Atucha: the pressure-vessel PHWR

Atucha is a heavy water reactor that uses no pressure tubes: the whole core sits inside a forged steel vessel, as in a PWR, but much larger, about 7 meters inner diameter at Atucha II, because natural uranium needs a bulkier core and more moderator space between channels.

The coolant is heavy water at about 115 bar. It enters through the side nozzles, flows down the annular downcomer and rises through 252 vertical channels (451 at Atucha II) that cross the moderator tank. It leaves at about 300 °C toward two U-tube steam generators, the same as a PWR's.

Inside the vessel there is a second tank, the moderator tank, also filled with heavy water and at the same pressure as the coolant: that way the channel walls carry no pressure difference and can be thin. What is different is the temperature: the moderator is kept at about 200 °C, cooler than the coolant.

To do that, moderator water is drawn off continuously and passed through external coolers. That heat is not wasted: the moderator coolers raise steam and preheat the feedwater, contributing close to 10% of the useful thermal power. In a CANDU, by contrast, the moderator heat is lost.

Like a CANDU, Atucha refuels at full power, but with a single machine mounted on the vessel head. The machine latches onto a channel closure, opens the lid under pressure, pulls out the complete fuel assembly, a bundle of 37 rods 5.3 meters long, and lowers a new one. Roughly one channel is changed per day.

Atucha I started with natural uranium (0.71%) and since 2001 uses slightly enriched uranium at 0.85%, which cut fuel consumption by about 40%. Atucha II still runs on natural uranium. The fuel is made by CONUAR at Ezeiza and the heavy water was produced by the Arroyito plant, in Neuquén.

The secondary side is conventional: light water boiling in the steam generators at about 44 bar and 255 °C, a turbine, a generator and a condenser cooled with water from the Paraná de las Palmas river. Atucha I delivers 362 MWe gross and Atucha II 745 MWe, and the containment is a steel sphere inside a concrete building.

Only three reactors of this type were built: Atucha I (1974), Atucha II (2014) and the MZFR at Karlsruhe, the German 57 MWe prototype closed in 1984. Siemens abandoned the heavy water line in the 1970s, so Atucha II ended up as the last of its family in the world, completed by Nucleoeléctrica Argentina after almost 20 years of halted construction.