Plasma-facing Materials
The "first wall" is the name given to the surface of the inner wall of a fusion reactor. This wall is in direct contact with the plasmy and is thus directly affected by the plasma and its constituents.
This means that very high temperatures - up to 1,000 °C - can occur. Moreover, the magnetic confinement of the ions in the plasma is not perfect, with the result that the surface of the first wall is also continuously bombarded by ions from the plasma. In fusion reactions involving deuterium and tritium atoms, the energy is mainly released in the form of the kinetic energy of the neutrons produced. In the blanket, this kinetic energy is then converted into heat which is used to produce electricity. Furthermore, the neutrons also perform the important task of producing the tritium fuel from lithium. Thus, the first wall must also be able to withstand neutron bombardment, neutron absorption in the wall must be kept low, and no materials can be used that form long-lived radioactive isotopes as a result of neutron bombardment.
Another key priority of the investigations in our field is hydrogen retention in and hydrogen permeation through the wall materials. This is important for two reasons. Firstly, the two hydrogen isotopes deuterium and tritium are used as fuel for the fusion reaction. Secondly, it is necessary to prevent too much tritium accumulating in the wall material or entering the structural materials and other external elements such as the cooling system, since tritium is a radioactive ß-emitter.
Identifying and analysing problems:
In order to identify problems that may occur with materials used in the reactor wall, all materials must be thoroughly examined with regard to the above-mentioned issues and solutions must be found for any weak points discovered. This is the only way to define material systems that meet the demands of later fusion power plants. A variety of analysis techniques are available for analysing, understanding, and improving the materials’ properties:
Ion beam analysis: When ions from the plasma hit the first wall and are implanted in the surface, it is important to know how deep the ions penetrate. To measure the depth distribution of different elements, a linear accelerator can be used to fire particles at the surface. In Rutherford backscattering spectrometry (RBS), the projectile is then scattered in the same way as when a shot is taken in a game of billiards. The energy of the backscattered projectile changes depending on the mass of the target atom and its depth in the sample under investigation, and this allows the element to be identified in the sample. Unfortunately, this method is not suitable for detecting the hydrogen isotope deuterium, which is why nuclear reaction analysis (NRA) is used in this case. In this method, helium-3 is used as a projectile. In a nuclear reaction with deuterium in the sample, the helium-3 is transformed into an alpha particle (= helium-4) and a high-energy proton. The number and energy distribution of the protons leaving the sample indicate the concentration and depth distribution of the deuterium..
X-ray photoelectron spectroscopy (XPS): X-ray photoelectron spectroscopy can be used to find out in what way the materials of the first wall change when exposed to thermal loads and ion bombardment. In this technique, a sample is irradiated using an X-ray source. This produces free electrons in the sample. The energy of these electrons can be measured, and this determines the chemical elements and compounds of which the sample is composed and how it changes, for example when heated to high temperatures.
Thermal desorption spectroscopy: In order to determine how much hydrogen (H) is bound in a sample, the sample is heated and the amount of H released from the sample is measured. By recording the amount of H released as a function of temperature, important indications are provided about the binding mechanisms of H in the sample. This enables us to develop methods for removing the bound H isotopes from the first wall.
Finding solutions:
Hydrogen permeation barriers: To keep the fuel in the reactor and prevent the radioactive tritium from passing through the reactor wall, we investigate hydrogen permeation barriers. These are thin layers made from oxides that can be integrated into the wall at suitable locations. These layers must, of course, also satisfy the requirements mentioned above, i.e. they must be thermally stable and have low neutron activation properties. A permeation facility has been developed to measure hydrogen permeation. This is composed of two vacuum chambers, with the sample – consisting of a barrier layer on a fusion-relevant wall material – placed in between. Deuterium is then fed into the first chamber, and in the second chamber, a mass spectrometer is used to measure the flow of deuterium through the sample. With the aid of temperature- and pressure-dependent measurements, conclusions can be drawn on the permeation mechanisms in the sample. The aim is to understand permeation mechanisms and to find suitable materials with very low permeabilities.
Yittrium oxide layer on a EUROFER97 substrate. The structure of the steel substrate is visible through the layer.
Tungsten-fibre-reinforced tungsten: nTungsten could be a suitable material for the first wall as it has a high melting point, suffers little damage from particle bombardment, and has low hydrogen permeability. Nevertheless, tungsten still presents some challenges that have yet to be resolved
One such challenge is the behaviour of tungsten under mechanical stresses. For instance, it is brittle at room temperature and only becomes more ductile at higher temperatures. Due to damage caused by neutrons, this transition may not occur until even higher temperatures are reached. To overcome this brittleness, a tungsten composite such as tungsten-fibre-reinforced tungsten can be used instead of pure tungsten.
SEM image showing an erbium oxide interface (dark) of a tungsten-fibre-reinforced tungsten composite sample. The cross section in the bottom half of the image was produced using a focused-ion beam.
A similar concept is found in fibre-reinforced plastics. Thus, a tungsten wire is coated and embedded in a tungsten matrix. The coating acts as a predetermined breaking point, at which energy can be dissipated under mechanical load without the material as a whole failing.
Self-passivating tungsten alloys: A further challenge presented by tungsten is how to prevent damage to the environment in the unlikely event that the cooling system fails. In such a case, nuclear decay heat can cause the first wall to heat up and reach temperatures of over 1,000 °C. If air enters the reactor, tungsten oxide (WO3) can form, a substance that is volatile at such temperatures. This can cause radioactive substances to be released into the environment.
To prevent the formation and release of volatile WO3, self-passivating tungsten alloys have been developed. Elements such as chromium and titanium are added to the tungsten, and in the event of a failure, these form a protective, stable oxide layer over the tungsten, thus preventing further oxidation and stopping radioactive substances from escaping.
The scientific publication Development of advanced high heat flux and plasma-facing materials gives an overview on the research topic.
Contact
Dr. Jan Willem Coenen
Tel. +49 2461 61-5536
Fax +49 2461 61-2660
j.w.coenen@fz-juelich.de