RH. Jones et al. /Jounal of Nuclear Materials 307-311(2002)1057-1072 Kefr=Km(Kr/Km-1-Kr/ah)Vr 63. Transmutation rates (1+Kr/Km+Kr/ah) x[(1-Kr/Km+Kr/ah)Ve Transmutation calculations were performed 1 +(1+Kr/Km+ Kr/ah) () REAC-3 code for pure SiC irradiated in the spectrum of the first wall of the ARIES-IV co usion energy device. The ARIES-IV first wall has a total neutron flux of 3. 6x 10 s n/cm s and a fast flux K are the thermal conductivity values of the matrix and (E>0.1 Mev) of 1.9 x 10 n/cms. Calculations were fiber constituents; and Vr and a are the fiber volume performed for a continuous irradiation of up to 12 ef- fraction and radius, respectively fective full power years(efpy) to a total neutron dose of For a woven 2D-SiC /SiC composite, the localized 1. 37 x 1024 n/cm. The elemental composition of the effects of dense fiber packing within individual tows material is not significantly affected by the post-irradi- (<0.6) and the occurrence of many direct fiber-fiber ation decay of radioactive isotopes, since they are either contacts at the numerous fiber bundle crossover points too short-lived or too long-lived to affect the composi will introduce positive deviations from Eq. (1). How- tion over a reactor lifetim ever, the analytic solution expressed by eq. () should be very appropriate to examine thermal conductivity 6.3.1. Sic burn-out degradation induced in these composites by neutron In this neutron spectrum, Si burns out somewhat radiation or by other mechanical or environmental more rapidly than C, Fig 9. Si burns out at a rate of treatments. If a 2D-SiCrSiC composite with initially about 0.0047/efpy. a 3% burnout of Sic has been sug high Kr- and h-values were irradiated, Ker could easily gested as a design limit for SiC composites In ARIES- be reduced by a factor of five or six due to the degra T, a 3% burnout occurs in about 6.5 efpy(total neutron dation of the interface conductance and the matrix fluence 7. 4 x 102n/cm2). The differing burnout rates of conductivity. Si and c result in an excess concentration of carbon To further examine this issue, the effects of temper- totaling about 3500 appm after 6.5 efpy. The Si and C ature and irradiation on Kefr were predicted for a hy- burnout rates are constant, and the excess C increases at pothetical 2D-SiCr/SiC composite made with high the rate of 540 appm excess Clefpy conductivity Tyranno SA fiber, a thin(0. 2-um) PyC fiber coating and a CVI-SiC matrix. For example, it was 6.3.2. Impurity burn-in predicted for this composite that Kefr would decrease The most abundant transmutation products, which from 34 W/mK before irradiation to <6 w/m k(at 200 ourn-in at constant rates, Fig. 10, are listed in table 3. C) after irradiation at 200 C. Similarly, Ker would After 6.5 efpy, although 3% of the Sic burns out, the decrease from 26 W/m K before irradiation to <10 W/ concentration of transmutant atoms totals almost 8% mk(at 1000C)after irradiation at 1000C This is because many transmutation reactions create 100 099 098 3% SiC burnout at 6.5 efp 095 Irradiation Time, efpy Fig. 9. The burnout of Si and C in Sic irradiated in ARIES-IV first wall as a function of dose in efpKeff ¼ Km½ðKf=Km 1 Kf=ahÞVf þ ð1 þ Kf=Km þ Kf=ahÞ  ½ð1 Kf=Km þ Kf=ahÞVf þ ð1 þ Kf=Km þ Kf=ahÞ 1 ; ð1Þ where h is the effective interfacial conductance; Km and Kf are the thermal conductivity values of the matrix and fiber constituents; and Vf and a are the fiber volume fraction and radius, respectively. For a woven 2D-SiCf/SiC composite, the localized effects of dense fiber packing within individual tows (f 6 0:6) and the occurrence of many direct fiber–fiber contacts at the numerous fiber bundle crossover points will introduce positive deviations from Eq. (1). How￾ever, the analytic solution expressed by Eq. (1) should be very appropriate to examine thermal conductivity degradation induced in these composites by neutron radiation or by other mechanical or environmental treatments. If a 2D-SiCf /SiC composite with initially high Kf- and h-values were irradiated, Keff could easily be reduced by a factor of five or six due to the degra￾dation of the interface conductance and the matrix conductivity. To further examine this issue, the effects of temper￾ature and irradiation on Keff were predicted for a hy￾pothetical 2D-SiCf/SiC composite made with high conductivity Tyranno SAe fiber, a thin (0.2-lm) PyC fiber coating and a CVI-SiC matrix. For example, it was predicted for this composite that Keff would decrease from 34 W/m K before irradiation to <6 W/m K (at 200
C) after irradiation at 200
C. Similarly, Keff would decrease from 26 W/m K before irradiation to <10 W/ m K (at 1000
C) after irradiation at 1000
C. 6.3. Transmutation rates Transmutation calculations were performed using the REAC-3 code for pure SiC irradiated in the neutron spectrum of the first wall of the ARIES-IV conceptual fusion energy device. The ARIES-IV first wall has a total neutron flux of 3:6  1015 n/cm2 s and a fast flux (E > 0:1 MeV) of 1:9  1015 n/cm2 s. Calculations were performed for a continuous irradiation of up to 12 ef￾fective full power years (efpy) to a total neutron dose of 1:37  1024 n/cm2. The elemental composition of the material is not significantly affected by the post-irradi￾ation decay of radioactive isotopes, since they are either too short-lived or too long-lived to affect the composi￾tion over a reactor lifetime. 6.3.1. SiC burn-out In this neutron spectrum, Si burns out somewhat more rapidly than C, Fig. 9. Si burns out at a rate of about 0.0047/efpy. A 3% burnout of SiC has been sug￾gested as a design limit for SiC composites. In ARIES￾IV, a 3% burnout occurs in about 6.5 efpy (total neutron fluence 7:4  1023 n/cm2). The differing burnout rates of Si and C result in an excess concentration of carbon totaling about 3500 appm after 6.5 efpy. The Si and C burnout rates are constant, and the excess C increases at the rate of 540 appm excess C/efpy. 6.3.2. Impurity burn-in The most abundant transmutation products, which burn-in at constant rates, Fig. 10, are listed in Table 3. After 6.5 efpy, although 3% of the SiC burns out, the concentration of transmutant atoms totals almost 8%. This is because many transmutation reactions create Fig. 9. The burnout of Si and C in SiC irradiated in ARIES-IV first wall as a function of dose in efpy. R.H. Jones et al. / Journal of Nuclear Materials 307–311 (2002) 1057–1072 1065