J. W. DAWSON, Consultant based in the UK, and M. PHILLIPS,

Nympsfield Nuclear Ltd, UK

Abstract: The key design features of commercial gas-cooled reactors are presented. The majority of gas-cooled reactors were built in the UK and in France, with the former country progressing their development into the 1980s and beyond whilst France concentrated on water-cooled reactors. The progression from uranium metal fuelled reactors (known as ‘Magnox’ in the UK) to Advanced Gas-Cooled Reactors is discussed, with further discussion of high-temperature gas-cooled reactors and proposed Generation IV plant. The topics discussed include: design of pressure vessels; design of fuel; refuelling; moderator and coolant chemistry; fuel storage and handling; and waste and decommissioning.

Key words: Magnox-type reactors, AGR-type reactors, uranium metal fuel, uranium oxide fuel, carbon dioxide coolant, steel pressure vessel, concrete pressure vessel, high-temperature reactors, generation IV reactors.

11.3 Introduction

Gas-cooled reactors have been used since the earliest days of nuclear power — indeed, it could be argued that Fermi’s first pile in a Chicago squash court was the first gas-cooled reactor, even if the power output was only a few watts.

Gas-cooled reactors were used in the early weapons programmes. The UK Windscale Piles operated in the 1950s until the Pile 1 fire in 1957. These reactors were fuelled with natural uranium clad in aluminium, and were air cooled on a single pass arrangement. This fire was the first major nuclear incident in the western world leading to substantial releases of activity and exposure of the general population.

While much of the rest of the world was pursuing water-cooled reactors for civil power generation (to a large extent on the back of PWR-type reactors developed for underwater propulsion), the UK and France were notable for developing gas-cooled reactors. Both countries developed Magnox-type reactors, France building nine reactors and the UK eventually building 24 reactors.

The name ‘Magnox’ comes from the alloy used for the fuel cladding, in the UK predominantly magnesium with a small component of aluminium, and in France magnesium combined with low levels of zirconium. (Throughout this chapter, the term ‘Magnox reactor’ is used as a generic description of CO2-cooled, uranium metal fuelled and graphite moderated reactors.) In both cases the fuel was natural enrichment uranium metal. The designs built on the technology of early piles used for the production of plutonium, which used aluminium-clad, natural uranium metal fuel. In the UK the first eight Magnox reactors (four each at Chapelcross and Calder Hall) were used for the manufacture of weapons grade plutonium, and in the case of Chapelcross were also used for the manufacture of tritium for military purposes until shut down. They were also used for producing electricity for the civil market. Due to the increased neutron absorption of lithium capsules used in the two Chapelcross tritium-producing reactors, the fuel was low-enriched uranium. Thermodynamic efficiency was not a strong consideration.

The reactor pressure vessel of gas-cooled reactors is enormous by comparison with typical water reactor plant. This results partly from the need to accommodate the massive graphite moderator, as well as from the reduced heat removal capacity of the coolant. The latter in turn leads to the need for larger heat transfer surfaces and/or lower heat fluxes across the clad-coolant boundary.

A few other Magnox-type reactors were built: Latina in Italy, Tokai Mura in Japan and Vandellos 1 in Spain, plus several similar plants of low power constructed in North Korea. However, only in the UK were gas-cooled reactors pursued with enthusiasm. Evolving designs moved from steel pressure vessel reactors to pre­stressed concrete pressure vessels with the construction of the later plants at Oldbury and Wylfa, and the last three French Magnox class reactors of Chinon A3, St Laurent des Eaux and Bugey 1 also used pre-stressed concrete pressure vessels.

The last of the UK Magnox reactors was completed in 1971 and in the UK was superseded by the Advanced Gas-Cooled Reactor (AGR). The reasons for the development of this reactor type were part political (development of a domestic technology) and part technical, in particular the increased thermodynamic efficiency associated with the much higher coolant temperatures of the AGR (exceeding 600 °C). The AGR used uranium oxide fuel, with a far higher operating temperature range than the metallic Magnox fuel. This improved the thermodynamic efficiency to around 42%, compared to around 28% in a Magnox plant (and 32% in a modern PWR).

A total of 14 AGR reactors were built on six sites in the UK (plus a small, prototype reactor). The original design life of these reactors has by the time of writing been exceeded across the fleet by an average of five years.

Other types of gas-cooled reactor have been designed or built. High temperature gas-cooled reactors (HTRs) such as the USA’s Peach Bottom were constructed in the 1960s, and a variety of other reactors such as Germany’s AVR and the UK’s Dragon were built. Many fuel designs exist, typically using small fuel pellets encapsulated in pyrolytic carbon and silicon carbide (TRISO fuel). Proposed future generation IV (gen IV) reactors include very high temperature reactors (VHTR) similar in concept to the HTR using either pebble or prismatic fuel, as well as variations on gas-cooled fast reactor designs.

In the following, the discussion centres on Magnox (Section 12.2) and AGR plant (Section 12.3), and follows with a brief discussion of HTR and future systems (Section 12.5).

302 Nuclear fuel cycle science and engineering