Abstract: This work presents the results of the fracture characterisation of the weld material of a nuclear vessel, currently in service, in the ductile to brittle transition region. The tests consisted of Charpy impact and tensile tests, performed in the framework of the surveillance programme of the plant. Moreover, in the context of this research, KJc fracture toughness tests on pre-cracked Charpy V notch specimens (evaluated according to the Master Curve methodology) together with some mini-tensile tests, were performed; non-irradiated and several irradiated material conditions were characterised. The analysis of the experimental results revealed some inconsistencies concerning the material embrittlement as measured through Charpy and KJc fracture tests: in order to obtain an adequate understanding of the results, an extended experimental scope well beyond the regulatory framework was developed, including Charpy tests and KJc fracture tests, both performed on reconstituted specimens. Moreover, Charpy specimens irradiated in the high flux BR2 material test reactor were tested with the same purpose. With this extensive experimental programme, a coherent and comprehensive description of the irradiation behaviour of the weld material in the transition region was achieved. Furthermore it revealed better material properties in comparison with the initial expectations based on the information obtained in the framework of the surveillance programme.

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