UNGG reactor

Two UNGG reactors at Saint-Laurent Nuclear Power Plant

The UNGG (Uranium Naturel Graphite Gaz) is an obsolete design of nuclear power reactor developed by France. It was graphite moderated, cooled by carbon dioxide, and fueled with natural uranium metal.

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.

Units

Cross section of UNGG fuel, showing internal cooling path

• 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

• Gas cooled reactor
• Nuclear power in France

References

• 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