وضعیت فعالیت های R & D اروپا درباره مواد مرکب SiCf / SiC جهت راکتورهای همجوشی

Silicon carbide composites are a candidate for fusion reactors structural material because of their low activation and after heat properties and good mechanical properties at elevated temperatures. These materials, to be more suitable with their use for fusion energy production, need a strong R&D effort in order to solve some critical issues such as thermal conductivity and radiation stability, hermeticity, chemical compatibility with the fusion environment, the capability to be formed in complex geometries, the joining process and long production time. Constant progress in the fibre quality and matrix–fibre interfaces contribute to support the use of SiCf/SiC composites as structural material for fusion application. This paper presents an overview of the current status of the European R&D activities on SiCf/SiC composites focussing on reactor design studies, composites manufacturing, material characterisation in particular after irradiation, chemical compatibility with different blanket environments and development of joining techniques.

The final goal of the Fusion Technology Programme is power generation under attractive economical and environmental conditions. In this pursuit the use of structural low activation materials (LAM) is fundamental as their use will reduce the risk related to accidents, will facilitate maintenance operations and will simplify decommissioning and waste management. Among LAMs, the SiCf/SiC composites are the leading candidates as structural material for fusion reactors due to their good mechanical properties at high temperature, low chemical sputtering, good resistance to oxidation at high temperature (≤1000°C) and very low short and medium term activation [1]. The SiCf/SiC composites have been conceived and developed mainly for aerospace applications. The optimisation of such material for fusion needs a strong coordination of R&D efforts among research institutes, including participation of industry through all the development stages. The development of more advanced fibres and the enhancement of composites processing methods as well as alternative solutions for fibre–matrix interphase lead to improved thermo-mechanical characteristics at elevated temperatures. Nevertheless critical issues related to the nuclear environment are still present; these issues are mainly connected to the fibres and matrix stability under neutron irradiation, to the poor thermal properties and to the residual porosity. In parallel, technology issues must be addressed, e.g. joining methodology and definition of design criteria. In parallel to the manufacturing development and materials characterisation, fusion reactors design studies using SiCf/SiC composites as structural material have been undertaken. These studies, relying on currently available materials data, provide also guidelines for improvements of ceramic composites, ranking the priorities of future developments. The present overview reports the main achievements of the European R&D activities on SiCf/SiC composites. A description of the design study of a SiCf/SiC composites blanket which uses liquid lithium lead as coolant is discussed. Advances in manufacturing routes dealing with chemical vapour infiltration (CVI) and polymeric infiltration and pyrolysis (PIP) are also presented including recent results on joining and coating techniques. Studies on radiation effects on thermal conductivity and mechanical properties including irradiation creep and compatibility with some solid breeders for long exposure time (10 000 h) are also reported.

A specific R&D effort on SiCf/SiC composites is currently ongoing in order to support the use of the material for FPRs. Significant higher effort both on the theoretical aspects, such as modelisation and analyses, and on experimental and manufacturing aspects will be required overt the next several years in order to achieve the necessary SiCf/SiC improvement and to establish its relevance for use as a structural material in in-vessel FPR components. On the theoretical side activities include the development of the TAURO blanket conceptual design with the double objective of: (i) improving behavioural modelisation and results interpretation and (ii) supplying useful guidelines for the material development and characterisation in order to relax some critical issues concerning the manufacturing of complex shapes. In this respect the production of small scale mock ups aimed at reproducing the main features of the blanket design will be necessary. Specific experimental campaigns will also be required to validate the models and established design criteria. Moreover, the set up of a verification methodology for structural analysis is mandatory for any reactor design study involving CMCs. The improvement of both thermal conductivity and stability of thermo-mechanical properties after irradiation remain the main issue of SiCf/SiC R&D: the constant progress in fibre quality which led to the fabrication of almost stoichiometric fibres is a good premise for reducing such concern. However, a large effort has to be made for the matrix–fibre interface and matrix processing itself in order to reduce the differences in performances with respect to the bulk CVD SiC: this objective may probably be achieved by using processing parameters which allow us to maximise the grain size. Hermeticity and joining techniques need further development to study their compatibility with fusion environments and to improve their performance. Finally it is mandatory for a strong coordination of R&D efforts among research institutes, including participation of industry through all the development stages.