a really severe chemical incident, said Prof Simon Middleburgh, a nuclear materials scientist, from Bangor University. If this uranium hexafluoride comes in contact with moisture in the air, it's really quite corrosive and nasty because it can form this very, very strong acid, he said. But it's not going to have a massive environmental impact beyond the very, very local area. The IAEA said its Incident and Emergency Centre had been working around the clock. The UN agency said it would provide further assessments on situation in Iran as more information became available.
