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Séminaire LGF – Giovanni Bonny (SCK CEN, Belgique) – 22 janvier 2024

Giovanni Bonny (SCK CEN, Belgique) délivrera le séminaire suivant.


Present status of the FRACTESUS project: Towards Fracture Toughness Miniaturization of Nuclear Steels


The fracture toughness of a material characterizes its resistance against crack propagation and is an important design parameter to guarantee the structural integrity of components. From several fracture toughness experiments within the ductile-to-brittle transition zone, the reference temperature, T0, can be derived by applying the Master Curve approach. Here, T0 defines the transition temperature for a median fracture toughness of 100 MPa·√m for a 1 inch fracture toughness specimen. It is important to note that a shift in T0 denotes a shift in transition temperature. For example, in the framework of irradiation embrittlement of steels, the shift in T0 provides a measure for the shift in ductile-to-brittle transition temperature (DBTT).

To assess irradiation embrittlement of reactor pressure vessel (RPV) steels, nuclear regulatory bodies base their evaluation primarily on the semi-empirical methodology based on impact tests on Charpy surveillance specimens. In the framework of long-term operation (LTO) of the present nuclear power plants, surveillance data is scarce, and irradiation space in materials test reactors (MTR) is limited and costly. In addition, the miniaturized size of the specimens limits their activity, which facilitates their handling. Therefore, the determination of irradiation embrittlement on miniaturized specimens is desirable.

Application of the Master Curve approach allows to determine T0 and hence the shift in T0 from different specimen sizes and geometries at different test temperatures. As such, miniature compact tension (MCT) specimens can be used to optimize irradiation space in MTR; or MCT specimens can be extracted from broken Charpy surveillance specimens. The latter can provide valuable additional data in the plant’s safety case.

The FRACTESUS project aims to determine the effect of specimen size on the fracture toughness properties. Finite element models (FEM) are used to investigate the difference between large-size and miniature compact tension (MC(T)) specimens and quantitatively assess the resulting loss of constraint due to size reduction. The optimal range of usability of MC(T) specimens can therefore be determined and evidenced with experimental results. Large inter-laboratory testing is included in the FRACTESUS project in an attempt to prove the repeatability and reproducibility of the small-scale testing of fracture toughness properties. Various materials relevant for most of the available reactor pressure vessel materials and irradiation conditions are investigated.

The present contribution provides an overview of the current status and achievements of the project so far, thereby presenting both experimental and FEM results.

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