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Fracture mechanics characterization of Russian WWER type reactor pressure vessel welding seams
Viehrig, H.-W.; Schuhknecht, J.;
The paper is focussed on Master Curve testing according to ASTM E1921-05 [5] of the Russian WWER-440 type reactor pressure vessel steel. Charpy size SE(B) specimens from the beltline welding seam SN0.1.4 of the Greifswald NPP Unit 1 (1st generation WWER-440/V-230) and Unit 2 (2nd generation WWER-440/V213) were investigated in this study. The specimens are TL and TS oriented in the RPV welding seam, this means specimen axis axial to the RPV wall and crack propagation circumferential and through the thickness, respectively. The WWER-440 welding seam consists of a welding root welded with an unalloyed wire Sv-08A and the filling material welded with the alloyed wire Sv-10KhMFT. Fracture toughness values at brittle failure, KJc, measured on Charpy size SE(B) specimens were evaluated following Master Curve analyses as specified in ASTM E1921-05. The results show a wavelike course of the evaluated reference temperature, T0, through the thickness of the welding seams of Unit 1 and Unit 8. The scatter is more pronounced for the irradiated annealed and reirradiated 1st generation RPV of Unit 1.
Another issue was the influence of the specimen orientation and mainly the crack extension direction. While the crack front of a TS specimen is located in a more or less uniform structure, the structure along the crack front of a TL specimen varies, because it usually spans several welding beads. Roughly speaking, TS and TL specimens have a differentiating and integrating behaviour, respectively. A difference in T0 was found also for TS and TL oriented specimens of Unit 8. The lowest T0 was measured on TS specimens from the welding root with 114°C, whereas TL specimens of the same thickness locations gave 62°C.
For the 2nd generation RPV of Unit 8, the KJc values measured on specimens of both orientations generally follow the course of the Master Curve and are enveloped by the fracture toughness curves for 2% and 98% fracture probability. This statement can generally be made also for the first generation RPV of Unit 1, but the scatter of the KJc values is larger compared to the Unit 8 RPV. More values than expected lie below the 2% fractile.
The reason for the scatter in the KJc values and T0 is found in the structure at and along the crack tip, which depends on the welding technology and the specimen orientation. The welding technology applied on the beltline welding seams of the 1st and 2nd WWER-440 RPV generation is different, which is clearly visible in macroscopic section in Figs. 3 and 4. KJc values measured on SE(B) specimens from the thickness locations beyond the welding root of Unit 8 RPV result in a valid T0. KJc values from specimens of the same thickness within the welding seam of Unit 1 show a larger scatter. The majority of KJc values from one thickness location falls below the fracture toughness curve for 2% fracture probability. The reason is seen in the brittle zone in the fusion region between two welding beads. The application of the SINTAP MC extension procedure leads to a conservative T0.
In comparison to the correlative and indirect approach of the fracture toughness estimation in the present codes, the results presented here show that the orientation of the surveillance specimens is crucial for the direct measurement of the fracture toughness. For fracture toughness determinations the specimen orientation should be chosen which either yields the most conservative fracture toughness values or which agrees with the loading and crack extension direction considered in the integrity assessment.
Keywords: reactor pressure vessel, multilayer welding seam, SE(B) specimen, specimen orientation, fracture toughness, Master Curve approach
  • Contribution to proceedings
    12th International Conference on Fracture, 12.-17.07.2009, Ottawa, Kanada
    Proceedings of the 12th International Conference on Fracture
  • Lecture (Conference)
    12th International Conference on Fracture, 12.-17.07.2009, Ottawa, Kanada

Publ.-Id: 11870 - Permalink