Heavy water

Heavy water, deuterium oxide or D₂O, is chemically the same as ordinary water but with a hydrogen nucleus that carries one extra neutron. That tiny difference changes everything: deuterium absorbs far fewer neutrons than light hydrogen, so a reactor moderated with heavy water can sustain the chain reaction with uranium exactly as it comes out of the mine, with no need to concentrate its 0.7% of uranium-235. That is the basis of Canada's CANDUs, where heavy water also cools the core, and of the German-designed PHWRs at Atucha I and Atucha II, which use it as both moderator and primary coolant. The light water in a PWR or BWR, by contrast, absorbs too many neutrons to sustain the reaction on natural uranium, which is why those reactors need it enriched.

Producing reactor-grade heavy water, at a purity of about 99.75% D₂O, means concentrating an isotope that occurs naturally in just one of every 6,500 water molecules. The near-universal industrial method is the Girdler-Sulfide (GS) process: water and hydrogen sulfide gas (H₂S) flow countercurrent between a cold tower and a hot tower, and deuterium migrates preferentially from the gas to the water at the cold stage through the simple chemical equilibrium of that isotopic exchange. It is a process that consumes a great deal of energy and handles a toxic, flammable gas at large scale, which explains why only a handful of countries ever built plants of this kind.

Argentina is one of them. The Heavy Water Industrial Plant (PIAP), in Arroyito, Neuquén province, is owned by CNEA and operated by ENSI, the company the Commission formed together with the province. It opened in 1993 with a nominal capacity of 200 tonnes a year, split across two 100-tonne lines, which at the time made it the largest in the world. With that output, Argentina supplied heavy water to Atucha I, Atucha II and Embalse, and even exported the surplus. The plant went years without producing, and since 2022 CNEA and Neuquén province have been advancing a reactivation plan aimed at bringing one of the two lines back on line, at a capacity of about 80 tonnes a year, while assessing whether to direct the second line toward a different industrial product, urea.