Fracture mechanics characterisation of the decommissioned WWER-440 reactor pressure vessel of the NPP Greifswald unit 4

Nuclear power plant operators must demonstrate that the structural integrity of a nuclear reactor pressure vessel (RPV) is assured during routine operations or under postulated accident conditions. The aging of the RPV steels is monitored via surveillance programs. Radiation loading, metallurgical and environmental histories, however, can differ between surveillance and RPV materials. Therefore, the investigation of RPV material from decommissioned NPPs offers the unique opportunity to evaluate the real toughness response.
The Greifswald units representing the first generation of WWER-440/V-230 reactors were shut down after 11 to 15 years of operation and the RPVs represent different material conditions as follows: Irradiated (Unit 4), irradiated and recovery annealed (Units 2 and 3), and irradiated, recovery annealed and re-irradiated (Unit1). The recovery annealing of the RPV was performed at a temperature of 475° for about 150 hours and includes a region covering ±0.70 m above and below the circumferential core weld.
Material samples of a diameter of 119 mm called trepans were extracted from the RPV walls. The working program is focused on the characterisation of the RPV steels (base and weld metal) across the thickness of the RPV wall. The paper presents an overview about test results measured on the trepans taken from the welding seam SN0.1.4. and forged base metal ring 0.3.1. located in the reactor core region of the Greifswald units 1, 2 and 4 RPV. It comprises chemical analysis, microstructure investigations (by means of metallography, electron microscopy and SANS) and mechanical testing (hardness measurements, tensile, Charpy-V), and fracture mechanics testing. The key part of the testing is focussed on the determination of the reference temperature T0 following the ASTM test standard E1921 to determine the facture toughness, and how it degrades under neutron irradiation.
Following results have been determined:

• The recovery annealing of the welding seams and base metal could be confirmed.
• KJc values of the weld metals generally follow the course of the MC though with a large scatter.
• There is a large variation in the T0 values evaluated across the thickness of the multilayered welding seams. The T0 measured on TS oriented SE(B) from different thickness locations of the welding seams strongly depends on the structure along the crack tip.
• A strong scatter of the fracture toughness KJc values of the irradiated (unit 4) and recovery annealed base (unit 1) metal of is observed with clearly more than 2% of the values below the fracture toughness curve for 2% fracture probability.
• It was demonstrated that T0 evaluated according to the SINTAP MC extension represents the brittle fraction of the data sets.
• The application of the Unified Curve concept gave T0 values comparable to the standard MC approach, but the slope of the fracture toughness temperature curve is more shallow for highly embrittled conditions.
• The embrittlement of the unit 4 base and weld metal does not follow the prediction according to the Russian code PNAE G-7-008-86.