[en] By mechanical loading, the behaviour of poly-granular graphites for industrial uses is globally brittle: when a pre-existing flaw becomes critical a crack initiates and then propagates more or less catastrophically. This scheme implies several features which are described in the present paper. First, as the crack will be initiated at a critical flaw, the ultimate stress appears as largely dispersed and the strength is not an intrinsic material's parameter. Secondly, the processing route introduces in the material some microstructure anisotropy, largely influencing the strength dispersion. Finally, the crack propagation is controlled by a bridging mechanism of the lips which depends on the microstructure. This effect can be described by the so-called crack growth resistance curve: the R-curve. (authors)

[en] Full text of publication follows.The modular VHTR is one of six advanced fission systems of interest for meeting the Generation IV goals of attaining highly economic, safe, reliable, sustainable, proliferation-resistant systems. The VHTR offers significant advantages for long-term development of sustainable energy and in particular for heat applications and hydrogen generation. This system can operate with either a direct or indirect cycle and makes use of the high efficiency Brayton cycle. Work on materials investigations for the HTR within Europe recommenced with the EU 5. Framework Programme (5FP) projects HTR-M and M1 [1] and together with other SFP projects (fuel, reactor physics, components, safety,..) and the establishment of the European High Temperature Reactor Technology Network HTR-TN, served as the main European platform for the co-ordination and development of VHTR issues. The HTR-M and M1 projects addressed material requirements for the key components of the direct cycle HTR. The work especially focused on the materials development for the pressure vessel, high temperature components (including turbine), and the graphite core. Alongside this, developments were undertaken on key component issues (HTR-E) associated with the gas turbine, the recuperator and other system developments (e.g. tribology, corrosion, bearings, seals, etc.) concerned the operation and performance of the power circuit components. Within this paper the main highlights from the results of the 5FP programmes affecting material issues are reviewed and examined. For the 6. Framework Programme activities the main European research focus on VHTR is through the RAPHAEL Integrated Project (IP). The project started in 2005 and addresses a range of issues (materials, components, fuel, code qualification, etc.), which are structured in a similar way to the corresponding GIF VHTR projects. The materials issues are addressed within one of the RAPHAEL, sub-projects with a focus on outstanding issues. The main emphasis on materials is for graphite development, materials for the heat exchangers, continuation of vessel qualification and work on design code requirements. The progress of the materials issues within the RAPHAEL-IP, which has reached its mid-term stage will be reviewed and the expected future orientations of the programme described. The main materials issues addressed in the paper are as follows: Reactor pressure vessel: - review and database actions covering existing and new vessel material options; - tests on Mod 9Cr 1Mo steel welded joints under irradiated and non-irradiated conditions to determine suitability for vessel application. High temperature materials: - review and database actions for the control rod and turbine; - review and database actions for the heat exchangers; - tests on selected materials (carbon/carbon (C/C) composites, high alloy steels) at temperature and under short and intermediate times in air, and simulated carburizing and de-carburizing environments. Graphite core: - review of experience plus data base actions for new graphites; - oxidation tests on graphites and C composites; - graphite selection and irradiation testing at 750 deg C and 950 deg C; - micro-structural modelling and development of guidelines. (authors)

[en] Good progress has been made on the Very High Temperature Reactor (VHTR) material and components investigations performed within the European Commission fifth and sixth Framework programmes. These programmes focus on the identification and investigation of key technological issues for this modular type of reactor, which offers significant advantages for the long-term development of sustainable energy and heat applications including hydrogen generation. For the fifth Framework programme two materials projects HTR-M and M1 and a component project (HTR-E) were undertaken to address requirements for the reactor pressure boundary, high temperature components (including turbine and recuperator), tribology issues and operational issues and the graphite core. For the sixth Framework programme the RAPHAEL-IP is underway addressing the remaining key issues plus extending the development and qualification to include heat exchangers, the gas circulator and design codes. The main highlights of the results from the key investigations and tests performed within these programmes are summarized. (authors)