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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - GEMMA (GEneration iv Materials MAturity)

Teaser

Summary

In order for nuclear fission to significantly contribute to the decarbonisation of Europe within next decades, a new generation of reactors with the highest performance in terms of safety, sustainability and economy must be developed. Within the framework of the GEN IV Forum, the most advanced countries, including the European Union, for years are developing, examining and implementing innovative reactor concepts that are scientifically interesting and technically promising. For the development of all these projects, which are called to face extremely heavy conditions of use, the bottleneck consists of the use of suitable structural materials that can withstand high temperature, high flow of fast neutrons and corrosive fluids.
Among all the possible metallic and non-metallic materials that can be used, in the GEMMA project the attention is focused on austenitic steels, i.e. on the most coherent solution with the tradition of sodium nuclear reactors. The sodium reactors were the first fast reactors to be built and in the landscape of GEN IV they are still the most credible solution. To guide the designers in the use of austenitic steels for sodium reactors, has been developed a body of regulations, best practices and design rules that now appears well established. However, there are still no design rules taking into account the specific needs of fourth generation reactors refrigerated by fluids other than sodium. Furthermore, also for the use in sodium, there are some behaviors of austenitic steels, especially in the presence of fast neutron flux which are not completely known. The goal of the Gemma project consists of research, development, qualification and standardization of austenitic steels for GEN IV reactors and technologies for their protection and their welding.
Through a wide use of experimental techniques the project intends:
To qualify the existing materials for the hostile conditions of use that are envisaged in GEN IV;
To perform screening for the selection of new austenitic steels being more resistant to the typical conditions encountered in GEN IV applications;
To develop protective coatings aimed to mitigate the material damage in GEN IV reactors;
To improve and qualify the predictive models of material damage through dedicated experiments and forthcoming model refinement

Work performed

Presently, the materials to be qualified and the welded joints of various kind as well as the protective coatings were developed and distributed to the partners to allow starting of qualification. Namely, slabs and plates of AISI 316L and 15-15 Ti steels were distributed both in the MYRRHA and ALFRED variants. The same materials were welded by TIG and SAW techniques which were optimized in the Project itself and also distributed to the partners for the experimental phases. The innovative GESA (Gepulste Elektronen Strahl Anlage), PLD (Pulsed Laser Deposition) and Detonation Gun coatings for structural materials were developed and employed to protect specimens to be subjected to mechanical and corrosion tests.
In order to fully understand the occurrence of mechanical stresses resulting from the welding of thick mechanical parts and in order to qualify stress models developed by GEMMA partners, one of the two envisaged tests of neutron diffractometry was carried out. The technique is able to accurately assess the level of residual stresses, within a welded piece, by detecting the minimal deformations of the crystalline lattice.
Furthermore, to build thermodynamic and kinetic models for Fe-Ni-Cr model alloys under irradiation, the experimental studies of elemental diffusion phenomena were started over a stratified sample, produced in the Project, whose layers contain different quantities of the three elements.

Final results

The greatest progress in acquiring new know-how has been in the development of aluminum austenitic steels and in the measure of residual stress.
The screening of over twenty different chemical compositions of aluminum-based austenitic steels (AFA Steels), developed by the collaboration between three different project partners, made it possible to compare the characteristics corresponding to different aluminum and nickel contents and to identify the best combinations. The participation of an important European steel maker will allow a rapid transposition of the results to the commercial sector if the encouraging results are confirmed and the transferability to large-scale production will be made possible.
Careful measurement of post-weld residual stresses was carried out on a welded steel piece accurately reproducing the welding of the main vessel of ASTRID. The assessment was conducted through the sophisticated method of high resolution neutron diffraction. The neutron diffraction of large welded pieces constitutes a novel application and permits a precise and volumetrically distributed evaluation of the tensional state within the joint.