Materials for Nuclear Fusion

I am staying two weeks in Padua for lectures on different topics on Nuclear Fusion Engineering and Physics. This is the opportunity to give a short overview on very interesting subjects. But before starting, a few words on the environment.

 

University of Padua

 

We are at the university of Padua, one of the oldest universities in the world, since it was founded in 1222. The weather is nice, students everywhere in the city (almost 60000 I heard). So a very beautiful and inspiring environment for work.

We will start this series of  posts with the issue of  materials for fusion reactors. Actually this question does not concern ITER because the flux of neutrons produced will be quite low. It will be a major problem for DEMO (a concept of demonstrator of reactors) and for the operational reactors. This subject faces the “dark” side of nuclear fusion, the one would could endanger the green advantage of fusion, if not properly treated. Consequently, a huge effort is put on the development of new materials with a better behaviour in presence of irradiation.

The main problems are the neutron generated by the principal reaction: Deuterium+Tritium->Alpha+Neutron. The neutron carries the major part of the released energy: 14.1 MeV. The neutrons are not affected by the magnetic fields, therefore they have straight trajectories finishing in the walls surrounding the plasma. There are two parts in the tokamak which have to deal with these neutrons: the blanket, on the walls and the divertor, at the bottom of the tokamak. The blanket has top stop the neutrons and to extract their energy through collisions with atoms and to use the reaction of neutrons with lithium to produce tritium which is reinjected as fuel. The divertor extracts the helium ashes to maintain the purity of the plasma.

The energy of 14.1 MeV is not easy to handle and is far beyond the knowledge acquired on fission reactors. To develop materials which are capable to deal with these neutrons, it is necessary to understand the interaction of these particles with the matter. A microscopic approach is required. The qualitative effects of irradiation are quite well understood but only quantitative  models make it possible the design of new materials.

Radiation in fusion materials gives place to primary responses:

  • displacement of atoms from equilibrium position which in turn leads to the formation of free vacancies and interstitials and to the sputtering of atoms from free surface.
  • transmutation reactions which can lead to the production of gas atoms (H, He) inside the material and to the change of chemical composition with a mixing of stable and radioactive elements. In a fission reactor, the transmutation rate is of 0.2 to 0.3 appmHe/dpa. On ITER, it should reach 10 appmHe/dpa.

Well, okay, you will probably mind there is no problem because these modifications occur at the microscopic level: it is too small to have an impact on the macroscopic properties. Error! And these are the long term effects, which concern almost all macroscopic properties of the material

– mechanical effects: swelling, hardening and increase of the ductile to the brittle transition temperature

– electrical effects: change of conductivity in insulators and degradation of critical current in supraconductors

– production of radiotoxicity and heat deposition

Consequently a lot of effort is put on the understanding of radiation defects generation: the type of defect produced, how the defect propagates in the material, one interesting are the dislocation and how they lead to swelling.

The response of the material depends on its type: ferritic-martensitic, ferritic steels, ODS steels…

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