Fuel assembly fabrication

Fabrication starts with uranium dioxide powder, which is pressed into cylindrical pellets under loads of several hundred megapascals and sintered at about 1,750 °C in a controlled atmosphere to become dense ceramic. Each pellet measures just under a centimetre in diameter and a little more than a centimetre long, and that tiny amount of uranium is equivalent to the energy of a tonne of coal. The pellets are stacked inside zircaloy tubes, a zirconium alloy with tin, niobium, iron, chromium and nickel, which are welded shut and pressurized with helium to several megapascals, leaving a plenum for the fission gases that build up during irradiation.

The final assembly depends on the reactor type. A PWR uses a 17 × 17 rod array: each assembly is four to five metres tall, about 20 centimetres across and weighs half a tonne, and the core of an 1,100 MWe plant holds 193 assemblies with more than 50,000 rods in total. A BWR uses smaller arrays, from 6 × 6 up to 10 × 10 or 11 × 11. A CANDU builds short bundles, of 28, 37 or 43 elements each, designed to be loaded horizontally with the reactor running: the core of a 790 MWe CANDU has 480 channels holding 5,760 bundles in total. And a VVER, the Russian PWR family, builds its assemblies in a hexagonal cross-section instead of square, to make better use of the core's space.

World fabrication of light-water fuel is dominated by four companies: Framatome, Global Nuclear Fuel, TVEL and Westinghouse. Argentina makes its own at home: CONUAR, a CNEA joint venture founded in 1981 and opened the following year at the Ezeiza Atomic Centre, has continuously supplied the fuel for Embalse, Atucha I and Atucha II, using pellets pressed from uranium dioxide that Dioxitek converts in Córdoba. Since 2012 CONUAR has also held the certifications that made it the only supplier of CANDU components outside Canada, and in 1986 it created the FAE business unit (Fabricación de Aleaciones Especiales), which makes zirconium and other alloy tubes for nuclear use and merged with CONUAR in 2019.