Generation-I reactors were built in the initial period of nuclear power expan­sion and generally had primitive design features. Most of these reactors have either been shut down or will be soon done so. Examples of such reactors are Magnox reactor (Calder Hall reactor in the United Kingdom) and first com­mercial power reactor at Shippingport in 1957 (in the state of Pennsylvania in the United States).

Magnox Reactor

This is a notable Generation-I gas-cooled reactor. Early breed of this reactor was used for the purpose of plutonium production (for atomic weapons) as well as elec­tricity generation. Figure 1.8a shows a cross section of a typical Magnox reactor. The

Calder Hall station in the United Kingdom was a Magnox type of reactor starting successful operation in 1956. Following that, several of these reactors were built and operated in the United Kingdom and a few elsewhere (e. g., Italy, France, and Japan). Generally, Magnox reactors were graphite moderated, and used the natural uranium as fuel clad in thin cylindrical tubes of a magnesium alloy (Magnox comes from the name of the magnesium-based alloy with a small amount of aluminum and other minor elements, magnesium nonoxidizing, for example, Mg-0.8Al — 0.005Be) and carbon dioxide (CO2) as coolant (heat transfer medium). Magnesium — based alloy was chosen since Mg has a very low thermal neutron capture cross sec­tion (0.059 b; lower than Zr or Al). The fuel elements were impact extruded with the integral cooling fins or machined from finned extrusions (Figure 1.8b). Also,

the alloy was resistant to creep and corrosion from CO2 atmosphere in the operat­ing temperature range, and contrary to Al, the alloy did not react with the uranium fuel. The addition of Al in the alloy provided solid solution strengthening, while the presence of minor amounts of Be helped improve the oxidation resistance. CO2 was circulated under pressure through the reactor core and sent to the steam gener­ator to produce steam that is then passed through a turbo generator system gener­ating electricity. These reactors could sustain lower temperatures (maximum coolant temperature of 345 °C) and, thus, has a limited plant efficiency and power capacity. This was mainly out of the concern of the possible reaction of CO2 with graphite at higher temperatures and the lower melting point of uranium fuel (1132 °C). Another problem was that the spent fuel from these reactors could not be safely stored under water because of its chemical reactivity in the presence of water. Thus, the spent fuels needed to be reprocessed immediately after taking out of the reactor and expensive handling of equipment was required. Only two Mag — nox reactors still operating in the United Kingdom are scheduled to be decommis­sioned soon. Magnox reactors were followed by an improved version of gas-cooled reactor known as Advanced Gas-Cooled Reactor (AGR) operating at higher temper­atures and thus improving the plant efficiency. The magnesium fuel element was replaced by stainless steels.