3a) Underlying Technology

3a1) Tough, layered SiC-based composites, using SiC needle-like powders made by combustion synthesis

Objectives: Of all engineering materials known, only SiC offers the critical properties needed as a first wall material in a fusion reactor, but its toughness is inadequate. Even after special processing, monolithic SiC rarely manages to exceed fracture toughness of about 6 MPam^1/2 as compared to a low acceptable limit of about 15 MPam^1/2. In an effort at reaching such a toughness SiC is toughened by fibres in a complicated process involving 3-D fibre mat weaving followed by chemical vapour infiltration by SiC, a process that is extremely time consuming and expensive. The work proposed here aims at developing a new layered material with good through-the-thickness toughness. Micro-plates of SiC will be made by slip-casting of elongated grains of SiC (offering toughness within the micro-layers) followed by layering of up any number of such plates to make up a layered SiC-based composite of high overall toughness. The method allows for high purity and it is flexible enough to enable the processing of materials of various thicknesses. The proposal includes full characterisation and studies of the effect of neutron-irradiation on the physical and mechanical properties.

Task description: High purity SiC with elongated morphology will be produced by combustion synthesis and fully characterized. By the slip-casting technique microplates of SiC will be produced. The irradiation resistance of the SiC grains will be also assessed. The work proposed will be carried out collaboratively between Greece and Slovenia, two teams having complementary expertise and facilities. SiC elongated grains will be fabricated by combustion synthesis and fully characterized. Also the irradiation damage on the material will be assessed. The SiC powders will be dispatched to the Slovenian team for the fabrication of microplates.

Progress report: The self-propagating high-temperature synthesis (SHS) method has been used to produce a variety of SiC powders, with the aim of stabilising elongated morphology. The emphasis of the work carried out in 2004 (see Annex XXIV for more details) was on determining the right conditions for heating and cooling which give high purity elongated grains. Over 30 experiments with various compositions and under various processing conditions have been carried out. The system offering greatest promise was found to be Si-C with the use of special additives, which produce grains with aspects ratios approaching 3:1. This is currently being optimized to increase the aspect ration to more than 5:1. Not all the grains synthesized have an elongated morphology, the origins of which are being investigated. The next 6 months will be devoted to optimizing the process with changes in heat treatments and the use of new additives. It must be noted that the project's full aims can be completed only as far as the production and testing of elongated SiC powders is concerned, unless the second part, proposed to be carried out with the Institute Jozef Stefan, which is to produce the composites using the powders produced in NCSR "Demokritos", is approved.

3a2) Gas impermeable coatings for SiC_f/SiC

Objectives: Among the most serious limitations of SiC_f/SiC materials to be used for the first wall of the fusion blanket, are sensitivity of the fibres and interfaces to the neutrons, low thermal conductivity across the thickness and high gas permeability, both owing to the large amount of open porosity. The first problem is being addressed by the use of stroichiometric fibres but the the "open" nature of the 3-D fibre weave and the use of chemical vapour deposition or infiltration methods to "built-up" the matrix between the fibres, means that the latter two limitations are more difficult to address satisfactorily. This proposal will address these last two problems by the development of a proven slip-casting method to fill in and coat the materials with a ceramic composition based on SiC by immersion. The aim is to improve both the thermal conductivity and the gas impermeability in one processing stage and determine the effect of n-irradiation on the physical and mechanical properties of the materials. The work proposed will be carried out collaboratively between two teams in Greece and Slovenia with complementary expertise.)

Task description: The coating method relies on the use of highly loaded aqueous suspensions of SiC-based ceramic powder mixtures which are used to coat the SiC_f/SiC parts by simple immersion. The use of appropriate chemical surface treatments and additives in the suspensions as well as special processing conditions will ensure good adhesion of the fine ceramic particles on the surface and within the pores of the SiC_f/SiC and, after sintering, creation of a dense, protective SiC-based ceramic layer. The slip-coated and infiltrated SiC_f/SiC is sintered under protective atmospheres which gives a "functionally-graded material", i.e., a material with good mechanical properties, high thermal conductivity and excellent gas impermeability. The coated and sintered materials will be characterized by a) electron microsopy, neutron and X-ray diffraction and other analytical methods, b) thermal conductivity and gas permeability, c) mechanical properties and fracture behaviour, and d) neutron irradiation and its effects on the physical and mechanical properties.

Progress report: Coated specimens of SiC-Al-Y were produced by immersing surface-prepared SiC_f/SiC samples into aqueous suspensions of powder mixtures of SiC with Al (Aluminium) and Y (Yttrium) oxides, following by heat treatments (conditioning and firing). A total of over 20 different compositions and heat treatments have been examined up to now and investigations with TEM and SEM have been carried out. The specimens thus prepared have been physically characterized with emphasis on the quality of adhesion, the open porosity and the overall crystallinity of the coating material. In general the slip-immersion method appears to offer a promising coating on SiC with good interfaces. The work was carried out partly during the visit of Dr. G. Vekinis to Ljubljana, Slovenia in March-April 2004. It is continuing with specimens being prepared using doped coatings with elements other than Y. The use of some low atomic number materials appears to offer similar coating behavior without the activation problems inherent in the use of Y. It is planned to carry out mechanical and further physical characterization in later stages. Nano-powders of SiC will also be used to obtain nanostructural coatings which are believed to offer specific benefits. Some of the Y specimens are currently being n-irradiated and will be examined at regular intervals after irradiation. Irradiations are also planned for the new doped materials. (More information on this activity is presented in Annex XXV.)

3a3) Structure evolution of SiCSiC composites under neutron irradiation

Objectives: The SiC_f/SiC composites are proposed to be used in high temperature and irradiation fields in a future fusion reactor. The macroscopic properties of these materials are strongly correlated with the underlying atomic structure and changes induced by temperature and/or irradiation. Thus in order to understand the behaviour of these materials in fusion applications and to fabricate new materials with improved properties the underlying micro-structure and its modifications need to be assessed. Neutron diffraction is a unique technique for this purpose since macroscopic samples can be investigated. A new type of SiC_f/SiC composites will be soon available and these samples will be investigated with different techniques from a number of Associations. This task will provide the basis of understanding the macroscopic properties measured from the different Associations.

Task description: The neutron diffraction spectrum of as received SiC_f/SiC samples prior to any treatment will be measured and from this the structural properties will be determined. Then a set of samples will be heated at different temperatures up to 1000°C. From the neutron diffraction spectra of the heat treated samples the changes of the structure due to the heat treatment will be assessed. Also a set of samples will be irradiated to different levels up to 0.5 dpa at 40°C. The irradiation induced structural changes will be assessed by neutron diffraction. In the future irradiations at higher temperatures will be performed.

Progress report: Composite specimens of Nicalon-S fibres in a SiC matrix, types N3 and N4, were subjected to neutron irradiation at 40ºC for irradiation times up to 1900 hours at 7.5×10¹³ n/cm²/s, at the Greek Research Reactor (GRR-1) at NCSR "Demokritos". This corresponds to a fast neutron fluence of 5.13×10²⁴ n/m² or to about 0.51 dpa. The samples after the irradiation were measured by neutron diffraction. The diffraction spectra revealed no indication of amorphization. Although lattice contraction takes place for both N3 and N4 types after irradiation at a neutron fluence of 1.6×10²⁴ n/cm² up to 4.3×10²⁴ n/cm², the lattice constant attains its value at the unirradiated condition at the fluence of 7.5×10¹³ n/cm²/s. Neutron irradiation is continuing for higher irradiation times. More details are presented in Annex XXVI.

3b) Technology Tasks

3b1) Detailed metallurgical characterisation (including ageing effects) of the EU ODS steel

Objectives: Metallurgical characterisation of EU ODS including ageing effects.

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°C 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, EELS. This will permit the identification of the different phases and the phase transformation behaviour. Further the isothermal ageing kinetics will be investigated.

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 to start around as soon as they are available.

3b2) Morphological characterisation of SiC/SiC composite: fibres, interface, matrix and porosities

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.

Progress report: The samples for this task, which were going to be produced by the Industry, have not been delivered yet to the Associations. The task will start when the samples are delivered.

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.

3b3) Neutron irradiation up to 0.8 dpa at 200-250°C of EUROFER plates

Objectives: Neutron irradiation up to 0.8 dpa at 200-250°C 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 300°C will be designed. The useful volume of the rig will be of diameter 10-15 mm and of height 400 mm (level of 50% flux reduction) or 300 mm (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 collaboration with the SCK-CEN Association and under secondment of a scientist of our team to MOL, work was carried out for the development of a high temperature irradiation and in the areas of a) mechanical strength calculations, b) hydraulic calculations, c) thermal assessment, d) safety aspects, e) nuclear calculations, f) drawings, g) specifications and e) instrumentation and data acquisition. Also is under way work on in-pile instrumentation which will be associated with the high temperature irradiation rig and in particular with the integration of sensors in rig assembly, calibration and testing and the data acquisition system. In conjunction experimental and modelling work was carried out for different neutronic and safety aspects associated with the high temperature irradiation rig. More details are presented in Annex XVII.

3b4) Measurement of the magnetic properties of ITER vacuum vessel materials and welds

Objectives: The ITER vacuum vessel is planned to contain plates of ferritic steel AISI 430 which are used to correct for the toroidal magnetic field ripple. A key property for this material is the saturation magnetic flux density Bs. Available data suggest that there could be a wide variation in the value of Bs for this material. The purpose of this work is to recommend -after systematic magnetic studies- the material properties for the procurement of this material in order that the total amount of shielding (magnetic and neutron) needed to be optimized. For this material from possible suppliers of AISI 430, both structural and magnetic properties will be assessed and the properties of the material for procurement will be recommended. The second part of the task concerns the magnetic permeability of the vacuum vessel welds. It is known, that fusion welds of austenitic steel have a higher ferrite content and as a result - higher magnetic permeability. Therefore the austenitic versus ferrite content and magnetic and structural properties in the weld and surrounding area will be determined. This could possibly permit the increase of the initial requirement for magnetic permeability of ITER welds of ~ 1.2. The effect of material heat treatments, operating temperature (between 20°C and 200°C) and material properties would also be evaluated.

Task description: Ferritic steel AISI 430 Procurement: Possible suppliers to be specified by EFDA. If not available known suppliers of the material will be chosen. Heat treatments as recommended by the suppliers or by EFDA. Evaluation of the material: a) saturation magnetization, coercive field and remanence from 4 to 900K with hysteresis loops, b) structure evaluation by X-ray diffraction, c) magnetic structure by Neutron diffraction and d) impurity content by Neutron activation analysis. Evaluation of the effect of the working temperature: Material will be heat treated at 100, 200 and 250°C for two weeks. a) measurement of Bs and hysteresis loops from 4 to 900K and up to 2 T magnetic field, b) structure evaluation by X-ray diffraction, c) magnetic structure by Neutron diffraction. Welded samples (supplied by EFDA) Specimens will be cut from the supplied welds Evaluation of the weld: For each specimen a) measurements of Bs and hysteresis loops from 300 to 600K, b) structure evaluation by X-ray diffraction, c) magnetic structure by Neutron diffraction Evaluation of the effect of the working temperature: The speciments will be heat treated at 100, 200 and 250°C for two weeks. a) measurement of Bs and hysteresis loops from 300 to 600K, b) structure evaluation by X-ray diffraction, c) magnetic structure by Neutron diffraction.

Progress report: Neutron and X-ray diffraction measurements of AISI 430 and welded samples have been carried out and the results evaluated, as described in detail in Annex XXVIII. Also, magentic measurements at room temperature have been carried out, while the corresponding measurements at higher temperatures are under way. The resistivity measurement apparatus has been fabricated and test measurements have been done.