[en] The Windscale Advanced Gas Cooled Reactor of the UKAEA is a prototype reactor. A decision to reverse the steam flow in the superheater banks of the heat exchangers resulted in a reversal of duties for the inlet and outlet headers. The original mild steel inlet headers were thus required to operate in the creep range at temperatures of the order of 450⁰C. A Code approach to the assessment of potential life was found to be inadequate for the purpose of system optimization and so stress-rupture data was analysed by a method which related failure probabilities to the operating parameters of stress and temperature. Whilst one can debate the absolute values of the derived numbers, it is believed that the method gives a feel for the changes of risk with variations in operating conditions. The method offers the considerable advantage of improving communications during inter-disciplinary discussions. New operational limits were established and, as a prudent check on predictions, header performance was monitored using a combination of strain measurement and ultrasonic testing during the annual shutdown. The creep strains developed during service were measured at ambient temperatures at shut-down by replication of orthogonal grids previously scratched on the header surfaces. These data were used to normalize predictive expressions originally derived from laboratory data and strain predictions are now made with some considerable confidence. On a basis of the original failure probability analysis, the rate of development of strain and the results of ultrasonic testing, the need to change the headers appears to be unlikely within the presently planned lifetime of the reactor. (author)