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Title of the thesis: Brittle fracture in weld microstructures

Thesis defender: Noora Hytönen
Opponent: Dr. Benoit Tanguay, CEA, France
Custos: Prof. Iikka Virkkunen,Aalto University School of Engineering

Materials ageing limits the lifetime of nuclear reactors, especially the ageing-induced embrittlement of the reactor pressure vessels. This dissertation investigates the phenomenon of brittle fracture in weld microstructures, with focus on nuclear safety and material ageing during extended long-term operation (ELTO) of nuclear power plants (NPPs). The research emphasizes the role of understanding the microstructural features of welds to ensure the integrity and safety of nuclear reactor components as they age. The study highlights the significant effects of thermal ageing and neutron irradiation on the mechanical properties of low-alloy steel (LAS), in terms of maintaining safe operational margins in NPPs.

The primary focus is on two critical weld components in a nuclear reactor: the welds in the reactor pressure vessel (RPV), as well as dissimilar metal welds (DMWs) in the pressurizer surge nozzle safe-end. The study on RPV welds is conducted on harvested material from a decommissioned boiling water reactor NPP combined with representative high-dose surveillance material. The studied DMW is a representative mock-up of a repair weld used in a pressurized water reactor. The brittle fracture initiation and crack propagation are investigated from a microstructural perspective. The methodology includes analysis of microstructural features through various characterization techniques combined with hardness and mechanical testing to evaluate the strength and toughness properties of the welds.

Key findings indicate that the microstructural characteristics of welds significantly influence their mechanical properties and susceptibility to brittle fracture. Two main types of second-phase particles are identified to initiate a cleavage fracture in a LAS weld: non-metallic oxide inclusions and carbides. In DMWs, three main types of fusion boundary microstructures were found: sharp fusion boundary, martensitic phase, and partially melted zone. The microstructures correlate with varying levels of dilution and local strength mismatch, significantly affecting the crack propagation path.

The dissertation highlights that understanding of the metallurgical features and microstructural evolution of welds can lead to improved assessment of material performance and safety during LTO. When performing testing of the weld metal, the location of the crack tip and the process zone must be known in terms of the local weld microstructure as it determines the local fracturing resistance.

Keywords: nuclear safety, reactor pressure vessel, brittle fracture, weld

Thesis available for public display 7 days prior to the defence at . 

Contact information: 
Noora Hytönen
E-mail: noora.hytonen@vtt.fi