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 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.
The first generation of French nuclear power stations were UNGGs, as was Vandellos unit 1 in Spain. Of ten units built, all were shut down by end 1994, most for economic reasons due to staffing costs. The largest was Bugey 1 with a net electrical output of 540 MW.
- G1, G2 and G3 at Marcoule. G1 was the first UNGG to go critical, in 1956.
- Chinon Nuclear Power Plant A1, A2, and A3 in Indre-et-Loire.
- Saint-Laurent Nuclear Power Plant A1 and A2 in Loir-et-Cher, operated from 1969 and 1971 to April 1990 and June 1992.
- Reactor 1 at Bugey Nuclear Power Plant in Ain, the last UNGG built in France, first criticality 1972, closed May 1994.
- Vandellòs Nuclear Power Plant Unit 1 in Spain, the only UNGG built outside France, closed July 1990.
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.
 See also
- French activities on gas cooled reactors, D Bastien, IAEA-TECDOC—899, pp:51-53, 30 Oct - 2 Nov 1995
- Twenty-nine years of French experience in operating gas-cooled reactors, D Bastien, IWGGCR—19, pp:113-119 21-23 Sep 1988
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