The H2020 project entitled “Fracture mechanics testing of irradiated RPV steels by means of sub-sized specimens (FRACTESUS)” started on the 1st October 2020. The aim of this project is to demonstrate the applicability of miniaturized compact tension specimens in fracture toughness testing of the reactor pressure vessel steels under hot cell conditions. Validation of this method in an industrially-relevant environment will be an important step towards achieving its acceptance by the nuclear authorities, and finally, to induce its prospective usage by the nuclear power plant operators. Successful implementation of a miniaturized specimens testing technique will result, among others, in the optimization of surveillance material usage and savings in irradiated materials testing. The general project overview showing its structure, partners involved and main deliverables was published elsewhere. Here, we focus on some technical aspects being of the utmost importance in the initial stage of the project and which will have a crucial impact on its overall progress. The general consideration on the selection of the best available materials for testing are discussed on the examples of 73W weld, A533B LUS and the WWER-440 base metal 15Kh2MFAA. Moreover, the general scheme of the testing process, which is planned within the project, is briefly presented as well as the basic assumptions about the numerical modeling task aimed for rationalizing experimental findings.
The H2020 FRACTESUS project is aimed at the validation of miniaturized compact tension (MCT) specimen. More specifically the usage of the MCT with the Master Curve (MC) oriented fracture toughness testing of reactor pressure vessel (RPV) materials in hot cell conditions will be examined. In the first stage of the project, a general strategy of material selection and testing processes has been established. The choice of the selected RPV materials is based on the widest possible range of mechanical properties expected for baseline materials, but also resulting from their exposure to neutron irradiation, in terms of different MC reference temperature T0, and properties determined from Charpy impact testing. Moreover, in order to validate the use of the MCT in a broad application space, it was decided to perform FT tests for both base metals and welds. The largest challenge was related to the availability of those materials. They need to be tested in numerous, planned round robin exercises. They should have already an existing extensive database of fracture toughness results obtained using large specimens. The final version of the test matrix was prepared, keeping all those requirements in mind. An irradiated round robin exercise is planned for one type of weld material, namely 73W, that will be tested by seven partners. Additionally, the FRACTESUS partners were divided into smaller groups with 3–5 participants who will test a sub-selection of unirradiated materials in six round robin exercises. This paper presents the summary of the material selection activities in the FRACTESUS project. The materials are briefly described and rationale for their usage within the project is provided.
The fracture toughness in the ductile-to-brittle transition region is determined for the heat-affected zone (HAZ) adjacent to the fusion boundary between a low alloy steel (LAS) and the weld metal of narrow-gap Alloy 52 dissimilar metal weld (DMW) after 15 000 h of thermal aging at 400 °C and of an Alloy 52 DMW with buttering in reference condition. The fracture toughness testing is done according to ASTM E1921, and fractography and cross-section metallography are applied to characterize the crack paths, crack locations and fracture type. The T0 transition temperature for the DMW with buttering is −117 °C, indicating marginally higher toughness compared to the narrow-gap DMW. The cracks close to the fusion boundary (approximately 200 μm) in both DMWs deviate from the HAZ towards the fusion boundary. The thermal aging treatment of the narrow-gap Alloy 52 DMW does not significantly affect the fracture toughness properties of the fusion boundary. Further research is needed to better understand the lower boundary fracture toughness behavior at approximately 300 μm from the fusion boundary. The results contribute to long-term operation assessment of nuclear power plants, and development of analysis and characterization methods for DMWs related to the effect of crack path and location.
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