Periodic Reporting for period 2 - FRACTESUS (Fracture mechanics testing of irradiated RPV steels by means of sub-sized specimens (FRACTESUS))
Okres sprawozdawczy: 2022-04-01 do 2023-09-30
The most promising way to provide sufficient amounts of surveillance material is the miniaturization of specimens for mechanical testing. The FRACTESUS project aims to optimize the surveillance material usage by introducing the miniature compact tension (MC(T)) specimens in fracture testing of irradiated materials. The current standard of so-called “Master Curve determination” do not put limitations on specimen size, however, many other restrictions are imposed that make miniature specimen testing much more demanding in comparison to tests done on larger specimens. There are still many concerns related with the reliability of the results of MC(T) tests. For example, the testing temperature range is limited. The production of smaller specimens is also much more difficult, especially in the case of irradiated material. Moreover, special testing set-ups and equipment have to be prepared for reliable testing. Having all this in mind, much work has to be done to convince the appropriate nuclear authorities for MC(T) usage, which is the ambition of the FRACTESUS project.
The project has entered its last year and many preliminary data and interpretations are available. The main achievements so far are summarized by: i) a numerical round robin; ii) an experimental round robin on unirradiated materials; and iii) an experimental round robin on irradiated material.
The numerical round robin is completed by nine different laboratories to ensure that the different numerical tools and set-ups used by the different laboratories provide comparable and consistent results. Two specimen geometries, 1T-C(T) and MC(T), and two temperatures, -100 °C and 23 °C are considered. The specimen geometries, minimum meshing parameters around the crack tip, the material constitutive law and boundary conditions are prescribed. This allowed a free choice of the simulation software (ANSYS, Abaqus, Cast3M20, XPER r2465 and MSC.Marc) the number of elements and nodes and pin material. Both macroscopic results (load, J-integral, plastic volume and correction factor versus pin displacement) and microscopic results (mechanical field at the crack tip) are compared. It can be concluded that both macroscopic and microscopic results are coherent and comparable between the different laboratories.
The experimental round robins on unirradiated materials has been completed by 13 different laboratories (>480 fracture toughness tests). The reference temperatures T0 obtained from MC(T) specimens by the different laboratories are compared to the ones obtained from larger (mostly 1T C(T)) specimens for six different materials. The considered materials are relevant for the nuclear industry and consist of four base (15Kh2MFAA, A533B LUS, A533B JRQ, A508 Cl.3) and two weld (ANP-5 and 73W) materials.
The inter-laboratory round robin showed the equivalence between T0 obtained by MC(T) and larger specimens, except for A533B JRQ and 73W. The reasons for the deviations of A533B JRQ and 73W are under investigation. As a general comment, it can be stated that the results from MC(T) specimens often lead to material inhomogeneity as compared to the results from larger specimens. The small sampling volume combined with the weakest link theory interpretation means that local differences in the material may result in a large variation from one tested MC(T) specimen to another. Thus, rather than MC(T) specimens misinterpreting homogeneous materials as being inhomogeneous, MC(T) specimens could be more adept at identifying inhomogeneity than larger specimens.
The experimental round robin on the irradiated 73W weld material irradiated at 288 °C up to 1.5×1019 n/cm2 (E > 1 MeV) has been performed by seven different laboratories (>112 fracture toughness tests). All experiments have finished and the results will be analyzed in RP3.
FRACTESUS output is expected to have a large socio-economic impact due to the fact that the testing methodology based on miniaturized specimens planned to be validated in the project is very important from the viewpoint of the LTO perspective of NNP. The surveillance material saving is crucial for assuring of the control on irradiation induced material properties changes in RPVs of reactors with prolonged service time.
The output of FRACTESUS project will have also a large scientific value. Miniaturized specimens might be produced from already tested ones. That opens up the possibility to perform new tests that will lead to the better understanding of material behavior and to enrichment of already existing databases. Another potential application of the validated MCT technology is to use it during a development of a new irradiation resistant materials. Small specimens are much easier to irradiate, that results in reduction of testing and waste management costs.