Heavy Water

Jeremy Bernstein

‘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.


  • 17 October 2017 at 9:38pm
    Codester says:
    This post is so disingenuous. Mr. Bernstein claims it's a mystery to him what the Iranians plan to do with 130 metric tons of heavy water after laying out the use of heavy water to enrich uranium to weapon grade plutonium.
    Mr. Bernstein also fails to recognize the agreement with Iran allows the Iranians to produce heavy water while Trump would seek an agreement that forbids Iran from producing any heavy water. The only use for heavy water is for the production of nuclear weapons. The agreement only restricts the Iranians ability to produce large quantities of plutonium quickly. It does not prevent them from producing weapon grade plutonium, however. The Iranians are perfectly capable of producing enough plutonium to build several nuclear weapons under the current agreement.

    • 17 October 2017 at 10:41pm
      Jeremy Bernstein says: @ Codester
      Would Codester care to explain how the Iranians are capable of producing plutonium now? Would he/she explain to us how plutonium is produced?

    • 18 October 2017 at 1:21am
      mr_hockey says: @ Codester
      According to Codester, "The only use for heavy water is for the production of nuclear weapons"

      So obviously the massive amount of heavy water used in Canadian CANDU nuclear reactors must be for the well-concealed weapons program of Soviet Canuckistan?

    • 18 October 2017 at 9:03am
      John Cowan says: @ Codester
      In addition to its use in CANDU-type reactors, heavy water also has biological applications. It is routinely used in metabolic testing with a dose of several grams per test (the human body naturally already contains about 5 grams of heavy water). It has also been used in the treatment of high blood pressure and as a (rather toxic) chemotherapy agent.

  • 18 October 2017 at 12:12pm
    coprolite says:
    I think Trump is right to be concerned but tearing up the agreement is not the answer.

    I am still less worried by Iran being a nuclear power than I am by Trump.

  • 18 October 2017 at 12:25pm
    stettiner says:
    "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".

    It seems.. And we are cool with that...

  • 18 October 2017 at 4:55pm
    Jeremy Bernstein says:
    Since Codester has not chosen to favor us with a reply perhaps I can help out. All plutonium s produced in nuclear reactors. Therefore the question of whether the Iranians can now produce significant quantities of plutonium devolves onto the status of their reactor program. All Iranian reactors including the ones under construction are supervised by the IAEA. The one reactor that was designed to produce plutonium-the IR40-at Arak is being converted from a heavy water reactor to a light water reactor-not adapted to producing plutonium. The power reactor at Bushehr a large light water reactor does produce significant quantities of plutonium. The IAEA tags all incoming and outgoing fuel elements which are supplied and removed by the Russians. Unless Codester can tell us about a reactor that has escaped the IAEA the comment is nonsense.
    On the other hand I do not have an explanation of why the Iranians tried to store heavy water in Oman but I am grateful that the IAEA spotted it.

  • 18 October 2017 at 6:54pm
    Jeremy Bernstein says:
    May I correct something I said and I thank the experts Scott Kemp and David Albright for this correction. The newly designed Arak reactor uses both heavy and light water but features in the design including the choice of fuel elements make it less suitable for plutonium production.

  • 18 October 2017 at 9:17pm
    Graucho says:
    Ever since Obama humiliated Trump at that press gathering, he has been hell bent on revenge by shredding the former president's legacy. These are the conclusions on which my alternate facts are based has been his consistent line of reasoning. Rational debate is alas pointless faced with this.

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