The UNGG and the Magnox are the two main types of GCR. A UNGG reactor is often referred to simply as a GCR in English documents, or sometimes loosely as a Magnox. It was developed independently of and in parallel to the British Magnox design, and to meet similar requirements. The main difference between the two designs is that UNGG used a horizontal fuel rod orientation, rather than the vertical orientation used in the Magnox reactor. The fuel cladding material was magnesium-zirconium alloy in the UNGG, as opposed to magnesium-aluminium in Magnox. As both claddings react with water, they can be stored in a spent fuel pool for short times only, making short-term reprocessing of the fuel essential, and requiring heavily shielded facilities for this.
The programme was a succession of units, with changes to the design increasing power output. In the experimental phase they were built by the Commissariat à l'Énergie Atomique (CEA), and later by Électricité de France (EDF). The largest UNGG reactor build was Bugey 1 with a net electrical output of 540 MW.
The earlier units, at Chinon and Marcoule, had heat exchangers outside the main pressure vessel; Later units (Saint-Laurent, Bugey and Vandellos) moved these heat exchangers to inside the pressure vessel.