Two basic layouts of advanced gas-cooled reactors (AGRs) are being built in the United Kingdom: a single-cavity arrangement in which platen boilers are positioned in an annulus around the core, and a multicavity arrangement in which pod boilers are symmetrically disposed within the walls of the concrete pressure vessel The steam side conditions for all the boiler designs are broadly in line with conventional power station practice, 160 bar/538°C/538°C at the turbine stop valve (2300 psig/1000°F/1000°F). The platen boilers are designed and manufactured by Northern Engineering Industries at Gateshead and Derby in England while the pod boilers are designed by Babcock Power and manufactured at their Renfrew works in Scotland. The boilers have carbon steel materials in the economizer sections but a 9%Cr—l% Mo steel is used in the evaporator sections to minimize CO2 corrosion on the gas side while retaining the resistance to stress corrosion cracking on the water side, which is associated with a ferritic steel Type 316 austenitic stainless steel is adopted in the superheater and reheater banks—the transition point in the superheater being defined to ensure a minimum superheat of ∼ 70°C during operation. The single-cavity design with platen boilers has been adopted for the latest AGRs at Hey sham II and Torness because the designs on which they are based (Hinkley Point B and Hunterston B) had been operating for two years at the time the decision was made