Optimisation of operating conditions in Magnox reactors of the United Kingdom Central Electricity Generating Board

In addition to the normal age degradation processes in nuclear power plant, the eight Magnox plants of the United Kingdom Central Electricity Generating Board suffer from breakaway oxidation of mild steel structural components and radiolytic oxidation of the graphite core. At six of the plants steel oxidation is the more likely of these two phenomena to dictate reactor lifetime and it is controlled by restriction of maximum operating temperatures and by the avoidance of sustained operation with a high concentration of water or hydrogen in the carbon dioxide coolant. Temperatures in most plants were reduced in 1969 to allow the planned lifetimes to be achieved but this caused a reduction in the power outputs. Choice of optimum operating temperatures is thus a compromise between power output and plant life. At the two later plants, Oldbury and Wylfa, which have higher coolant pressures, graphite oxidation could be life determining. The life of the graphite core may be prolonged by raising the hydrogen content of the coolant gas but this conflicts with the requirements to restrict rates of steel oxidation. The optimum hydrogen content is thus a compromise between these competing requirements. The paper describes a method which has been developed for deriving optimum combinations of temperature and hydrogen and gives an illustration of its application with a specimen calculation for the Wylfa plant. (author). 2 refs, 1 fig