Fracture mechanics testing of irradiated RPV steels by means of sub-sized specimens (FRACTESUS)

Project Information

FRACTESUS

Grant agreement ID: 900014

DOI

10.3030/900014

Project closed

EC signature date 25 May 2020

Start date 1 October 2020

End date 30 September 2024

Funded under

• Euratom

Total cost

€ 4 981 811,25

EU contribution

€ 2 986 814,61

Coordinated by

STUDIECENTRUM VOOR KERNENERGIE / CENTRE D'ETUDE DE L'ENERGIE NUCLEAIRE
Belgium

Periodic Reporting for period 3 - FRACTESUS (Fracture mechanics testing of irradiated RPV steels by means of sub-sized specimens (FRACTESUS))

Reporting period: 2023-10-01 to 2024-09-30

Summary of the context and overall objectives of the project

Safety of nuclear power plants (NNP) is governed by the structural integrity of their components. Fracture toughness is one of the most important properties taken into account during NNPs designing. The largest and practically irreplaceable component of any NPP is the reactor pressure vessel (RPV). Mechanical properties of RPV materials change due to their exposure to neutron irradiation resulting in the decrease of fracture toughness. Appropriate codes and standards describe the minimum required fracture toughness levels that have to be assured in the specific reactor operating conditions. Mechanical properties of the RPV are periodically monitored by means of surveillance specimens stored inside the RPV. However, the space in such capsules is limited, thereby limiting the amount of surveillance specimens. This limitation causes problems when RPV properties have to be monitored during a longer service time than the initially planned one. Many of the currently used reactors in NPPs are close to the end of their initially planned service time. Safe extension of the existing NPPs service time (so called long term operation perspective - LTO) is crucial for satisfying the rising demand on electricity.

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The present 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 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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The FRACTESUS project is expected to provide an unique database of fracture toughness test results delivered with MCT specimen usage for RPV materials of different properties (initial-dependent on material batch and resulted from neutron irradiation). Those results will be compared to the existing database of large specimen tests, with a support of numerical modelling. This will be an important step before the approval of MCT testing by nuclear authorities and, finally, the usage by NPPs operators in surveillance programs. Detailed guidelines of MCT specimen usage will be prepared and changes in standard proposed, if necessary.

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.

Miniature compact tension specimens to be tested in the FRACTESUS project