Microstructural and microchemical characterization of ferritic ex-service Magnox control rod sheath material

Microstructural characterization has proven an essential part of a Post Irradiation Examination (PIE) program for ex-service Magnox Reactor Safety Control Rods material. Clad ductility of the component is a key parameter and this can be assessed microstructurally by Transmission Electron Microscope examination. After service, one specific control rod was found to have suffered damage to its outer ferritic steel sheath. Using an understanding of the mechanisms for copper precipitation and dislocation loop formation under reactor operating conditions, it was possible to make an estimate of the operating temperature of the component. This was calculated to be 285 ± 15 C, considerably lower than the nominal operating temperature of ∼350 C. In support of this measurement, direct comparison with work on dislocation loop formation in ferrovac iron, irradiated to a comparable neutron dose over a range of temperatures, suggests an irradiation temperature of ∼280 C, derived from the comparison of mean dislocation loop size