Fracture Mechanics Testing of Materials
The investigation of Reactor Pressure Vessel (RPV) material from the decommissioned Greifswald Nuclear Power Plant (NPP) offers a unique opportunity to understand the irradiation behaviour under real operation conditions.
These first generation WWER-440 RPVs were not monitored with surveillance specimen program to assess the aging during the operation. The embrittlement forecast is based on trend curves, which may not reflect the reality.
WWER-440 RPVs are composed of several forged rings, which are connected by X-butt multilayer submerged arc circumferential welding seams. The welding seams consist of a welding root welded with an unalloyed wire and the filler material welded with the Cr, Mo and V alloyed wire. The base metal is a low alloyed Cr, Mo and V alloyed carbon steel with a structure of tempered bainite.
The four Greifswald NPP units were shutdown in 1990 after 11 – 17 years of operation and the RPVs represent the following conditions:
From the RPV of the units 1, 2 and 4 trepanned plug specimens were taken with an especially designed drilling machine. Trepans of a diameter of about 120 mm were taken from both the beltline welding seam and the forged base metal ring located in the region
of the reactor core.
The trepans are cut into discs through the thickness using a wire travelling electro erosive discharging machine (EDM). From these discs plane tensile and Charpy size SE(B) specimens are machined.
In addition specimens are reconstituted from halves of tested specimens to extend the test matrix.
The testing and investigation programme includes
Vickers hardness measurements (HV 10),
fracture toughness testing in the lower ductile to brittle transition and ductile region,
instrumented Charpy-V testing according to DIN EN 10045-1 (1991) and EN ISO 14556,
determination of chemistry profiles in different thickness locations, and
Small Punch testing.
The main focus is on the Master Curve testing according to ASTM Test Standard E192109a.
Previous results of trepan from the beltline welding seam of the Greifswald unit 1 RPV:
The left figure illustrates the course of reference temperature T0 through the thickness of the beltline welding seam SN0.1.4 of the Greifswald Unit 1 RPV representing the irradiated, annealed and reirradiated (IAI) condition. As depicted the T0 results vary through the thickness of the welding seams. There is an initial increase of T0 from 10°C at the inner surface to 49°C at a distance of 22 mm from it followed by a continual decrease to 35°C at a distance of 84 mm, and finally increasing again to a maximum of 61°C near the outer RPV wall. Beyond the welding root T0 shows a span of about 50 K. The re-embrittlement during an operation period of 2 campaigns can be assumed to be low. Hence, it can be inferred the variation of T0 measured through the thickness of the multilayer welding seam SN0.1.4 results basically from differences in the structure of the welding beads.
In the right figure the KJc(1T) values versus the test temperature normalised to T0 of the individual thickness location of the trepans 1-1 and 2-3 are summarized. The KJc(1T) values generally follow the course of the MC, though the scatter is large and more than 2% of the KJc(1T) values lie below the 2% fracture probability curve. That strongly indicates that the weld metal is not fully homogeneous, even at the same thickness location.
Taking into account a critical temperature of brittleness of 186°C estimated by the Russian code the recovery annealing of the welding seam could be confirmed with the test results.