B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 is mandatory. A tentative route for improving the ceramic breeder LiSiO4 industrially produced by composites thermal properties relies, first of all, Glaswerke Schott( Germany) using the melt spray upon the use of quasi stoichiometric advanced technique has a granular form with an average fibres(Hi-NICALONTM S Type, Dow Corning diameter of 0.55 mm and it contains about 2% Sylramic) with larger grain size. The Cvi process TeO2. Li,TiO, pebbles have a shape and dimen- has to be improved in order to obtain a Sic sions similar to the Li4SiO4 ones and have been matrix with larger grain size. The fibre matrix manufactured by ENEA-Casaccia, starting from interface volume, usually amorphous and lamella commercially available Li2T1O3 powder. The ce- shaped, should be reduced because it acts as a ramic composite investigated was the 3D SEP- thermal barrier between the crystalline phases. In CERASEP N3-1; the composite samples order to face the thermal conductivity decrease (50 x 10 x 3 mm) were subjected to a final uni- fter irradiation and to stay within the design form CVD SiC coating( about 100 um thick). requirements the SiC crystalline morphology of Three cells were operated with LigSiO4 and three fibres and matrix should likely assure, at room with Li,TiO, for 216, 1000 and 10000 h at 800C temperature, a thermal conductivity of 100 W containing 1000 ppm H2; do stream operating samples( so called,)and immersed samples were provided for each cell. The evolution of the mechanical properties was 5. Compatibility nvestigated on immersed and blank samples for each cell before and after test by means of a three As part of the long term European programme point bending test n low activation materials the compatibility of After 10000 h of test, on blank sample SiC /SiC commercial composites with a solid Li, SiO4, lithium meta-silicate Li2SiO3 and other breeders bed in fusion relevant conditions was complex silicates were found indicating lithium investigated [19]. Lithium orto-silicate(Li SiO4), migration out of the reaction chamber. On im foreseen in the European helium cooled pebble mersed samples the original Sic coating was still bed blanket concept and lithium titanate, consid oresent after exposure: its thickness ranged be ered the most promising alternative to it, have tween 80 and 100 um Over it a layer of about 30 been then used for the exposure experiments. The um of lithium meta-silicate (Li2SiO3), not adher ent and heavily cracked, was observed(Fig The transformation of the Sic coating is not protective since a linear law of weight increase was observed but no changes have been detected inside the specimens. Slight changes of the me- chanical properties were observed at different temperatures: in particular, no variations within the experimental error were detected for Young modulus. As far as the flexural strength is con- cerned only the values obtained at 800C showed a worsening of about 25% for samples after 10 000 h of exposure. Therefore, for Li4SiO4 ex posure, a long life time can be foreseen for such kind of composite depending on the SiC coating layer thickness On blank samples over LiTiO, only silica was DOSE LEVEL (dpa) found up to 10 000 h whilst on immersed samples Fig 4. Thermal conductivity of neutron irradiated 2D Nicalon the original Sic coating is not pr half the whole surface and a heavily cracked layer18 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 is mandatory. A tentative route for improving the composites thermal properties relies, first of all, upon the use of quasi stoichiometric advanced fibres (Hi-NICALON™ S Type, Dow Corning Sylramic) with larger grain size. The CVI process has to be improved in order to obtain a SiC matrix with larger grain size. The fibre matrix interface volume, usually amorphous and lamella shaped, should be reduced because it acts as a thermal barrier between the crystalline phases. In order to face the thermal conductivity decrease after irradiation and to stay within the design requirements the SiC crystalline morphology of fibres and matrix should likely assure, at room temperature, a thermal conductivity of 100 W m−1 K−1 . 5. Compatibility As part of the long term European programme on low activation materials the compatibility of SiCf /SiC commercial composites with a solid breeders bed in fusion relevant conditions was investigated [19]. Lithium orto-silicate (Li4SiO4), foreseen in the European helium cooled pebble bed blanket concept and lithium titanate, consid￾ered the most promising alternative to it, have been then used for the exposure experiments. The ceramic breeder Li4SiO4 industrially produced by Glaswerke Schott (Germany) using the melt spray technique has a granular form with an average diameter of 0.55 mm and it contains about 2% TeO2. Li2TiO3 pebbles have a shape and dimen￾sions similar to the Li4SiO4 ones and have been manufactured by ENEA-Casaccia, starting from commercially available Li2TiO3 powder. The ce￾ramic composite investigated was the 3D SEP￾CERASEP® N3-1; the composite samples (50×10×3 mm) were subjected to a final uni￾form CVD SiC coating ( about 100 mm thick). Three cells were operated with Li4SiO4 and three with Li2TiO3 for 216, 1000 and 10000 h at 800°C in flowing He containing 1000 ppm H2; down￾stream operating samples ( so called ‘blank’) and immersed samples were provided for each cell. The evolution of the mechanical properties was investigated on immersed and blank samples for each cell before and after test by means of a three point bending test. After 10000 h of test, on blank samples over Li4SiO4, lithium meta-silicate Li2SiO3 and other complex silicates were found indicating lithium migration out of the reaction chamber. On im￾mersed samples the original SiC coating was still present after exposure: its thickness ranged be￾tween 80 and 100 mm. Over it a layer of about 30 mm of lithium meta-silicate (Li2SiO3), not adher￾ent and heavily cracked, was observed (Fig. 5). The transformation of the SiC coating is not protective since a linear law of weight increase was observed but no changes have been detected inside the specimens. Slight changes of the me￾chanical properties were observed at different temperatures: in particular, no variations within the experimental error were detected for Young’s modulus. As far as the flexural strength is con￾cerned only the values obtained at 800°C showed a worsening of about 25% for samples after 10 000 h of exposure. Therefore, for Li4SiO4 ex￾posure, a long life time can be foreseen for such kind of composite depending on the SiC coating layer thickness. On blank samples over Li2TiO3 only silica was found up to 10 000 h whilst on immersed samples the original SiC coating is not present over about half the whole surface and a heavily cracked layer Fig. 4. Thermal conductivity of neutron irradiated 2D Nicalon CG™f /SiC vs. dose level