Fusion Materials

Fusion Materials Challenges

An ambitious programme on fusion materials is one of the main keys to the successful development of fusion. Indeed one of the attractive features of fusion stems from the fact that no radioactive products result from the reaction itself: the fusion of deuterium and tritium produces helium and neutrons. However, these neutrons are very energetic: their 14 MeV energy is typically one order of magnitude higher than that of neutrons produced in fission reactors. This results in the production of a significant amount of transmutation products, especially helium (and hydrogen) in the bulk of the materials which can result in swelling and alteration of the mechanical properties. The question of materials is therefore both an opportunity and a challenge for fusion. The fact that there is freedom to chose and optimise the materials surrounding the plasma in order to minimise activation on the long term (and thereby avoid creating long live radioactive waste) constitutes indeed an opportunity. However, the specificity of the reactions produced by the 14 MeV neutrons together with the operating conditions required for the materials (large power fluxes and operating temperatures in the range 400 to 600 oC) constitute a challenge which fusion materials R&D has to take up.

Thermal He-desorption from pre-implanted Fe-C samples is a particularly sensitive experiment to test kinetic models describing the diffusion of He in presence of point defects. The figure below shows the agreement between experimental desorption from Fe-C and the Rate Theory Modelling (Model) where the energetics of He and point defects is determined using first principle calculation based on the Density Functional Theory (DFT).

EFDA Fusion Materials R&D Objectives

The Fusion Materials activity under EDFA concentrates on long term developments in connection with DEMO design studies (DEMO is the future fusion demonstration reactor foreseen after ITER): e.g. new materials, research areas with high risk, physical understanding to master the evolution of materials properties during operation. These activities will complement the project-oriented developments of Fusion for Energy and will therefore be prepared and executed in close cooperation with F4E. Whenever developments within EFDA will have reached an appropriate stage of maturity, an assessment will be made in view of a possible transfer to F4E.

The R&D activities address 3 main research objectives:

1. Materials development closely linked with DEMO design

   EFDA activities here will focus on materials for DEMO applications. In particular the area of structural materials, functional materials, and advanced joining technology will be addressed.

2. Modelling of radiation effects and experimental validation

   A reliable prediction of radiation effects necessitates a good understanding and modelling of the physical mechanisms. This requires the use and development of modelling tools and their experimental validation at the relevant scale. Some of the modelling tools need intensive computation capability and are considered in the definition of future fusion computing centres presently under discussion.

   The quality of understanding and the reliability of the predictive capability based on computational material science strongly depends on experimental validation. This requires using well defined materials and irradiation conditions and conducting physical, chemical and mechanical characterisation at the relevant scale. Fission neutron and spallation-neutron mixed spectrum irradiations will be used for exploring the evolution of mechanical properties and microstructures.

   This document gives a summary of key recent results achieved by the EFDA materials modelling and experimental validation programme, and outlines specific future objectives that, if achieved, will bring the programme to the goal of having available a comprehensive yet conceptually transparent predictive model for EUROFER-type steels.

   The breakdown of objectives given in the document goes beyond stating general needs, and highlights the key physical phenomena, the development of mathematical description and experimental validation for which should advance the program to a level where computer modelling becomes capable of driving innovative development of promising candidate fusion materials.

3. Irradiations

   Irradiation campaigns are also required to constitute an engineering database of materials in real operational conditions. The irradiations will be carried out on fission reactors, spallation sources, multiple beam irradiation facilities and on the future International Fusion Materials Irradiation Facility (IFMIF) projected in the mid-term. Post-irradiation examination will include the measurement of physical and mechanical properties, the characterisation of microstructure, He & H retention measurements.

The Fusion Materials Topical Group

To achieve these objectives three Research Projects and one Research Area have been set up (the Research Projects have well defined objectives, deliverables and milestones, while the Research Area presently requires only looser coordination):

The Research Projects are as follows :

• Tungsten and tungsten-alloy development for plasma facing components, structural application, heat sink and armour materials
• Nano-structured oxide dispersion strengthened (ODS) ferritic steel (where oxide clusters have dimensions in the nanometer range) development
• Radiation effects modelling and experimental validation

W and W-alloys have too a low fracture toughness which can be improved as the grain-size is refined by alloying using for instance dispersion of La oxide (WL10) and rhenium addition (W26%Re). Nevertheless this refinement is not stable in these alloys for the foreseen in-service temperature range up to ~1200 0C or higher.(see figure below). New alloys with better stability are being developed: W-Ti, W-V for structural applications, W-Y2O3, W-TiC for armour.

The Research Area will deal with the development of SiCfSiC (silicon carbide composite made of SiC fibres embedded in SiC matrix) and associated joinings & coatings for fusion reactors.

The Fusion Materials Topical Group is under the co-chair of Sergei Dudarev (UKAEA) and Michael Rieth (FZK). Including material development and materials science & physical modelling activities it should provide a reinforced coordination among the EU Associations, Universities and Research Institutes to develop fusion materials with higher radiation and heat resistance and predict their behaviour under fusion environments. Seventeen Associations out of twenty seven are presently contributing to the Topical Group.