UNGG reactors use natural uranium as fuel, graphite as moderator, and carbonic gas (CO2) under pressure as coolant. In French reactors, the core was in the form of a pile of graphite bricks in hexagonal shape. Each brick was pierced by a canal in which the fuel was placed in the form of cartridges along which gas circulated.

No UNGG operates in France today. Apart from research reactors, the UNGG were G1, G2, G3 at Marcoule (Gard), Chinon A1, A2, and A3 (Indre-et-Loire), Saint-Laurent-des-Eaux A1 and A2 (Loir-et-Cher), and Bugey 1 (Ain). At the end of the sixties, EDF decided to abandon the UNGG line to the benefit of pressurized water rectors. The first UNGG reactor, G1, was put into service in 1956: the last, Bugey 1, in 1972.