L Giancarli et al Fusion Engineering and Design 61-62(2002)307-318 with the potential for high energy conversion neutron multiplier and breeder, respectively, and efficiency(>50%) SiCSiC for joints(e.g. bolts). The difficulty in this Starting from the different strategies which can case is to keep the same low-activation require- be adopted for FPR safety and from the r&d ment for all the other in-vessel components next section, this paper presents an assessment of reactivity, low afterheat materials should be used the most recent proposals of breeding blanket This strategy has been adopted by the TauRo designs with particular focus on fabrication issues. and ARIES-AT blanket designs, which use low In particular, two self-cooled lithium-lead (SCLL) pressure, low reactivity, low afterheat Pb-17Li as blanket designs, ARIES-AT [ and TAURO [2]. coolant, neutron multiplier and breeder. The and one helium-cooled be/ceramic(HCBC)design, difficulty in this case is to fulfill the same require- DREAM 33], will be considered ments for the other in-vessel components which imply for instance the use Pb-17Li(or equivalent) as coolant for divertor and shield 2. Attractiveness and development risks for SiCr Sic structures 2. 2. High plant efficiency The attractiveness of Fpr breeding blankets using SiCSic structures is based on the achiev- Maximum acceptable working temperature of able high safety standards and high plant effi- SiC/Sic under irradiation is about 1000C. de- ciency. These significant advantages of SiC!Sic have been developed with the aim of exploiting this favorable feature for having high compared to other structural materials can be fully coolant outlet temperature and, as a consequence, exploited by Dy making coherent design choices concerning the other materials required in the high overall plant efficiency. Moreover, high blanket temperature coolant gives the potential of an efficient hydrogen production in combination 2.1. High safety standards with the standard electricity production. The three designs considered in this paper have High safety standards can be potentially aid particular attention to this aspect. In part achieved because of the low short term activation cular, He-coolant outlet temperature in DREAM blanket is about 900C leading to a net thermal and decay heat which minimize accidental releases, efficiency greater than 45%. For the Tauro facilitates the accommodation of loss-of-coolant (LOCA)and loss-of-flow (LOFA)events, and blanket the Pb-17Li parameters have been opti- simplifies maintenance procedure mized in order to reach an outlet temperature of In particular, in order to limit to an acceptable about 950C and a corresponding net thermal level the accidental release of activation products efficiency of about 55%. In case of ARIES-At the two different strategies can be envisaged (4), that choice of having an annular Pb-17Li flow allows is. either to minimize the in-vessel overall activa to reach an outlet temperature of about 1100C tion inventory and control the release, or to leading to a net thermal efficiency as high as 58.5% minimize the available energy within the safety vessel and keep the activation products confined In the first case, all materials present within the 2.3. R&d requirements and development risks vessel should have low activation characteristics and for the SiCsic a minimization of the Present-day SiCASic composites are not ade- impurity contents should be pursued. This strategy quate to be used directly as structure of nuclear has been adopted by the dream blanket design, components. A comparison between measured which uses only low-activation materials, such as properties on present-day Sic!SiC and require- high-pressure He as coolant, and Be and Li2O ments are given in Table 1. In fact, there are somewith the potential for high energy conversion efficiency (
/50%). Starting from the different strategies which can be adopted for FPR safety and from the R&D needs for SiCf/SiC structures, summarized in the next section, this paper presents an assessment of the most recent proposals of breeding blanket designs with particular focus on fabrication issues. In particular, two self-cooled lithium/lead (SCLL) blanket designs, ARIES-AT [1] and TAURO [2], and one helium-cooled be/ceramic (HCBC) design, DREAM [3], will be considered. 2. Attractiveness and development risks for SiCf/ SiC structures The attractiveness of FPR breeding blankets using SiCf/SiC structures is based on the achiev￾able high safety standards and high plant effi￾ciency. These significant advantages of SiCf/SiC compared to other structural materials can be fully exploited by making coherent design choices concerning the other materials required in the blanket. 2.1. High safety standards High safety standards can be potentially achieved because of the low short term activation and decay heat which minimize accidental releases, facilitates the accommodation of loss-of-coolant (LOCA) and loss-of-flow (LOFA) events, and simplifies maintenance procedure. In particular, in order to limit to an acceptable level the accidental release of activation products, two different strategies can be envisaged [4], that is, either to minimize the in-vessel overall activa￾tion inventory and control the release, or to minimize the available energy within the safety vessel and keep the activation products confined. In the first case, all materials present within the vessel should have low activation characteristics and for the SiCf/SiC a minimization of the impurity contents should be pursued. This strategy has been adopted by the DREAM blanket design, which uses only low-activation materials, such as high-pressure He as coolant, and Be and Li2O as neutron multiplier and breeder, respectively, and SiCf/SiC for joints (e.g. bolts). The difficulty in this case is to keep the same low-activation require￾ment for all the other in-vessel components. In the second case, only low pressure, low reactivity, low afterheat materials should be used. This strategy has been adopted by the TAURO and ARIES-AT blanket designs, which use low pressure, low reactivity, low afterheat Pb/17Li as coolant, neutron multiplier and breeder. The difficulty in this case is to fulfill the same require￾ments for the other in-vessel components which imply for instance the use Pb/17Li (or equivalent) as coolant for divertor and shield. 2.2. High plant efficiency Maximum acceptable working temperature of SiCf/SiC under irradiation is about 1000 8C. De￾signs have been developed with the aim of exploiting this favorable feature for having high coolant outlet temperature and, as a consequence, high overall plant efficiency. Moreover, high temperature coolant gives the potential of an efficient hydrogen production in combination with the standard electricity production. The three designs considered in this paper have paid particular attention to this aspect. In parti￾cular, He-coolant outlet temperature in DREAM blanket is about 900 8C leading to a net thermal efficiency greater than 45%. For the TAURO blanket the Pb/17Li parameters have been opti￾mized in order to reach an outlet temperature of about 950 8C and a corresponding net thermal efficiency of about 55%. In case of ARIES-AT the choice of having an annular Pb/17Li flow allows to reach an outlet temperature of about 1100 8C leading to a net thermal efficiency as high as 58.5%. 2.3. R&D requirements and development risks Present-day SiCf/SiC composites are not ade￾quate to be used directly as structure of nuclear components. A comparison between measured properties on present-day SiCf/SiC and require￾ments are given in Table 1. In fact, there are some 308 L. Giancarli et al. / Fusion Engineering and Design 61/62 (2002) 307/318