Fusion Engineering and design ELSEVIER Fusion Engineering and Design 51-52(2000)11-22 www.elsevier.com/locate/fusengdes Status of the European r&d activities on Sicf Sic composites for fusion reactors B. Riccardi a,.P. Fenicib. A. Frias Rebelo b. L. giancarlicg. Le marois d E. Philippe e Associazione EURATOM-ENEA CR FRASCATI, C P.65-00044 Frascati, Rome, Italy b European Commission, JRC Ispra, Tp 202, 21020 Ispra(VA), Italy CEA Saclay, DRN/ DMT, F-91191 Gif-sur-Yuette Cedex, Franc CEA Grenoble, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, france SEP Division of SNECMA, Le Haillan-BP 37-33165 St Medard en Jalles Cedex, france Abstract 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 Ich 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 Sicr sic composites as structural material for fusion application. This paper presents an overview of the current status of the European r&d activities on SiCSic 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. c 2000 Elsevier Science B V. All rights reserved Keywords: Status: European R&D activities; SIC / SIC composites; Fusion reactors 1. Introduction als(Lam) is fundamental as their use will reduce the risk related to accidents will facilitate mainte- The final goal of the Fusion Technology Pro- nance operations and will simplify decommission gramme is power generation under attractive eco- ing and waste management nomical and environmental conditions. In this Among LAMs, the SiCr Sic composites are the pursuit the use of structural low activation materi- ading candidates as structural material for fu- sion reactors due to their good mechanical prop- erties at high temperature, low chemical Corresponding author. Tel: +39-6-94005159: fax: +39. puttering, good resistance to oxidation at high 94005799 temperature(≤1000° and very low short and E-mail address: riccardia frascati. enea. it(B. Riccardi). medium term activation l] 0920-3796/00/S- see front matter c 2000 Elsevier Science B.v. All rights reserved. PI:s0920-3796(00)00311-2
Fusion Engineering and Design 51–52 (2000) 11–22 Status of the European R&D activities on SiCf /SiC composites for fusion reactors B. Riccardi a,*, P. Fenici b , A. Frias Rebelo b , L. Giancarli c , G. Le Marois d , E. Philippe e a Associazione EURATOM-ENEA CR FRASCATI, C.P.65-00044 Frascati, Rome, Italy b European Commission, JRC Ispra, Tp 202, 21020 Ispra (VA), Italy c CEA Saclay, DRN/DMT, F-91191 Gif-sur-Y6ette Cedex, France d CEA Grenoble, 17 rue des Martyrs,F-38054 Grenoble Cedex 9, France e SEP Di6ision of SNECMA, Le Haillan-BP 37-33165 St Medard en Jalles Cedex, France Abstract 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. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Status; European R&D activities; SICf /SIC composites; Fusion reactors www.elsevier.com/locate/fusengdes 1. Introduction 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 (51000°C) and very low short and medium term activation [1]. * Corresponding author. Tel.: +39-6-94005159; fax: +39- 6-94005799. E-mail address: riccardi@frascati.enea.it (B. Riccardi). 0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0920-3796(00)00311-2
B. Riccardi et al./ Fusion Engineering and Design 51-52(2000)11-22 The SiC SiC composites have been conceived ous design studies which differ on the assumed and developed mainly for aerospace applications safety strategy [2-4]. within the eu the TAURO The optimisation of such material for fusion blanket concept [4] has been proposed by the needs a strong coordination of R&D efforts Commissariat a l' Energie Atomique (CEA).In among research institutes, including participation this concept the chosen safety strategy is based on of industry through all the development stages the minimisation of the energy available within The development of more advanced fibres and the reactor so that the amount of radioactive the enhancement of composites processing meth- material released. in case of severe accident. can ods as well as alternative solutions for fibre -ma e limited. This passive safety concept, which has trix interphase lead to improved thermo- to be applied to all in-vessel components, leads to mechanical characteristics at elevated tempera the use of low pressure coolants with reduced tures. Nevertheless critical issues related to chemical reactivity in air. Liquid Pb-17Li is the nuclear environment are still present; these issues most promising candidate. The main objectives of are mainly connected to the fibres and matrix the TAuRo Study were to find a credible alterna- stability under neutron irradiation, to the poor tive to existing high-pressure He-cooled blanket thermal properties and to the residual porosity In designs, to develop models and design criteria parallel, technology issues must be addressed, e.g. joining methodology and definition of design adapted MC) structural materials. to determine the main issues In parallel to the manufacturing development for a self cooled Pb-17Li using SiC/SiC materials and materials characterisation. fusion reactors de- and to evaluate the limit for fusion application of sign studies using SiCr Sic composites as struc the existing industrial SiCr/SiC composites tural material have been undertaken. These The tauro design is based on the reactor studies, relying on currently available materials specifications defined for the SEAFP Study [5]: 3 data. provide also guidelines for improvements of Gw of fusion power, neutron and heat wall load- ceramic composites, ranking the priorities of fu- Ing of, respectively, 2 and 0.5 MW m-2 and 5 ture developments years of full power continuous operation. The The present overview reports the main achieve- SEAFP conceptual reactor has 6 toroidal field ments of the European R&D activities on Sic/ coils and 48 outboard and 32 inboard segments SiC composites. a description of the design study (about 10 m high) of a SiCr/Sic composites blanket which uses liq The tauRo blanket consists essentially of a uid lithium lead as coolant is discussed. advances SiCSic box containing the Pb-17Li which in manufacturing routes dealing with chemical works as coolant. tritium breeder. neutron multi vapour infiltration(CVI) and polymeric infiltra plier and tritium carrier. The maximum velocity tion and pyrolysis(PIP)are also presented includ- of Pb-17Li is about I m s-l in the channel ing recent results on joining and coating located just behind the Fw. The design is based techniques. Studies on radiation effects on ther- on the assumption, to be experimentally verified, mal conductivity and mechanical properties in that the Sic Sic has enough low electrical con- cluding irradiation creep and compatibility with uctivity to avoid large magneto-hydro-dynamic some solid breeders for long exposure time (MHD) induced pressure drops. Recent studies (10 000 h)are also reported carried out at JRC Ispra [6] indicate, for commer- cially available Sicr/Sic composites, a measured electrical conductivity ranging from 350(@2m) 2. Design at 200oC to 550(Q2m)at 1000C. Therefore, the item need further investigations taking into ac The use of SiC/SiC composite as structural count that the neutron irradiation tends to in- material for fusion power reactor has been pro- crease the electrical conductivity, being the design posed by different institutions by means of vari- reference value 500(Q2m)
12 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 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 thermomechanical 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. 2. Design The use of SiCf /SiC composite as structural material for fusion power reactor has been proposed by different institutions by means of various design studies which differ on the assumed safety strategy [2–4]. Within the EU the TAURO blanket concept [4] has been proposed by the Commissariat a l’Energie Atomique (CEA). In this concept the chosen safety strategy is based on the minimisation of the energy available within the reactor so that the amount of radioactive material released, in case of severe accident, can be limited. This passive safety concept, which has to be applied to all in-vessel components, leads to the use of low pressure coolants with reduced chemical reactivity in air. Liquid Pb–17Li is the most promising candidate. The main objectives of the TAURO Study were to find a credible alternative to existing high-pressure He-cooled blanket designs, to develop models and design criteria adapted to ceramic matrix composites (CMC) structural materials, to determine the main issues for a self cooled Pb–17Li using SiCf /SiC materials and to evaluate the limit for fusion application of the existing industrial SiCf /SiC composites. The TAURO design is based on the reactor specifications defined for the SEAFP Study [5]: 3 GW of fusion power, neutron and heat wall loading of, respectively, 2 and 0.5 MW m−2 and 5 years of full power continuous operation. The SEAFP conceptual reactor has 16 toroidal field coils and 48 outboard and 32 inboard segments (about 10 m high). The TAURO blanket consists essentially of a SiCf /SiC box containing the Pb–17Li which works as coolant, tritium breeder, neutron multiplier and tritium carrier. The maximum velocity of Pb–17Li is about1ms−1 in the channel located just behind the FW. The design is based on the assumption, to be experimentally verified, that the SiCf /SiC has enough low electrical conductivity to avoid large magneto-hydro-dynamic (MHD) induced pressure drops. Recent studies carried out at JRC Ispra [6] indicate, for commercially available SiCf /SiC composites, a measured electrical conductivity ranging from 350 (Vm)−1 at 200°C to 550 (Vm)−1 at 1000°C. Therefore, the item need further investigations taking into account that the neutron irradiation tends to increase the electrical conductivity, being the design reference value 500 (Vm)−1
B. Riccardi et al./ Fusion Engineering and Design 51-52(2000)11-22 The design activities have been focussed on the about 1.5 MPa). Stiffeners have also the functions blanket outboard segments. In the most recent of Pb-17Li flow separators design version [7 each outboard segment is di- Because of the low pressure of the coolant fle vided in the poloidal direction in four straight 2.5 FW thickness of 3 mm could be acceptable m high modules, attached on a common thick However, plasma erosion has to be taken into back plate but cooled independently(Fig. 1). Pre- account and the minimum thickness has still to be liminary estimation of the back plate thickness present at the submodule end of life. At this stage leads to the choice of 80 mm. The feasibility of of the design uncertainties are present concerning such a structure has not been analysed in detail the erosion rate of SiC/SiC and the acceptability but a possible solution could be the use of a of direct exposure of Sic/sic to the plasma multilayer plate of SiC/Sic or, because of the regarding the last point the use of protective lower neutron flow at the back plate location, the coatings or monolithic Sic armour could be use of composites with lower resistance to neutron envisaged dose such as the carbon-SiC composites. Each A preliminary fabrication sequence was deter modulus is divided in the toroidal direction in five mined for the TAURO submodule. The different submodules and each of them is supported by the basic components to be manufactured and then back plate and cooled in parallel through a com assembled are shown in Fig. 2. The joining tech mon top horizontal collector formed by two let nique require relatively large contact surfaces els. one for the inlet and one for the outlet flow possible technologies are textile assembling by stitching and co-infiltration during manufacturing module. Within each submodule. the Pb_ILi or brazing. The use of half finished product T flows, at first, poloidally downwards (U=lm )in a thin channel (1.25 cm thickness) located Joints and to improve the stiffness of the module 2-D neutronic analysis was performed by means just behind the FW (6 mm thickness), at the of Montecarlo code TRIPOLI 4, by using the bottom turns in a second channel and flows up, ENDBF-VI transport cross section library [8] then down and up again(at gradually reduced The analysis was aimed at evaluating the overall elocity down to 0.06 m s-) for entering in the performance of the blanket in terms of tritium outlet collector. Toroidal stiffeners plates are re breeding ratio TBR), power density deposition quired for reinforcing the submodule box in order and coolant temperature at the outlet. The results to enable it to withstand the Pb-17Li hydrostatic obtained have shown that the tauro blanket pressure (whose maximum estimated value with 90% enrichment Li can widely fulfil TBR and that lower enrichment can be envisaged. The power density deposition distribution has been used for successive thermal-mechanical analysis For the initial TaURo blanket version [4, the design criteria used were those defined in the ARIES reactor studies [2]. More more severe criteria has recently be defined based mental results on 2D-CERASEPs N2-1 composite [9]. These new criteria take into ac count the orthotropic characteristics of the com- osite and do not distinguish between primary (mechanical) and secondary (thermal)stresses. The assumed limits, which depends on the specific composite and, therefore, are likely to change when more advanced composites will be evalu- Fig. I. The TAURO blanket concept. ated. are 145 MPa for the von mises stresses in
B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 13 The design activities have been focussed on the blanket outboard segments. In the most recent design version [7] each outboard segment is divided in the poloidal direction in four straight 2.5 m high modules, attached on a common thick back plate but cooled independently (Fig. 1). Preliminary estimation of the back plate thickness leads to the choice of 80 mm. The feasibility of such a structure has not been analysed in detail but a possible solution could be the use of a multilayer plate of SiC/SiC or, because of the lower neutron flow at the back plate location, the use of composites with lower resistance to neutron dose such as the carbon–SiC composites. Each modulus is divided in the toroidal direction in five submodules and each of them is supported by the back plate and cooled in parallel through a common top horizontal collector formed by two levels, one for the inlet and one for the outlet flow (Fig. 1). The feeding pipes are located behind the module. Within each submodule, the Pb–17Li flows, at first, poloidally downwards (6=1 m s−1 ) in a thin channel (1.25 cm thickness) located just behind the FW (6 mm thickness), at the bottom turns in a second channel and flows up, then down and up again (at gradually reduced velocity down to 0.06 m s−1 ) for entering in the outlet collector. Toroidal stiffeners plates are required for reinforcing the submodule box in order to enable it to withstand the Pb–17Li hydrostatic pressure (whose maximum estimated value is about 1.5 MPa). Stiffeners have also the functions of Pb–17Li flow separators. Because of the low pressure of the coolant flow, a FW thickness of 3 mm could be acceptable. However, plasma erosion has to be taken into account and the minimum thickness has still to be present at the submodule end of life. At this stage of the design uncertainties are present concerning the erosion rate of SiC/SiC and the acceptability of direct exposure of SiC/SiC to the plasma: regarding the last point the use of protective coatings or monolithic SiC armour could be envisaged. A preliminary fabrication sequence was determined for the TAURO submodule. The different basic components to be manufactured and then assembled are shown in Fig. 2. The joining technique require relatively large contact surfaces: possible technologies are textile assembling by stitching and co-infiltration during manufacturing or brazing. The use of half finished product (T and L shape) is useful to reduce the number of joints and to improve the stiffness of the module. 2-D neutronic analysis was performed by means of Montecarlo code TRIPOLI 4, by using the ENDBF-VI transport cross section library [8]. The analysis was aimed at evaluating the overall performance of the blanket in terms of tritium breeding ratio (TBR), power density deposition and coolant temperature at the outlet. The results obtained have shown that the TAURO blanket, with 90% enrichment 6 Li can widely fulfil TBR and that lower enrichment can be envisaged. The power density deposition distribution has been used for successive thermal-mechanical analysis. For the initial TAURO blanket version [4], the design criteria used were those defined in the ARIES reactor studies [2]. More realistic and more severe criteria has recently be defined based on experimental results on 2D-CERASEP® N2-1 composite [9]. These new criteria take into account the orthotropic characteristics of the composite and do not distinguish between primary (mechanical) and secondary (thermal) stresses. The assumed limits, which depends on the specific composite and, therefore, are likely to change when more advanced composites will be evaluFig. 1. The TAURO blanket concept. ated, are 145 MPa for the von Mises stresses in
B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 DETAIL C二 Fig. 2. TAURO blanket exploded view the plane and 110 MPa for the stresses through cific component with fibre architecture. The most the thickness. The thermo-mechanical analysis has widely studied and used fibre are the been performed by using the above new criteria NICALONTM and an elastic behavioural model for the com Since the beginning of 1990s the SEP Division posite. A behavioural model capable of simulating of SNECMA has been involved with the manu- the non linear stress-strain relationship and of facturing of Sic/Sic composites for fusion power predicting the damage status of the composite is reactors [ll]. A standard 2-D composite named under development [9]. Moreover the data used CERASEP N2-1 was used to carry out the initial for the design calculation are related to the sep evaluation work at the start of the European CERASEP N3-I with the further assumption of programme and demonstrate the an improved thermal conductivity in the thickness such material. One of the characteristics of this direction(15W mK ) The calculated maxi- particular SiC/SiC composite was its two dimen- mum shear in the joints between the sub module sional strengthening feature, achieved using a fab- side wall and bottom cup is 60 MPa: this value is ric of NiCaloN CG fibre (with about 12% the limiting design parameter for the join xygen) and by increasing the density of the pre strength. A recently performed parametric study form by a SiC CVI matrix. Due to the geometric has shown that with a FW-thickness of 3 mm and complexity of the parts making up the TauRo a module height of about 80 cm a FW surface blanket and in order to improve the material's heat flux between 0. 7 and I MW m could be shear related properties, CERASEP N3-1 was tolerated by the tauRo design [10] subsequently developed. This material, also pro- duced with NICaloNTM CG fibres offered an innovative 3-D strengthening feature: the 3. Manufacturing GUIPEX texture. CERASEP N2-1 and N3-1 materials offered fairly similar mechanical and Today's industry has a large installed capa hermal properties (Table 1) but the main advan- for full scale production of Sic SiC comp tages of the 3-D material are as follows: improved parts. Moreover the industry is capable of thermal conductivity in the Z direction, increased neerability' of such materials by optimising a spe and more consistent interlaminar shear failure
14 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 Fig. 2. TAURO blanket exploded view. the plane and 110 MPa for the stresses through the thickness. The thermo-mechanical analysis has been performed by using the above new criteria and an elastic behavioural model for the composite. A behavioural model capable of simulating the non linear stress–strain relationship and of predicting the damage status of the composite is under development [9]. Moreover the data used for the design calculation are related to the SEP CERASEP® N3-1 with the further assumption of an improved thermal conductivity in the thickness direction (15 W m−1 K−1 ). The calculated maximum shear in the joints between the sub module side wall and bottom cup is 60 MPa: this value is the limiting design parameter for the joint strength. A recently performed parametric study has shown that with a FW-thickness of 3 mm and a module height of about 80 cm a FW surface heat flux between 0.7 and 1 MW m−2 could be tolerated by the TAURO design [10]. 3. Manufacturing Today’s industry has a large installed capacity for full scale production of SiCf /SiC composites parts. Moreover the industry is capable of ‘engineerability’ of such materials by optimising a specific component with fibre architecture. The most widely studied and used fibre are the NICALON™. Since the beginning of 1990s the SEP Division of SNECMA has been involved with the manufacturing of SiC/SiC composites for fusion power reactors [11]. A standard 2-D composite named CERASEP® N2-1 was used to carry out the initial evaluation work at the start of the European programme and demonstrate the capability of such material. One of the characteristics of this particular SiC/SiC composite was its two dimensional strengthening feature, achieved using a fabric of NICALON™ CG fibre (with about 12% oxygen) and by increasing the density of the preform by a SiC CVI matrix. Due to the geometric complexity of the parts making up the TAURO blanket and in order to improve the material’s shear related properties, CERASEP® N3-1 was subsequently developed. This material, also produced with NICALON™ CG fibres, offered an innovative 3-D strengthening feature: the GUIPEX® texture. CERASEP® N2-1 and N3-1 materials offered fairly similar mechanical and thermal properties (Table 1) but the main advantages of the 3-D material are as follows: improved thermal conductivity in the Z direction, increased and more consistent interlaminar shear failure
B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 stress with the absence of interlaminar delamina en and the increase of the matrix crystallinity tion during manufacturing and use, and lower and properties. Recently, the use of an hybrid dispersion of the shear strength process combining CVI and PIp processes al CERASEP N3-1 features can be improved by lowed the fabrication of composites with relevant high purity SiC fibres virtually stoichiomet thicknesses(6 and 10 mm) with an intermediate ric and oxygen freeand with thermal conductiv- matrix structure [13]. Flat panels were produced ity intrinsically higher than that of the with different duration of CVI process and num- NICALONTM CG. Benefits with respect neutron ber of PIP cycles. The maximum density for 10 irradiation induced change in the fibre properties mm thick panels was reached at 60 h of CvI and are expected. Other positive aspects related to the seven PIP cycles( 2.05 g cm-2) use of such fibres are: an increasing in the maxi mum operating temperature from 1100 to 1300oC and improvements in mechanical properties 4. radiation In parallel with Sic, SiC composite manufac turing by the CVI technique, alternative matrix Neutron and particle irradiation induces dis- processing routes are going to be investigated, in placement damage and helium production in SiC particular polymer infiltration and pyrolisis(Pip) based materials, In order to assess the effect of [12]. This technology is less expensive than CVI, irradiation damage and He production on the can be carried out at lower temperature and al- mechanical properties of Sic /SiC structural com- lows the manufacturing of more complex shapes posites a testing campaign was carried out at the ENEA has extensively used preceramic polymers Institute of Advanced Materials/Joint Research with SiC nanopowders. This composites exhibited Centre, Ispra-Italy up to 1998. In particular the interesting mechanical properties in particular material tested was produced by the SEP Division high strain to failure and toughness. As all com- of SNECMA using a 2-D woven laminate of posites produced by PIP, the material exhibits NICALONTM fibres and a CVI infiltration of lower thermal conductivity, poor crystallinity and B-SiC matrix. The neutron irradiation of SiC/ SiC high residual oxygen content. The availability specimens was carried out at the high flux reactor more advanced fibres allowing higher pyrolysis (HFR), Petten-NL up to accumulated damage temperature will permit the reduction of free oxy- doses of 1. 29, 2.69 and 5.23 dpa at 750C irradia- Table I SEP SiCSiC composites main properties roperty Temperature CERASEP N2-1 CERASEP N3. (2-D) 2.5 2.4 10±2 Fibre content (%) Tensile strength(in plane)(MPa) ile strain (in plane)(%) ung module Trans-laminar shear strength Inter-laminar shear strength(MPa) 00-0000000 0.8±0.25 200 200±20 200 MPa 200+20GPa Thermal conductivity (in plane)(Wm-K-) 15 Thermal conductivity(trough the thickness)(W m-K 3±2 Thermal conductivity(trough the thickness)(W m-K Thermal conductivity(trough the thickness)(W m-K Thermal expansion coefficient (in plane)(K-) 4.0×10=6 4.0×10 Thermal expansion coefficient(trough the thickness)(K-) 20 2.5×10
B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 15 stress with the absence of interlaminar delamination during manufacturing and use, and lower dispersion of the shear strength. CERASEP® N3-1 features can be improved by using high purity SiC fibres virtually stoichiometric and ‘oxygen free’ and with thermal conductivity intrinsically higher than that of the NICALON™ CG. Benefits with respect neutron irradiation induced change in the fibre properties are expected. Other positive aspects related to the use of such fibres are: an increasing in the maximum operating temperature from 1100 to 1300°C and improvements in mechanical properties. In parallel with SiCf /SiC composite manufacturing by the CVI technique, alternative matrix processing routes are going to be investigated, in particular polymer infiltration and pyrolisis (PIP) [12]. This technology is less expensive than CVI, can be carried out at lower temperature and allows the manufacturing of more complex shapes. ENEA has extensively used preceramic polymers with SiC nanopowders. This composites exhibited interesting mechanical properties in particular high strain to failure and toughness. As all composites produced by PIP, the material exhibits lower thermal conductivity, poor crystallinity and high residual oxygen content. The availability of more advanced fibres allowing higher pyrolysis temperature will permit the reduction of free oxygen and the increase of the matrix crystallinity and properties. Recently, the use of an hybrid process combining CVI and PIP processes allowed the fabrication of composites with relevant thicknesses (6 and 10 mm) with an intermediate matrix structure [13]. Flat panels were produced with different duration of CVI process and number of PIP cycles. The maximum density for 10 mm thick panels was reached at 60 h of CVI and seven PIP cycles ( 2.05 g cm−2 ). 4. Irradiation Neutron and particle irradiation induces displacement damage and helium production in SiCbased materials. In order to assess the effect of irradiation damage and He production on the mechanical properties of SiCf /SiC structural composites a testing campaign was carried out at the Institute of Advanced Materials/Joint Research Centre, Ispra-Italy up to 1998. In particular the material tested was produced by the SEP Division of SNECMA using a 2-D woven laminate of NICALON™ fibres and a CVI infiltration of b-SiC matrix. The neutron irradiation of SiCf /SiC specimens was carried out at the high flux reactor (HFR), Petten-NL up to accumulated damage doses of 1.29, 2.69 and 5.23 dpa at 750°C irradiaTable 1 SEP SiCf /SiC composites main properties Property Temperature CERASEP CERASEP ® N3-1 ® N2-1 (°C) (3-D) (2-D) Density (g/cm 20 2.5 3 ) \2.4 Porosity (%) 20 10 1092 Fibre content (%) – 40 40 Tensile strength (in plane) (MPa) 300 20 285 920 Tensile strain (in plane) (%) 0.75 0.8 20 90.25 Young modulus (in plane) (GPa) 200 20 200920 Trans-laminar shear strength 20 200 MPa 200920 GPa Inter-laminar shear strength (MPa) 44 20 – Thermal conductivity (in plane) (W m 1000 15 15 −1 K−1 ) Thermal conductivity (trough the thickness) (W m 20 9 1392 −1 K−1 ) Thermal conductivity (trough the thickness) (W m 5.8 7.6 −1 K−1 ) 800 Thermal conductivity (trough the thickness) (W m 1000 5.7 7.5 −1 K−1 ) 20 4.0×10−6 Thermal expansion coefficient (in plane) (K−1 ) 4.0×10−6 2.5×10 – −6 Thermal expansion coefficient (trough the thickness) (K−1 ) 20