Fracture mechanics and mechanical characterization of the beltline welding seam of the decommissioned WWER-440 reactor pressure vessels of nuclear power plant Greifswald Unit 4

The paper presents data measured for two trepans sampled from the beltline welding seam of the decommissioned WWER-440/V-230 reactor pressure vessel (RPV) of the nuclear power plant Greifswald Unit 4. Greifswald Unit 4 representing the first generation of this reactor type was shut down after 11 operating cycles in 1990. The aging of the RPV steels was not monitored by any surveillance program. The main focus of this work is put on fracture toughness characterization according to test standard ASTM E1921. Charpy size SE(B)-specimens were machined from different locations through the thickness of the multilayer beltline welding seam. SE(B) specimens machined from the same thickness are a set of specimens for which one reference temperature T0 was defined according to ASTM E1921. The pre-cracked and side-grooved Charpy-size SE(B) specimens were monotonically loaded until they failed by cleavage instability. T0 values were evaluated with the measured cleavage fracture toughness values, KJc, by applying the multi-temperature procedure. The neutron fluence at the RPV outer and inner RPV wall was determined with 4.1•1019 und 0.87•1019, respectively. That is a decrease for about 80% over the RPV wall.
Large variations in the evaluated T0 values across the wall of the multilayer beltline welding seams were observed. At the inner wall just beyond the cladding and in the welding root region comparatively low a T0 were evaluated with 28°C and 6°C, respectively. After the welding root towards the outer wall the average T0 amounts 81°C with a span (maximum value minus minimum value) of 57 K. It is demonstrated that the ductile-to-brittle transition temperature shift predicted by the Russian code for the existing content of deleterious elements P and Cu and the accumulated neutron fluences lies within the scatter of the measured T0 values. The irradiation induced shift of T0 expected due to the decrease of the neutron fluence by about 80% from the inner to the outer RPV wall is not visible. It is overlapped by the strong variation of KJc values caused by the intrinsic weld bead structure and the different filling materials used for weld root and the main weld.
In addition Charpy-V tests were performed to compare the results of fracture mechanics testing with the current integrity assessment code practice. The transition temperatures TT47J vary also through the thickness of the multilayer welding seam. At the inner wall just beyond the cladding and in the welding root region TT47J was measured with 73°C and 40°C, respectively. After the welding root the average TT47J amounts to 124°C with a span of 16 K. Compared with T0 the variation of TT47J within the filling layers after the welding root is considerably smaller.
Compact and reconstituted Charpy-V and SE(B) specimens were thermally annealed at 475°C for 152 h. This temperature time regime corresponds to the one applied to the Greifswald Units 1 to 3. The comparison of test of thermally annealed and non-annealed specimens provides irradiation induced shifts of 114 K for TT47J and 86 K for T0. The testing of annealed specimens confirms the results measured with samples from thermally annealed RPV of Greifswald Unit 2.