The paper analyses data derived from surveillance specimens withdrawn periodically throughout the life of Magnox reactors with the objective of establishing a predictive equation for the end of life Charpy impact properties of the submerged-arc welds in the ferritic steel pressure vessels. Theoretical arguments and experimental data show that the transition temperature shifts in welds exposed in high temperature (360°C and above) low irradiation dose (∼0.4 mdpa) locations are controlled by thermally-induced intergranular fracture due to phosphorus segregation. The available data have been analysed as a function of time-at-temperature and a trend curve has been developed, taking into account both the time and temperature of exposure before and after a temperature decrease early in life. Data analysis showed that a threshold time condition should be allowed for, which must be exceeded before a shift in transition temperature is observed. This was interpreted as a threshold level of phosphorus segregation which had to be reached before intergranular fracture could take place in preference to cleavage fracture.