3a) Underlying Technology

3a1) Radiation resistance of SiCf/SiC composites and SiC fibres (Principle Investigator: Vekinis G.)

Objectives: The work is focused on understanding the decohesion mechanisms at the fibre and ceramic material interfaces and failure mechanisms after irradiation aiming at improving the fibres and composite materials radiation resistance. Both standard material (N3-1) and advanced composites (N4-1) will be studied. Specific attention will be put on the in-plane mechanical properties, through thickness properties, matrix fibre debonding and toughness. The work on fracture micromechanisms will be carried out using a special in-situ observation rig inside the SEM able to measure stresses and displacements and the whole process is captured in video. Also the irradiation resistance of advanced fibres (Hi-Nicalon, Hi-Nicalon-S and Tyranno) to be used for the fabrication of the new generation of composites will be assessed. This work is a continuation of similar Technology and Underlying Technology tasks which we had been assigned in the past and in order to have meaningful results these studies have to be continued on samples irradiated at high fluence.

Task description: Two specimen types of SiCSiC (N3 and N4) and three types of fibres (Hi-Nicalon, Hi-Nicalon-S and Tyranno) will be used. The samples will be irradiated for different periods for fluence between 3.2 ´1020 and 5.4´1020 n/cm2. Irradiation will be performed at GRR-1 “Demokritos” research reactor. After each irradiation period one specimen of each type will be recovered and its mechanical properties, matrix-fibre debonding and toughness will be studied. Also micromechanisms will be assessed by in situ SEM.

Progress report: SiCSiC (N3 and N4) specimens have been irradiated to high fluences. Already samples irradiated for total of 1600hrs have been recovered and now their irradiation levels make mechanical and electron microscopy tests feasible. Another set of samples after being irradiated for above to 2000 hrs will be recovered in the middle of December and examined early next year. Also fibres (Hi-Nicalon, Hi-Nicalon-S and Tyranno) have been irradiated up to 800hrs and examined.

3a2) Synthesis of SiC and in-situ bonding of SiCf/SiC by rapid combustion processing (Principle Investigator: Xanthopoulou G.)

Objectives: If SiCf/SiC (plates or other forms) is to be used as a plasma-facing material, it must be able to be joined successfully to itself and to other materials. For potential use on the plasma-facing blanket of a fusion reactor, the joints must satisfy a number of criteria: the joint material chemically and physically compatible with SiCf/SiC, the joints must have adequate mechanical and physical properties and no open porosity, the method used must not adversely affect the properties of the materials being joined, must be applicable over relatively large areas and to enable joining both between segments of SiCf/SiC and with the backing material. Ordinary brazing methods suffer from a major drawback: the material to be joined needs to remain at high temperatures for a relatively long period of time, in contact with the molten brazing materials, in order to allow diffusion. This can adversely affect the properties of the SiC fibers or the matrix. A rapid method which can potentially satisfy all the above criteria is "Controlled Combustion" where SiC is synthesised in-situ between the parts to be joined (under some pressure).

Task description: Already work carried out in the previous year has shown that the rapid combustion method can lead to satisfactory bonding of joining SiCf/SiC materials. A number of systems have now been identified on which more work is justified with the following objectives:

• Finding ways of reducing the pre-heating temperature needed for the SHS synthesis of pure SiC

• Reduce the porosity of bonds while maintaining good overall integrity and mechanical properties.

Progress report: As is presented in Annex XVII, three systems were studied during 2002:

1) SiO2 +C with additives of Mg and Al, under various conditions

2) Al-Fe with additives of Mg under various conditions

3) Al-Ni with additives of Mg under various conditions

The above systems were used to bond plates of sic/sic under medium pressure and the resulting bonded sandwiches were analysed by SEM, XRD, microstructural and mechanical analysis. A total of 110 specimens were made by SHS and analyses were carried out: SEM (50), XRD (35), mechanical (25), porosity (35), microstructure (80).

3b) Technology Tasks

3b1) Impurity determination of advanced SiC fibers and validation of European Activation Data (TW2-TTMN-002: Deliverable 11, Principal Investigator: Stamatelatos I.E.)

Objectives: Silicon carbide as a ceramic matrix composite (SiCf/SiC) is a candidate low activation material for the first wall and blankets of the future fusion power plants. However, SiC composites may contain impurities as a result of their manufacture procedure. Since activation of these impurities may result in undesirable dose-rates, the choice of fiber sources and manufacture processes without impurity contamination are required. It is apparent that monitoring of the impurity levels of SiC fibers tested by the Associations is of paramount importance, since this will contribute to the choice of source fiber material and will assist in the evaluation of the SiC composite produced. Neutron activation analysis enables the determination of several of the active impurity concentrations. Knowledge of the impurities will allow the calculation of the response of the fibers irradiated by fusion reactor spectra. Moreover, the response of the fibers irradiated by fast fission reactor neutrons will be measured and compared with the prediction of the European Activation System codes and data.

Task description: It is proposed to monitor the impurity levels of SiC fibers tested by the Associations using Instrumental Neutron activation analysis. Once the impurities will be determined the response of the fibers irradiated by fusion reactor spectra will be calculated. Moreover, the response of the fibers irradiated by fast fission reactor neutrons will be measured and compared with the prediction of the European Activation System codes and data.

Progress report: Validation studies of the European Activation File (EAF) library are carried out against measurements of gamma ray spectra and dose rates at fission reactor neutrons. SiC fiber, a candidate first wall low-activation material, is studied. Emphasis is given on the contribution of the inherently present material impurities on the gamma emitting radionuclide production. Instrumental neutron activation analysis of SiC fibers (UBE-Tyranno, Hi-NICALON and S-NICALON) were performed and impurity levels were determined. The European Activation System (EASY) software package was used to validate gamma emitting reaction cross-sections and decay data. The results of the computations were compared with experiments performed by irradiating fibers at fission reactor neutrons. (Details are presented in Annex XVIII.)

3b2) Neutron irradiation up to 0.8 dpa at 200-2500 oC of EUROFER plates (TW2-TTM2-001b: Deliverable 7, Principal Investigator: Messoloras S.)

Objectives: Neutron irradiation up to 0.8 dpa at 200-2500C of EUROFER plates for modelling purposes. The details will be discussed with the group undertaking modelling of the irradiation effects on Fe. Part of the effort will be devoted in the development of a high temperature rig.

Task description: An irradiation rig capable of reaching 3000C will be designed. The useful volume of the rig will be of dia 10-15 mm and of height 400 mm (level of 50% flux reduction) or 300mm (level of 75% flux reduction) - 200 mm gives almost constant flux. Samples will be irradiated at different levels up to 0.8 dpa, will be withdrawn and will be despatched for PIE.

Progress report: In order to construct the high temperature irradiation rig calculation of the gamma heating for different positions of the reactor core were carried out and for different materials. Some experimental work on the measurement of the heating arising from the gammas has been initiated. Also it was calculated the fast neutron flux for different possible positions in the reactor core in which the high temperature rig could be installed. Different ideas for materialising the high temperature rig has been discussed and information was sought from other partners.

3b3) Tungsten: preliminary experiment (TW2-TTMA-002: Deliverable 8, Principal Investigators: Messoloras S. and Vekinis G.)

Objectives: Tungsten (W) is a potential candidate for the plasma-facing blanket in the fusion reactor. W is a body-centre cubic metal with very high temperature capability (melting point in excess of 3400 C). The wrought or cast material is sensitive to grain boundary impurities and there is evidence to suggest that it suffers from hydrogen embrittlement, which results in reduced its mechanical strength. Similarly, neutron irradiations affect adversely its properties.

Due to its high melting point, tungsten is difficult to form but various methods now exist enabling its wider utilisation, such as Chemical Vapour Deposition (CVD), which offers very small grain size and low oxygen, hydrogen and nitrogen content. These properties increase its fracture toughness (reduce its brittleness) and should enhance its neutron irradiation capability No data or other information appears to exist on the neutron irradiation resistance of CVD-W in the open literature. Such data are considered very important in determining the feasibility of using this material for fusion applications. The irradiation behaviour is thought to depend on many factors among which is its microstructure and the purity of the material.

The objectives are

• Assessment of effects of neutron irradiation on the physical properties of different W alloys including CVD-W

• Studies of microstructural changes and changes in mechanical properties after different irradiation fluences

Task description: 1) Development of a high temperature irradiation rig: Feasibility and Safety analysis and 2) Effects of neutron irradiation on mechanical and microstructural properties of W alloys including CVD-W which includes a) Irradiations of different alloys up to 0.2 dpa, b) Metallography (OM and SEM), c) Transmission Electron Microscopy, d) Mechanical tests in tension and e) Fracture behaviour and mechanisms.

Progress report: Work on the feasibility and safety analysis for the installation of a high temperature rig continued this year. The CVD-W alloys were not fabricated by CEA so the irradiations could not start early in the year. Since it was apparent that the fabrication of these samples was problematic it was decided that industrially fabricated W should be examined. Samples have been ordered and the irradiations are expected to start in the middle of November.

3b4) Thermal, electrical and ferro-magnetic properties of standard Eurofer plate (TW2-TTMS-002: Deliverable 23, Principal Investigators: Mergia K. and Papastaikoudis K.)

Objectives: The electrical resistivity, the thermal conductivity and the magnetic properties of the standard Eurofer plate will be measured in the temperature range RT to 600 oC. The study of these basic physical properties is very important as, apart from characterising Eurofer plate, they are essential design parameters. The different contributions to the electrical resistivity will be distinguished. Also various magnetic coefficients will be derived such as saturation magnetisation, Curie temperature and/or other transition temperatures (if within temperature range investigated) etc.

Task description: Samples of the appropriate shape and size for each kind of measurement will be prepared from the Eurofer plates. All measurements will be carried out under a controlled inert atmosphere in order to eliminate oxidation effects. The electrical resistivity will be measured by the four-point method. Special attention will be paid to the preparation of the high temperature electrical contacts. The thermal conductivity will be measured with a specially designed apparatus. The magnetic properties will be measured with a vibrating sample magnetometer.

Progress report: The electrical resistivity of Eurofer was measured as a function of temperature between room temperature and 650 oC. Magnetic measurements were performed in the temperature range from room temperature up to 900 oC. Hysteresis loops at 100 oC intervals up to 600 oC are being measured in order to determine the coercivity as a function of temperature. Thermal conductivity measurements are under way. (More details are presented in Annex XIX.)

3b5) Complete Eurofer characterisation of the typical microstructures and modelling of the phase transformation behaviour (TW2-TTMS-002: Deliverable 3, Principal Investigator: Mergia K.)

Objectives: In order to specify an improved EUROFER heat-treatment it is necessary the different crystallographic, magnetic and precipitate structures to be identified as function of temperature. Further it is important the phase transformation rules to be modelled. In order to accomplish this, a wide range of experimental techniques will be used.

Task description: The phase structure of the as received alloy will be assessed. A DSC study will be undertaken in order to understand the precipitation and dissolution sequence of the alloy. Then different heat treatments will be applied in order to obtain a homogeneous as much as possible system (identification of the solution treatment temperature). Then different heat treatments in the temperature range RT-900 oC will be applied (isochronal) to the solution treated material, The microstructure of the system in the above mentioned conditions will be investigated by DSC (phase kinetics), X-rays, neutron diffraction (crystallographic and magnetic structure), SEM with EDX (coarse precipitation), TEM with EDX and EELS. This will permit the identification of the different phases and the phase transformation behaviour.

Progress report: Differential scanning thermographs were obtained between room temperature and 600 oC for different scanning rates. X-rays and neutron diffraction measurements were performed at room temperature. TEM with EDX and EELS are underway. Isochronal heat treatments in the temperature range from RT to 900 oC will be applied and the microstructure will be assessed by DSC and neutron diffraction.