3a) Underlying Technology

3a1) Structure evolution of SiCSiC composites under neutron irradiation (Principal Investigator: Mergia K.)

Objectives: Aim is to correlate micro-structure of SiCSIC composites and the changes induced by neutron irradiation with their mechanical and physical properties before and after irradiation. This would provide fundamental and engineering basis for devising appropriate composites and for defining approaches in order to improve their irradiation resistance. There is a real need to establish the effect of the neutron radiation damage in the microstructure of SiC and its composites which results in modifications of their properties such as hermicity, electrical and thermal conductivity, chemical compatibility, dimensional stability and mechanical properties. SiCSiC composites are considered for fusion reactor applications however many outstanding problems have to be solved in the next decade. Modelization in order to understand the effects of the irradiation on the materials and consequently to devise approaches of eliminating its effects is becoming an important aspect of the EU Fusion programme. Models have to be tested against experimental evidence and a main requirement for a successful model is to explain the structural transformations under irradiation and in SiC mainly the amorphization under low temperature irradiation. The results of this work will be important in testing models either phenomenological or from first principles and assist in their development.

Task description: Neutron diffraction is proposed as the main techniques to examine the microstructural changes of SiCSiC composites after neutron irradiation. XRD due to the short length of penetration of X-rays is mainly a surface technique whereas neutrons due to its deep penetration can give structural information for both matrix and fiber simultaneously. Areas of attention will be lattice expansion, loss of crystallinity and lattice strains induced by neutron radiation damage. Both SEP and the new 2D and 3D EU reference SiC/SiC and the LPS/NITE SiC/SiC composites to be delivered in 2003 will be examined and also fibres UBE Tyranno and Hi-Nicalon-S. The samples will be irradiated to different doses up to 5x1020 n/cm2. In order to ascertain the damage dependence on neutron energy samples will be irradiated in both fast neutron rich and slower neutron rich spectrum.

Progress report: Both types of samples N3 and N4 were irradiated at different doses (irradiation times from 200 to 1600hrs at 3x1013 n/cm2/s). The irradiated samples were measured by neutron diffraction. Structural changes are observed to occur mainly after 800 hrs irradiation times. These indicate lattice expansion and defect accumulation. No amorphization has been observed. However there are some lattice constant changes, which point out that at high doses amorphization might occur. Since these experiments are important in understanding the irradiation degradation of SiC/SiC composites and fabricating irradiation resistant materials this task would be continued and will mainly be financed by our Institute.

3b) Technology Tasks

3b1) Detailed metallurgical characterisation (including ageing effects) of the EU ODS steel (TW3_TTMS_006-deliverable 2, Principal Investigator: Mergia K.)

Objectives: Metallurgical characterisation of EU ODS including ageing effects.

Task description: (see work proposed for the year 2004)

Progress report: The work for this task has not started because the samples, which were going to be produced by CEA, were not delivered. It is expected this task to start next year.

3b2) Morphological characterisation of SiC/SiC composite: fibres, interface, matrix and porosities (TW3_TTMA_001- deliverable 1, Principal Investigator: Vekinis G.)

Objectives: SiC/SiC composites offer excellent high temperature resistance and are considered as the most promising long-term candidates for the fusion front-wall blanket. Previously tested composites however (CERASEP N3-1 and N4-1) showed that the fibers used (Hi-Nicalon) were not resistance to damage induced by neutron irradiation – the fibers and the interfaces were considerably damaged and the materials lost a great amount of the toughening effect.

Task description: The new materials to be delivered in 2003 (2D and 3D EU reference SiC/SiC and the LPS/NITE SiC/SiC composites) will be made using the near-stroichiometric fibres UBE Tyranno and Hi-Nicalon-S. These are expected to offer better resistance to neutron irradiation than the previous fibers tested (Hi-Nicalon) and therefore they represent an important step in the eventual development of SiC/SiC for fusion. The use of SiC fiber toughening imparts a measure of flaw-tolerance to the materials, giving them a measure of energy dissipation (toughness) during fracture. The proposed work will offer a full characterisation of the new materials and will form the basis for comparison with the materials’ properties measured at higher temperature irradiation.

Progress report: The samples for this task, which were going to be produced by the Industry, have not been delivered yet to the Associations. It is expected that the samples will arrive at the end of the year and thus the task will start when the samples are delivered.

3b3) Development of Joining techniques for SiC/SiC (TW3_TTMA_001_ deliverable 7a, Principal Investigator: Xanthopoulou G.)

Objectives: Investigation of the rapid joining technique on the new 2D and 3D EU reference SiC/SiC and the LPS/NITE SiC/SiC composites to be supplied in 2003. The study will be based on the Combustion Wave method (based on the Self-Propagating High Temperature Synthesis technique) developed previously for the CERASEP N3-1 and N4-1 SiC/SiC materials. Joining Parameters to be studied include: pre-heating temperature, composition of bonding material, external pressure, morphological characteristics of starting powders. The bonds will be characterised by examination of their morphology, microstructure, mechanical strength in shear, atomic structure (XRD).

Task description: Rapid joining technique for the new 2D and 3D EU reference SiC/SiC and the LPS/NITE SiC/SiC composites.

The study will be based on the Combustion Wave method developed previously for the CERASEP N3-1 and N4-1 SiC/SiC materials.

The main advantages of this method over traditional brazing methods are:

• Very short duration of bonding: combustion is initiated at one edge of a joint and propagates rapidly, as a combustion wave, in all directions until the joint is formed.

• The method allows bonding of large surfaces together.

• Environment temperature can be kept low, thereby reducing the possibility of damage to the SiC/SiC.

• Joints can be formed in air and do not need special atmospheres.

• A choice of 3 bonding compositions is available, developed previously.

• The method can be adapted readily for bonding SiC/SiC to refractory metals such W, Mo but also to Cu, Ni and other materials.

The method can also be used for coating of SiC/SiC.

Progress report: Nickel, iron, cobalt, chromium, titanium, copper based systems and there mixtures were studied for search of optimum composition, additives and temperature for bonding SiC .It was found that for all occasions is possible to bond plates of SiC with method SHS. It is actually possible to receive melted layer at room temperature between 2 SiC plates by method SHS, but bonding with surface is not very strong and after some efforts plates can be separated. At temperatures about 1000oC we received very strong bonding. Minimum temperatures, which were achieved for bonding was 710C for iron system. It is very significant result, because this bonding can be reached in few seconds at such low temperature. It is possible to try to drop this preheating temperature even more. For all systems were studied minimum thickness of the layer, which is able to burn and bond SiC plates at different temperatures. Best result was1mm (for iron system with additives at 700C). XRD analysis measurement of physical properties was done for the above mentioned systems.

The task will finish at the end of the year.

3b4) Validation experiments for selected EFF/EAF data evaluations (E<20 MeV): Measurements and analysis of gamma activation in EUROFER irradiated in low energy (fission) neutron spectrum (TW3_TTMN_002_ deliverable 4, Principal Investigator: Stamatelatos I.E.)

Objectives: EUROFER is a candidate low activation material for the blankets of future fusion power plants. It is apparent that a detailed evaluation of the induced activity and gamma dose rate under fusion neutron spectrum characteristics is essential in order to evaluate the performance of this material. However, nuclear data need to be validated to assure they give reliable results. In this work validation of the activation data for EUROFER in the neutron energy range of a fission reactor facility will be performed.

Task description: Induced activity and gamma dose rate of EUROFER will be evaluated using the European Activation System (EASY) for fission neutron spectrum irradiation. The evaluation will be based on the determination of the elemental composition of the samples using Neutron Activation Analysis (NAA) and accurate evaluation of the reactor neutron spectrum at the irradiation position. The results of the gamma dose-rate evaluation will be experimentally verified. Analysis of the results will provide C/E (calculation/experimental) data and associated uncertainty estimates.

Progress report: EUROFER is a candidate structural material of future fusion power plants. Induced activity and gamma dose rate of EUROFER was evaluated using the European Activation System (EASY) for fission neutron irradiation. The evaluation was based on the experimental determination of the elemental composition of EUROFER samples by instrumental neutron activation analysis and the prediction of GRR-1 reactor neutron spectrum at the irradiation position using a detailed Monte Carlo model of the core and irradiation facility configuration. The results of the gamma dose evaluation using EASY system were experimentally verified by irradiation of a set of EUROFER samples at the reactors’ fast neutron irradiation trap (lattice position D-4) and measurement of the gamma emission by means of gamma spectroscopy. Analysis of the results provided calculated to experimental ratios (C/E) and associated uncertainty estimates for the main active isotopes of EUROFER for fission neutron irradiation. The results of the study provide data complimenting work performed at higher neutron energies (fusion spectrum) and assist on a detailed characterisation of the EUROFER material properties

3b5) Neutron irradiation up to 0.8 dpa at 200-2500C of EUROFER plates (TW2_TTMS_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: The gamma heating was measured at different positions in the reactor and with different materials and it was compared with calculations carried out the previous year. Also the neutron flux was measured at different positions and was compared with calculations. Initial design of the high temperature irradiation rig was performed. Two experts from MOL (Belgium Association) came to "Demokritos" to discuss different design aspects. It was decided to send one of our engineers to MOL in order to carry out collaborative work towards the complete design of the high temperature irradiation rig.

3b6) Tungsten: preliminary experiment (TW2_TTMA_002_ deliverable 8, Principal Investigator: Messoloras S./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 irradiation 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 is 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

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: The safety analysis of the high temperature rig has been completed. Concerning the effects of irradiation on tungsten the task has been delayed because CVD-W alloys were not fabricated by CEA so the irradiations could not start. In order to obtain some information of the effects of irradiation on tungsten, and since it was unsure if the samples would be fabricated, industrially produced samples were ordered and an irradiation campaign started around September 2002. Early 2003 when we recovered the first sample it was observed that the samples were accidentally oxidised. New samples were ordered and before starting a new irradiation campaign we asked the GCSU for an extension of the due date. No answer was received.

3b7) Thermal, electrical and ferro-magnetic properties of standard Eurofer plate (TW2_TTMS_002_ deliverable 23, Principal Investigator: Mergia K., 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-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 remaining measurements were carried out and the task was completed beginning of the year. Final report sent to GCSU and accepted.

3b8) 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-9000C 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: The remaining measurements were carried out and the task was completed beginning of the year. Final report sent to GCSU and accepted.