‘The Iranian regime has committed multiple violations of the agreement,’ Donald Trump said last week. ‘For example, on two separate occasions, they have exceeded the limit of 130 metric tons of heavy water.’

In 1931, the American physical chemist Harold Urey discovered deuterium, the isotope of hydrogen that has a neutron in its nucleus along with a proton. He manufactured some ‘heavy water’ (D2O) and, I think, drank some. Heavy water remained an interesting laboratory phenomenon until the Second World War, when it took on new importance since it plays a role in the production of plutonium, which does not exist naturally on earth.

In a nuclear reactor, a uranium-238 nucleus that absorbs a neutron becomes U-239 which decays into neptunium which in turn decays into plutonium. The neutrons absorbed by the U-238 are produced by the fission of the less common isotope U-235. To keep the reaction going, some of the neutrons flying around need to hit U-235 nuclei, causing them to split and releasing more neutrons – a chain reaction. The role of the heavy water is to moderate or slow down the neutrons, which increases their ability to produce fission of U-235. Heavy water is such a good moderator that you can use natural uranium – which is more than 99 per cent U-238 – in the fuel elements. During the Manhattan Project the US built three heavy water production plants. The role of heavy water was also understood by the Germans but their attempts to produce it were sabotaged.

In the 1980s the Iranians began working on heavy water and when in the 1990s they started building a plutonium-producing reactor in Arak, they placed a heavy water production facility nearby. One of the great successes of the multinational agreement with Iran is that the Arak reactor is being reconstructed as a light water reactor with a greatly diminished capacity for plutonium production. The construction of heavy water reactors is forbidden for fifteen years. ‘All excess heavy water,’ the treaty says, ‘will be made available for export to the international market.’ Iran is allowed to keep 130 metric tons. On two occasions they stored some twenty tons in Oman. This was discovered by IAEA inspectors and it seems that all the permitted heavy water is now back in Iran. What the Iranians plan to do with it is a mystery to me. What Trump fails to recognise is that this is a triumph of the agreement. Without the deal, the Iranians could produce unlimited amounts of heavy water which they could have used in their Arak reactor or a future one to make plutonium.