The objective of BREDA is to extend the understanding the service induced ageing effects of Reactor Pressure Vessel, RPV, materials subjected to thermal and irradiation conditions relevant to operation of the nuclear power plants. The irradiation effects are traditionally followed by a surveillance program simulating the actual RPV where-as thermal ageing is not specifically adressed. No previous study is known on how well the surveillance program reflects the actual ageing behaviour.
Through the work, the coupled behaviour of thermal and irradiation induced ageing, and the emergence of ageing related defects and the resulting degradation have been studied, i.e. how the damage to the microstructure influence the resulting mechanical properties.
In the initial stages, mechanical and microstructural testing of archive materials, collected from the original testing of the RPV, and thermally aged material, TA, harvested from the RPV Head, RPVH, of Barsebäck 2 has been performed at VTT. Testing of irradiated samples have recently started and preliminary results are becoming available. Mechanical testing of TA material from a retired pressurizer, PRZ, from Ringhals, aged at higher temperature (operating-T~345°C) compared to a BWR RPV (operating-T~270°C), have also been performed at KTH. Chalmers has performed atom probe tomography investigations of irradiated samples from the Barsebäck RPV and on TA materials, as well as comparative studies on irradiated material, from Ringhals and relevant archive material collected from Ringhals and Barsebäck. The second step involved testing of irradiated samples harvested from the RPV at Barsebäck. The harvesting of all sources of materials were executed under co-operative programs and a total of 8 trepan samples from the irradiated belt line region of the Barsebäck RPV, and the RPVH (operating-T~280°C) was transferred to VTT for testing. In addition to this, samples from TA PRZ-welds (operating-T~345°C)) was shipped to KTH. Impact testing shows an insignificant effect on the mechanical properties caused TA at the lower operating temperature RPVH weld metal where-as testing of samples collected from the replaced Pressurizer of Ringhals 4 showed significant influence on the ductile to brittle transition temperature, as well as exhibiting both hardening and non-hardening effects during mechanical testing. The microstructural features of un-irradiated, irradiated and TA material have been mapped laying a thorough foundation for continued studies on the materials. Of specific interest is the fact that observations were made on the less irradiated material from a BWR, similar to what is observed in more extensively irradiated samples taken from the Ringhals PWR-plants. So far 3 Ph.D. students participating in the project have successfully defended their respective theses and 3 more students have been enrolled to continue the work.
The project is a continuation of currently on-going studies on mechanical and microstructural testing of material harvested from the RPV of Barsebäck 2 and thermally aged materials from Ringhals PWR. The main deliverable during 2023 is a collection of mechanical test data of irradiated material from the beltline area. A test matrix for testing of the recently acquired thermally aged material form the decommisioned pressurizer of Ringhals 2 will be completed. This work will be supported by an enhanced modelling framework to study the evolution of the initiation sites and their effect on the measured mechanical properties. The project also includes an enhanced microstructural evolution of RPV material to study possible segregation effects partially being responsible for the non-hardering ageing effect. This study includes Light Optical, Scanning Electron, Transmission Electron Microscopy and Atom Probe Tomography at Chalmers and VTT. The initial parts of the assessment of the obtained data will be laid out and this effort will be outlined to allow for a comprehensive study of the results and possibly influencing decision making at the utilities as well as the regulators regarding ageing of large pressure vessels.
The extension of the program will allow for comparative mechanical and microstructural studies coupled with modelling of fracture behaviour to enhance the understanding of combinatory effects of thermal and irradiation induced ageing.