The monitoring of oxidation takes the form of com­ponent inspection, the acquisition of material speci-

Fio. 3.71 Prediction of oxide thickness

mens, and the measurement of oxide thickness on components within the reactor or on representative components in autoclaves. (An autoclave is an oven filled with pressurised CO] and operated to simulate reactor conditions.) The reactor and boiler inspections are aimed at confirming the integrity of components by visual, photographic, ultrasonic and television tech­niques. The results of the oxidation assessments are used to direct attention to the areas and components of greatest risk. The use of material samples, which have been oxidised in the reactor environment, allows correlation with laboratory data on the variables which effect time to breakaway and its rate of oxidation. This data is collected for all reactors, to determine the dependency of oxidation rate on such variables as water or hydrogen concentration, operating tempera­ture and materia! composition. From the collected data, predictive equations can be formulated enabling oxide thicknesses for future operation to be estimated. In addition, the continuity of thermocouples and BCD extrapolation pipework, refuelling checks and control rod freedom of movement are all used to monitor for the effects of oxidation during normal operation.

It is expected that any serious component failure in the charge pan or core areas would affect some or all of these systems.

To control oxidation, de-rating of the CEGB re­actors was carried out in 1968 by limiting the gas out­let temperatures to 360°C. On the grounds of safety alone there is no real problem since assessments are for one year predictions only. A new base-line can be established each year at the annual inspection. The condition of the inspected reactor is taken as indica­tive of the running reactor.