There have been several studies on the effect of neu­tron irradiation on the strength of various types of SiC forms including reaction-bonded, sintered, pressure­less sintered, and CVD SiC materials.1’11’13’14’58’60-65

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Figure 13 Irradiation temperature dependence of irradiated elastic modulus of CVD SiC, at ambient temperature, normalized to unirradiated values. The error bars are showing standard deviations for all the neutron data points and ranges of data scatter for the ion data points. Reproduced from Snead, L. L.; Nozawa, T.; Katoh, Y.; Byun, T-S.; Kondo, S.; Petti, D. A. J. Nucl. Mater. 2007, 371, 329-377.

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Figure 14 Irradiation-induced change of elastic modulus versus swelling of CVD SiC. An estimation of the influence of lattice relaxation on elastic modulus is calculated using Tersoff potential. Reproduced from Snead, L. L.; Nozawa, T.; Katoh, Y.; Byun, T-S.; Kondo, S.; Petti, D. A. J. Nucl. Mater. 2007, 371, 329-377.

Si for reaction-bonded SiC, which typically segregate to grain boundaries during sintering, tends to have a significant influence on strength under neutron irradiation. For the case of sintered SiC with boron
compounds as sintering additives, the reaction of 10B(n, a)7Li causes the accumulation of helium bub­bles at and near the grain boundary phases under neutron irradiation.60-63 In contrast, unmatched

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Figure 17 Fluence dependence of irradiated flexural strength of hot-pressed and sintered SiC normalized to unirradiated strength. Irradiation is variable but in the saturable swelling regime. Reproduced from Newsome, G. A.; Snead, L. L.;

Hinoki, T.; Katoh, Y.; Peters, D. J. Nucl. Mater. 2007, 371, 76-89.

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Figure 18 Flexural strength of CVD SiC at ambient temperature as a function of irradiation dose. Reproduced from Newsome, G. A.; Snead, L. L.; Hinoki, T.; Katoh, Y.; Peters, D. J. Nucl. Mater. 2007, 371, 76-89.

swelling between Si and SiC for reaction-bonded SiC causes disruption at the grain boundary, severely reducing the strength.11,19,60-64 Meanwhile, the high — purity materials such as CVD SiC exhibit superior irradiation resistance.

The irradiation effect on flexural strength of Rohm and Haas CVD SiC in a fluence range of 0.15-30 dpa is summarized in Figure 18. In comparing Figure 18 with Figure 17, it is clear that CVD SiC retains stability in strength to a much higher dose than the sintered and reaction-bonded forms of SiC. It is to be noted that in Figure 18, the data of Dienst does indicate a significant as-irradiated degradation in strength around 15 dpa. However, such degradation is not seen for the ^30 dpa irradiation of Snead. It is speculated that the degradation in the Dienst data may
have been due to statistical limitations of the study and/or due to issues with sample handling postirradi­ation. This issue is discussed in the Dienst reference.65 A compilation of strength data as a function of irradia­tion temperature is given in Figure 18, indicating no apparent correlation for the dose and temperature ranges studied. However, as with the fracture tough­ness data, irradiation-induced strengthening seems to be significant at 573-1273 K. The large scatter in flex­ural strength of brittle ceramics is inevitable, as the fracture strength is determined by the effective frac­ture toughness, morphology, and characteristics of the flaw that caused the fracture. Irradiation possibly modifies both the flaw characteristics and the fracture toughness through potential surface modification, relaxation of the machining-induced local stress, modifications of elastic properties, and fracture energy.

A typical means of describing the failure of cera­mics is through the use of Weibull statistics, which is a departure from the analysis of data that is assumed to follow a normal Gaussian distribution. In the two — parameter Weibull formalism, sometimes referred to as a weakest-link treatment, the failure probability F is described as

F(x) = 1 — e-(x"/x0)

where m is the Weibull modulus and xo is the distribution size parameter. A change in the Weibull statistics, indicating a higher scatter in as-irradiated flexural strength has been observed by previous authors, although the point could not be made convincingly because of limitations in the number of tests observed. In the earliest work known to the authors, Sheldon66 noted a 14% decrease in crushing strength of highly irradiated CVD SiC shells with an increase of the coefficient of variation from 8% to 14%. Price63 went on to a 4-point bend test using relatively thin (~0.6 mm) strips of CVD SiC deposited onto a graphite substrate. In his work, the flexural strength following a ^9.4 x 1025nm-2 (E > 0.1 MeV) irradiation was unchanged within the statistical scatter, but the scatter itself increased from about 10 to 30% of the mean flexural strength as described assuming a normal distribution. Unfortu­nately, there were not sufficient samples in Price’s work to infer Weibull parameters. In more recent work by Dienst,65 the Weibull modulus was reported to decrease from about 10 for irradiation of ~1 x 1026 nm — (E > 0.1 MeV). However, it is worth noting that the Dienst work used a very limited sample population (about 10 bars.)

Statistically meaningful data sets on the effect of flexural strength of CVD SiC have been re­ported by Newsome and coworkers14 and Katoh and coworkers.58,67 Figure 19 shows a compilation Weibull plot of the flexural strength of unirradiated and irra­diated Rohm and Haas CVD SiC taken from the two separate irradiation experiments carried out by New­some and cowokers14 and Katoh and coworkers.58,67 The sample population was in excess of 30 for each case. In Figure 19(a), the data was arranged by irradia­tion temperature, including data for unirradiated samples and 1.5—4.6 x 1026nm-2 (E > 0.1 MeV) dose range. It is likely that the Weibull modulus decreased by irradiation, appearing to be dependent on irradia­tion temperature. This is not easily visualized through inspection of Figure 19(a) unless one notes that there are significantly more low stress fractures populating the 573 K population. The scale parameters of flexural strength of unirradiated materials and materials irra­diated at 573, 773, and 1073 Kwere 450, 618, 578, and 592 MPa, respectively. The Weibull modulus of the flexural strength of unirradiated materials and materi­als irradiated at 573, 773, and 1073 Kwere 9.6, 6.2, 5.5, and 8.7, respectively, with significant uncertainty.

The work of Katoh, on identical material irra­diated at the same temperature as in the Newsome work, is at a slightly higher irradiation dose than the data of Newsome. As seen in Figure 19(b), the effect on the Weibull modulus undergoes a trend similar to that of Newsome, although the modulus for the 773 K and 1073 K irradiation of Katoh remained almost unchanged. Given the data discussed on the effect of irradiation on the Weibull modulus and scale parameter of CVD SiC bend bars, it is clear that the material is somewhat strengthened and that the Weibull modulus may undergo irradiation-induced change, with the greatest decrease occurring for the lowest temperature irradiation.

The fracture strength and failure statistics of tubular SiC ‘TRISO surrogates’ have been deter­mined by the internal pressurization test and the results are plotted in Figure 20. Thin-walled tubular SiC specimens of 1.22 mm outer diameter, 0.1 mm wall thickness, and 5.8 mm length were produced by the fluidized-bed technique alongside TRISO fuels.68 The specimens were irradiated in the HFIR to 1.9 and 4.2 x 1025nm-2 (E > 0.1 MeV) at 1293 and 1553 K. In the internal pressurization test, tensile hoop stress was induced in the wall of the tubular specimens by compressively loading a polyurethane insert.68,69

In Figure 20, Weibull plots of the flexural strength and internal pressurization fracture strength

of unirradiated and irradiated samples are presented. As with the Newsome and Katoh data, the sample population is large enough to be considered statistically meaningful. From this data, the mean fracture stress of tubular specimens is seen to increase to 337 MPa (from 297 MPa) and the Weibull modulus slightly decreased to 3.9 (from 6.9) after irradiation to 1.9 x 1025nm~2 (E> 0.1 MeV) dpa at 1293 K. The mean fracture stresses and Weibull moduli at 4.2 x 1025nm~2 (E> 0.1 MeV) were similar to those at 1.9 dpa. The results for 4.2 dpa irradiation indicate

that by increasing the irradiation temperature from 1293 to 1553 K, no discernible change in fracture stress distribution occurred. The horizontal shift indicates a simple toughening or an increase in frac­ture toughness alone. While the data for these surro­gate TRISO samples, irradiated through internal compression, are somewhat limited, the findings indi­cate that the trend in strength and statistics of failure are consistent with those found for the bend bars. Therefore, the general findings of the bend bar irra­diation on strength and Weibull modulus appear

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appropriate for application to TRISO fuel model­ing. Specifically, a slight increase in the mean strength is expected (although it may be less signifi­cant at higher temperatures), and the statistical spread of the fracture data as described by the Weibull modulus may broaden. Unfortunately, a precise description of how the Weibull modulus trends with irradiation dose and temperature is not yet possible, although within the dose range and temperature covered by the data in Figures 19 and 20, a modest reduction is possible.