In-service embrittlement of the pressure vessel welds at the Doel I and II nuclear power plants

In-service embrittlement of the welds at the Doel I and II pressure vessels displays an apparent "outlier" behaviour as compared to the predictions of US NRC Regulatory Guide 1.99 Rev.2 This issue is being addressed by a systematic R&D effort, supplementary to the mandatory surveillance programmes. The status of this effort is reviewed in this paper. The anomalous trends can be outlined as follows. Extensive characterization of the chemical composition of representative test specimens reveals that these low nickel welds, although being nominally identical (same fabrication practice, heat treatment, microstructure, ...), do feature an appreciable variability of their bulk copper content - covering a range of 0.13% to 0.35%. High copper Charpy-V weld specimens are however not available in the surveillance capsules. The variability could be identified only because the weld part of some HAZ (heat affected zone) Charpy-V specimens, as well as some tensile specimens, consist of the high copper melt For the low copper contents, (0.13-0.18%), the 41 J Cv transition temperature shift, as obtained from seven capsules exposed in the neutron fluence range of 1 to 3.9 10¹⁹ n/cm² (>1 MeV), is rather constant, but significantly larger than predicted (by US NRC Regulatory Guide 1.99 rev.2).This has caused some concern on the upper bound shift to be adopted, given the possibility of higher copper content in the actual pressure vessel welds. High copper samples were manufactured by applying the stud welding reconstitution technique to remnants of the HAZ specimens, and the results cannot be distinguished from the low copper data Such insensitivity to copper,confirmed by tensile and hardness measurements, is at odds with current literature knowledge and with Regulatory guidelines. A further anomaly observed for this type of weld metal is an appreciable sensitivity to thermal ageing. The surveillance results will be evaluated in accordance with the USNRC Regulatory guide-lines, and the embrittlement trends described above will be thoroughly evaluated from a micromechanical perspective, in the light of a high resolution microstructural investigation.