3a) Underlying Technology

3a1) Tough, layered SiC-based composites, using SiC needle-like powders made by combustion synthesis (Principal Investigators: G. Vekinis, K. Mergia)

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 MPam1/2 as compared to a low acceptable limit of about 15 MPam1/2. 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: Due to personnel problems and the reactor shutdown for the period for which this task had to be completed this task was discontinued.

3a2) Gas impermeable coatings for SiCf/SiC (Principal Investigators: G. Vekinis, K. Mergia)

Objectives: Among the most serious limitations of SiCf/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 SiCf/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 SiCf/SiC and, after sintering, creation of a dense, protective SiC-based ceramic layer. The slip-coated and infiltrated SiCf/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, c) neutron irradiation and its effects on the physical and mechanical properties.

Progress report: Due to personnel problems and the reactor shutdown for the period for which this task had to be completed this task was discontinued.

3a3) Structure evolution of SiCfSiC composites under neutron irradiation (Principal Investigator: K. Mergia)

Objectives: The SiCf/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 SiCf/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 SiCf/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: Due to reactor shutdown, work for this task was not carried out. When the reactor becomes operational, work on this task will be continued.

3b) Technology Tasks

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

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-900oC 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 as soon as they are available.

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

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 fibres used (Hi-Nicalon ) were not resistance to damage induced by neutron irradiation – the fibres 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 fibres tested (Hi-Nicalon) and therefore they represent an important step in the eventual development of SiC/SiC for fusion. The use of SiC fibre 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 characterization of the new materials and will form the basis for comparison with the materials’ properties measured at higher temperature irradiation.

Progress report: Due to personnel problems and the reactor shutdown for the period for which this task had to be completed, this task will be discontinued.

3b3) Neutron irradiation up to 0.8 dpa at 200-2500C of EUROFER plates (TW2_TTMS_001b_deliverable 7, Principal Investigator: S. Messoloras)

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 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 dispatched for PIE.

Progress report: Taking into account the gamma heating inside the reactor, a prototype irradiation rig is under construction. In this irradiation rig all the parameters for reactor operation have been assessed as gas flow, gaps, sample holders etc. In order to evaluate the performance the rig against the calculation, experiments using a heater instead of the reactor gamma heating will be carried out. With these experiments the gas pressure and temperature controlling parameters will be evaluated. Also with a gamma thermometer the gamma flux inside the irradiation position will be measured. After these measurements and experiments, the irradiation rig will be inserted in the irradiation position for further evaluation. All the necessary safety calculations and procedures have been carried out. (See Annex XXVIII.)