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A total of 565 instrumented impact tests (232 performed on full-size and 333 on subsize Charpy-V specimens) have been analyzed in order to derive meaningful assumptions on the correlations existing between test results obtained on specimens of different size. Nine materials (pressure vessel steels) have been considered, in both the as-received and irradiated state, for a total of 19 conditions examined. For the analysis of data, conventional as well as novel approaches have been investigated; the former ones, based on a review of the existing literature, include predictions of upper-shelf energy (USE) values by the use of normalization factors (NF), shifts of index temperatures related to energy/lateral expansion/shear fracture levels, and a combination of both approaches (scaling+shifting of energy curves). More original and recent proposals have also been verified, available in the literature but also proposed by SCK•CEN in the frame of enhanced surveillance of nuclear reactor pressure vessels. Conclusions are drawn regarding the applicability and reliability of these methodologies.

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.

BCR-661 is a Nimonic 75 Certified Reference Material for tensile testing, available from the European Institute for Reference Materials and Measurements. The certification is based on specimens with a diameter of 10 mm. Since for several purposes small specimens are desirable, an attempt was made to qualify a specimen with a diameter of 2.4 mm. It was found that for cylindrical specimens made by turning neither the 0.2 % proof stress, nor the tensile strength values corresponded with the certified values. Rectangular section specimens made by electro-discharge machining gave correct stress values but too high elongations. The reduction of area values were for all specimens satisfactory. It was shown that the stress deviations found in the turned specimens are due to the presence of a work hardened layer caused by machining. Annealing of turned specimens resulted in correct strength values. With regard to the elongation values the influence of the specimen shape and possible specimen head deformation seem to be important factors.