Development of a Coated Fuel Particle Failure Model under High Burnup Irradiation

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1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 33, No. 9, p (September 1996) Development of a Coated Fuel Particle Failure Model under High Burnup Irradiation Kazuhiro SAWAt, Shusaku SHIOZAWA, Department of HTTR Project, Oarai Research Establishment, Japan Atomic Energy Research Institute * Kazuo MINATO and Kousaku FUKUDA Department of Chemistry and Fuel Research, Tokai Research Establishment, Japan Atomic Energy Research Institute** (Received April 22, 1996) In high temperature gas-cooled reactors (HTGRs), coated particles are used as fuels. For upgrading HTGR technologies, Japan Atomic Energy Research Institute has been developing high burnup Tri-isotropic (TRISO) coated fuel particles. The TRISO coatings consist of a low-density, porous pyrolytic carbon (PyC) buffer layer adjacent to the spherical fuel kernel, follwed by an isotropic PyC layer, a SiC layer and a final PyC layer. In safety design of HTGR fuels, it is important to retain fission products within the particles so that their release to primary coolant does not exceed an acceptable level. Therefore the basic design criteria for the fuel are to minimize the failure fraction of as-fabricated fuel coating layers and to prevent significant additional fuel failures during operation. In the high burnup coated fuel particle, stress due to fission gas pressure and irradiation-induced PyC shrinkage is introduced into the coating layers and consequently the stress could cause failure of coating layers under high burnup irradiation condition. Some models have been developed to evaluate failure fraction of coated fuel particles, however, they are regarded as a guideline for fuel particle design rather than as a predictive tool for coated fuel particle performance. Then the behavior of coated fuel particles has been examined only by many irradiation experiments and development of reliable model has been needed. A failure model is newly developed to predict failure fraction of TRISO-coated particle under high burnup irradiation. In the model, it is assumed that the failure fraction depends not only on failure of the SiC layer but also on that of the PyC layers. The failure fractions of through-coatings failed particles and the SiC-failed particles are calculated based on the failure probability of each coating layer. Based on the model developed here, parameter calculations for the first-loading-fuel of the High Temperature Engineering Test Reactor and for the high burnup fuel were carried out. KEYWORDS: HTGR type reactors, coated fuel particles, failures, probability, stresses, irradiation, neutron beams, physical radiation effects, model, high burnup I. INTRODUCTION In High Temperature Engineering Test Reactor (HTTR), which in under construction in Japan Atomic Energy Research Institute (JAERI), a fuel assembly consists of fuel rods and a hexagonal graphite block as shown in Fig. 1. A fuel rod contains fuel compacts in which coated particles are dispersed. In modern high temperature, gas-cooled reactors (HTGRs), Triisotropic (TRISO)-coated fuel particles are employed. The TRISO coatings consist of a low-density, porous pyrolytic carbon (PyC) buffer layer adjacent to the spherical fuel kernel, followed by an isotropic PyC layer (inner PyC; IPyC), a silicon carbide (SiC) layer and a final PyC * Oarai -machi, Higashiibaraki-gun, Ibaraki-ken ** Tokai-mura, Naka-gun, Ibaraki-ken Corresponding author, Tel , t Fax , sawa httr.oarai.jaeri.go.jp (outer PyC; OPyC) layer as shown in Fig. 2. In safety design of the HTGR fuels, it is important to retain fission products within particles so that their release to primary coolant does not exceed an acceptable level. From this point of view, the basic design criteria for the fuel are to minimize the failure fraction of as-fabricated fuel coating layers and to prevent significant additional fuel failures during operation(1). In order to predict failure fraction during irradiation, some failure mechanisms have been investigated(2)(3). Under high temperature irradiation, coating failure by the kernel migration, or so-called amoeba effect, and/or the corrosion of the SiC layer by a fission product of palladium is important(1)(2). On the other hand, under high burnup irradiation, stress induced in the coating layers and consequently so-called pressure vessel failure is understood to be important failure mechanism. Therefore some models were developed to calculate coating layer 712

2 Development of a Coated Particle Failure Model under Irradiation 713 Fig. 1 Fuel element of the first-loading-fuel of the HTTR the failure fraction depends not only on the SiC pressure vessel failure but also on the failure of the PyC layers. Therefore, failure probability of each coating layer is calculated at first, then the failure fraction of coated fuel particles is evaluated. This paper describes newly developed analytical model to calculate failure fraction of the TRISO-coated fuel particle under high burnup irradiation condition. Parameter calculations for the first-loading-fuel of the HTTR and for future high burnup fuel were also carried out to discuss the prediction results by the model developed here. FAILURE MODEL Fig. 2 TRISO-coated fuel particle stresses based on classical rigid SiC model(4)(5). Recently, the finite element method has been applied to calculate detail stress distribution in coating layer(6). However, these models are regarded as a guideline for fuel particle design rather than as a predictive tool for coated fuel particle performance because calculated results has not been able to represent the experimental data(7). Moreover, in Japan, since the maximum burnup of the first loading fuel of the HTTR is low (33 GWd/t), no analytical model has been prepared to evaluate failure behavior due to the stresses. Then the behavior of coated fuel particles has been examined only by many irradiation experiments and development of a reliable model has been needed. We considered that the problem of these models is caused by an assumption that failure of the TRISOcoated particles simply depends on the SiC layer intactness. Then a failure model is newly developed to predict failure fraction of TRISO-coated particles under high burnup irradiation. In this model, it is considered that The essential features of a fuel failure model developed here are as follows. (1) The conventional failure models have assumed that failure of coated fuel particles is determined only by failure of the SiC layer. This means that they can not describe failure behavior of SiC-failed particle which has intact PyC layer. Therefore the model calculate failure probability of each coating layer. (2) The failure probability of each coating layer is described by a Weibull distribution with microscopic surface flaws considered to be critical failure initiation sites. The failure criterion of each coating layer is determined based on experimental observations accumulated by irradiation tests. (3) The stress act on the coating layers of the fuel particle are assumed to be caused by pressure of fission gases and CO gas from UO2 kernel and by fast neutron-induced shrinkage of the PyC layers. The maximum stress is evaluated by rigid SiC model with spherical shell(8) because recently fabricated fuel particles have good sphericality. (4) Two types of failed particles are categorized, i.e., through-coatings failed particle and SiC- failed particle. The through-coatings failed particle results in VOL. 33, NO. 9, SEPTEMBER 1996

3 714 K. SAWA et al. IPyC, SiC and OPyC layer failure. The SiC-failed particle has failed IPyC and SiC layers but an intact OPyC layer. The gaseous fission products are released from through-coatings failed particle. On the other hand, since OPyC layer is capable of retaining the gaseous fission products, the SiC-failed particle does not release the gaseous fission products. The failure probability that as-fabricated SiC-failed particle becomes the through-coatings failed particle is also modelled. 1. Failure Probability of Each Coating Layer The fracture strength distribution of coating layers is expressed by a Weibull distribution. The failure probability of each layer is calculated by the following basic equation: The tensile stress is introduced into the coatings due to the pressure of fission gases and CO gas as burnup proceeds. On the other hand, the IPyC and OPyC layers undergo irradiation-induced shrinkage as a result of fast neutron exposures. Consequently, the OPyC layer places a compressive load on the SiC layer, which counteracts the SiC tensile stress due to internal pressure. Similarly, the IPyC layer acts to reduce the SiC tensile stress by tending to contract while maintaining contact with the inner SiC surface. The shrinkage of the PyC layers results in tensile stresses on themselves. Based on above-mentioned behavior, failure probability of each coating layer is modeled as follows. (1) IPyC Layer It is assumed that the IPyC layer fails by tensile stress only in the SiC layer failed particle. When the SiC layer is intact, the IPyC layer is supported by the SiC layer, which is much stronger than the PyC layer, and the IPyC failure does not occur (fipyc-=0). For the SiC-failed particle, in which the SiC layer cannot support the IPyC layer, tensile stress due to internal gas pressure would cause failure of the IPyC layer (f'ipyc). (2) SiC Layer The SiC layer fails by internal gas pressure in high burnup irradiation condition. The compressive stress caused by the IPyC and the OPyC layers shrinkage mitigates the stress on the SiC layer. When the OPyC layer is intact, the failure probability of the SiC layer (fsic) is evaluated based on tensile stress which is a balance of the stress due to internal gas pressure and the compressive stresses by both the IPyC and the OPyC shrinkage. Since a compressive load by the OPyC layer cannot be expected, the tensile stress of the SiC layer and the failure probability in the OPyC-failed particle (f'pyc) becomes larger then that in the OPyC intact particle. (3) OPyC Layer The OPyC layer fails only by tensile stress. When (1) the SiC layer, which is inside of the OPyC layer, is intact, the failure of the OPyC does not occur (fopyc=0) because it is supported by the stronger SiC layer. In the SiC-failed particle, the failure of the OPyC layer occurs by the internal gas pressure (f'opyc). 2. Failure Probability and Failure Fraction of Coated Fuel Particles Based on above-mentioned failure probabilities of coating layers, probability that the intact particle becomes the through-coatings failed particle (FTC) can be expressed as follows: The probability that the intact particle becomes the SiCfailed particle (Fsic) becomes same as the failure probability of the SiC layer: (2) Fsic = fsic (3) In the fabrication of coated fuel particles, a few particles are made as initially SiC-failed particles(9). In this model, the SiC-failed particle is defined as a particle whcih has failed SiC layer but has an intact OPyC layer. The probability that as-fabricated SiC-failed particle becomes the through-coatings failed particle (F'Tc) is written as 3. Stress Calculation (1) Stress by Internal Gas Pressure The tensile stress induced in the coating layer is calculated based on a thick-walled spherical pressure vessel model(10)(11). The maximum stress becomes as The internal pressure generates from stable gaseous fission products and CO gas due to excess oxygen by UO2 fission. The internal pressure is calculated by The number of free oxygen atoms is calculated by the experimentally obtained equation(12): The number of stable noble gases is calculated by the following equation(13): (4) (5) (6) (7) (8) nfp 0.31 x F x FR. (9) The temperature dependence of release fraction of fission gases from UO2 kernel to the buffer layer is considered in the model by JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

4 Development of a Coated Particle Failure Model under Irradiation 715 (10) Recoil release is neglected in the model because release fraction by recoil is about 2%. In Eq.(10), the following diffusion coefficient is employed(3) It is simply assumed that the free volume in the buffer layer is calculated by densities of as-fabricated porous buffer layer and of theoretical PyC: (11) (12) (2) Stress by PyC Shrinkage The model formalism describes the j-layer stress contribution from PyC shrinkage as a function of anisotropic irradiation-induced strains(14) and creep in the PyC layers. A detailed analytic description of the formalism is described in Ref.(8). The stress is defined as Wj and Gj are calculation parameters written as follows: 4. Strength of Layers Several data were obtained to represent the strength (13) (14) (15) of coating layers(3). For this model, 834 MPa of mean strength of unirradiated SiC is employed(16). For irradiated SiC, 480 MPa of mean strength was obtained(3)(16). In this model, fast neutron fluence dependence of the SiC layer strength is assumed by It is also assumed that Weibull modulus do not depend (16) on fast neutron fluence and the value of 8 is employed for the SiC layer(3)(16). For PyC layers, 160 MPa of mean strength and 4 of Weibull modulus were employed(17). M. DISCUSSIONS In order to perceive coated fuel particle behavior under high burnup irradiation condition, parametric calculations were carried out using the model described in previous chapter. 1. Stress Calculation Calculated stresses acting on the coating layers of the intact particle for the first-loading-fuel of the HTTR are shown in Fig. 3 as a function of fast neutron fluence. In the figure, tensile and compressive stresses are expressed by positive and negative values, respectively. Major specifications of the first-loading-fuel are summarized in Table 1. In the calculations, burnup rate and the fast neutron flux were selected as the highest values of the first-loading-fuel of the HTTR, 1/20 GW/t and 2.6 x 1017 m-2.s-1, respectively. The fuel temperature was assumed to be 1,300dc. When irradiation starts, compressive stress acts on the SiC layer by fast neutron-induced shrinkage of the PyC layers. Then, the stress on the SiC layer turns from compression to tensile in accordance with gaseous fission products and CO gas buildup in the buffer layer. Tensile stresses act on the PyC layers by their shrinkage. For the first-loading-fuel of the HTTR, which will be irradiated up to 1.5x 1025 M-2 of the maximum fast neutron fluence, the maximum tensile stress on the SiC layer in Fig. 3 Calculated stress on each coating layer VOL. 33, NO. 9, SEPTEMBER 1996

5 716 K. SAWA et al. Table 1 Major parameters for calculation of the HTTR first-loading-fuel the intact particle is almost zero, and it is predicted that no pressure vessel failure will occur. On the other hand, in the OPyC failed particle, no compressive load by the OPyC layer is expected on the SiC layer, then the stress of the SiC layer becomes to be about 200 MPa at the end of irradiation of the first-loading-fuel. It means that the irradiation-induced shrinkage of the PyC layers effectively prestressed the SiC layer thereby delaying the change from compression into tension and this effect is quantified by the model developed here. 2. Calculation of Failure Probabilities The failure probabilities that the intact particle and the as-fabricated SiC-failed particle become to the through-coatings failed particle were calculated for 1,000, 1,300 and 1,600dc of irradiation temperatures. The burnup rate and the fast neutron flux were selected the highest values of the first-loading-fuel. The results are shown in Fig. 4 as a function of burnup. Solid and dashed lines show the results for the intact and the SiCfailed particles, respectively. Since the through-coatings failure probabilities of the SiC-failed particle, E'TC, represent the failure probabilities of the OPyC as shown in Eq.(4), the differences between the failure probabilities of the intact and SiCfailed particles reflect the failure probabilities of the SiC and the IPyC layers. This model predicts that the as-fabricated SiC-failed particle results in the throughcoatings failed particle much earlier than the intact particle does. However, the SiC-failed particle can keep is OPyC integrity and does not fail immediately after irradiation starts. This result has not been obtained by the conventional simple pressure vessel failure models. When the irradiation temperature is 1,000dc, the significant failure of the intact particle starts at beyond 100 GWd/t of burnup. The significant failures of the intact particle start at the burnup of about 60 and 35 GWd/t when the irradiation temperatures are 1,300 and 1,600dc, respectively. Note that this model predicts the failure fraction only by the stress calculation. When fuel is irradiated at 1,600dc, amoeba effect and/or the corrosion of the SiC layer would additionally occur. The amoeba effect would cause thinning of the IPyC and the SiC layers. The SiC layer would be locally corroded by Pd-SiC reaction and the strength of SiC would be decreased. Then, at 1,600dc, the failure fraction would be higher than that predicted here. On the other hand, the as-fabricated SiC-failed particle fails to the through-coatings failed particle from about 20, 10 and 5 GWd/t of burnup when the irradiation temperatures are 1,000, 1,300 and 1,600dc, respectively. The internal pressure is proportional to fuel temperature and the fission gases and free oxygen release fractions from the kernel increase with fuel temperature as shown in Eqs.(6), (8), (10) and (11). Therefore, the results show that the higher irradiation temperature is, Fig. 4 Burnup dependent failure probabilities as a function of irradiation temperatures JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

6 Development of a Coated Particle Failure Model under Irradiation 717 the earlier failure probabilities increase significantly. 3. Buffer Layer Thickness Dependence Since the calculated stresses depend on the coating layers thicknesses, sensitivity calculation was carried out. Recent fabrication data of coated fuel particles show that the deviations of thicknesses of the PyC layers and the SiC layer are relatively small, however,the ls deviation of the buffer layer thickness is as large as about 15% of the median thickness(18). Moreover, the internal pressure depends strongly on the free volume in the buffer layer as shown in Eqs.(6) and (12), the calculation was carried out focusing on the buffer layer thickness. The calculated failure probabilities of the firstloading-fuel of the HTTR are shown in Fig. 5 as a function of the buffer layer thickness. Irradiation temperature, burnup and fast neutron fluence are 1,300dc, 33 GWd/t and 1.5x 1025 m-2, respectively. In the figure, solid and dashed lines also indicate the results for the intact and the SiC-failed particles, respectively. In this condition, the intact particle fails significantly if it has a buffer layer thinner than 30 gm, which corresponds to about 3o- limit of the buffer layer thickness distribution of the first-loading-fuel. On the other hand, it is predicted that the as-fabricated SiC-failed particle easily becomes to the through-coatings failed particle and when the buffer layer thickness is thinner than 45 pan, it becomes to be 100%. This result indicates that the buffer layer in the intact particle should be thicker than 30 ttm to prevent the through-coatings failure for the first-loading-fuel. When the buffer has closed pore, the free volume in the buffer layer becomes smaller than the calculated value by Eq.(12) and the failure probabilities become higher than the values predicted here. 4. Failure Behavior of High Burnup Fuel For upgrading of HTGR technologies, Japan Atomic Energy Research Institute has been developing a high burnup TRISO-coated fuel particle(")("). The high burnup fuel is designed to keep its integrity up to high burnup condition. The specifications of the high burnup fuel are shown in Table 2 comparing with the firstloading-fuel of the HTTR. In order to mitigate the internal pressure, the high burnup fuel has been designed to be thicker buffer layer than that of the first-loadingfuel. The target burnup of the high burnup fuel is twice or three times of the first-loading-fuel. Failure behavior of the high burnup fuel was investigated by the model developed here. The comparison of predicted failure probabilities to the through-coatings failure of the intact high burnup fuel (solid line) and the first-loading-fuel (dashed line) are shown in Fig. 6 as a function of burnup. The irradiation temperature and the fast neutron flux are 1,300dc and 1.5 x 1025 m-2, respectively. It is predicted Table 2 Specifications of high burnup fuel and HTTR first-loading-fuel Fig. 5 Failure probability as a function of buffer layer thickness VOL. 33, NO. 9, SEPTEMBER 1996

7 718 K. SAWA at at. Fig. 6 Linear view of failure probabilities of first-loading-fuel and high burnup fuel that no significant failure of the first-loading-fuel occurs up to about 50 GWd/t and this result seems to be consistent with experimental observations(2). At about 80 GWd/t, the first-loading-fuel is predicted to fail completely. On the other hand, the remarkable failure of the high burnup fuel starts about 90 GWd/t of burnup and the failure probability increases more gradually than the first-loading-fuel does. This result also qualitatively consistent with an irradiation test of the high burnup fuel in which significant failure did not occur up to about 70 GWd/t of burnup(20). Figure 7 shows logarithm view of the failure probabilities. If an acceptable throughcoatings failure fraction is assumed to be 10-5, the high burnup fuel is evaluated to maintain its integrity up to 70 GWd/t of burnup, which is more than twice of the burnup limit of the first-loading fuel. N. CONCLUSIONS A failure model was newly developed to predict failure fraction of TRISO-coated particles under high burnup irradiation. In this model, it is considered that the failure fraction depends not only on the SiC pressure vessel failure but also on the failure of the PyC layers. Therefore, failure probability of each coating layer is cal- Fig. 7 Logarithm view of failure probabilities of first-loading-fuel and high burnup fuel JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

8 Development of a Coated Particle Failure Model under Irradiation 719 culated at first, then the failure fraction of coated fuel particles is evaluated. Based on the model, the following results were obtained through parametric calculations for the firstloading-fuel of the HTTR and the high burnup fuel. (1) Calculated stress on each layer of the intact particle showed that the maximum tensile stress on the SiC layer in the intact particle is almost zero, and it is evaluated that no pressure vessel failure is predicted in the HTTR. On the other hand, in the OPyC failed particle, no compressive load by the OPyC layer is expected on the SiC layer, then the stress of the SiC layer becomes to be about 200 MPa at the end of irradiation of the first-loading-fuel. This means that the irradiation-induced shrinkage of the PyC layers effectively prestressed the SiC layer thereby delaying the change from compression into tension and this effect is quantified in the model developed here. (2) The failure probabilities that the intact particle and the as-fabricated SiC-failed particle become to the through-coatings failed particle were evaluated. This model predicted that the as-fabricated SiCfailed particle results in the through-coatings failed particle much earlier than the intact particle does. However, the SiC-failed particle can keep its OPyC integrity for a while and does not fail immediately after irradiation starts. This result had not been obtained by the conventional simple pressure vessel failure models. The results also showed that the higher irradiation temperature is, the earlier failure probabilities increase significantly. (3) The parameter calculations of the buffer layer thickness predicted that the intact particle fails significantly if it has a buffer layer thinner then 30 pm, which corresponds to about 3u limit of the buffer layer thickness distribution of the first-loading-fuel. This result recommended that the coated fuel particles should have the buffer layer thicker than 30 pm to prevent the through-coatings failure for the first-loading-fuel. (4) Failure behavior of the high burnup fuel was investigated by the model developed here. It is predicted that no significant failure of the first-loadingfuel occurs up to about 50 GWd/t and this result is consistent with experimental observations. On the other hand, the remarkable failure of the high burnup fuel starts about 80 GWd/t of burnup and the failure probability increases more gradually than the first-loading-fuel does. In summary, newly developed model can represent failure behavior of the TRISO-coated particles and research and development of future fuels will be carried out by using the model. [NOMENCLATURE] f i:: Failure probability of i-layer at irradiation time t fipyc: Failure probability of the IPyC layer in the SiC intact particle fi'pyc: Failure probability of the IPyC layer in the SiC failed particle fsic: Failure probability of the SiC layer in the OPyC intact particle c: Failure probability f'si of the SiC layer in the OPyC failed particle fopyc: Failure probability of the OPyC layer in the SiC intact particle.f'opyc: Failure probability of the OPyC layer in the SiC failed particle,: Stress on s the i i-layer at irradiation time t (MPa) o,i: Strength of the i-layer (MPa) s mi: Weibull modulus for the i-layer strength tj: Thickness of j-layer (mm) Tj: Radius to the inner surface of the j-layer (Am) 1j: Stress on the j-layer by internal gas pressure s (MPa) P: Internal pressure (MPa) n: Mole number of stable gaseous fission products and free oxygen atoms (mol) R: Gas constant (J/mol K) T: Irradiation temperature (K) V: Free volume in buffer layer (m3) nfp: Mole number of stable gaseous fission products (mol) no: Mole number of free oxygen (mol) FR: Fractional release of stable gaseous fission products from fuel kernel F: Fission number (fission) D': Reduced diffusion coefficient of stable gaseous fission products from fuel kernel (s-1) ti: Irradiation time (days) Puffer: Density of buffer layer (g/cm3) PPyc,Th: Theoretical density of PyC (g/cm3) 22,j: Stress on the j-layer by PyC sshrinkage (MPa) C: Creep coefficient (=2.9x m2/pa(15)) Sr: Dimensional change of PyC in the radial direction St: Dimensional change of PyC in the tangential direction v: Poisson ratio in creep (=0.5) Calculation parameters of Wj,Gj: the j-layer in Eq.(13) ri,j: Distance from kernel center to the inner surface of the j-layer (m) r2,j: Distance from kernel center to the outer surface of the j-layer (m) P : Fast neutron fluence (x1025 m-2) ACKNOWLEDGMENTS The authors wish to express their gratitude to Mr. T. Tanaka, Director of Department of HTTR Project, Mr. 0. Baba, Deputy Director of HTTR Project and Dr. M. VOL. 33, NO. 9, SEPTEMBER 1996

9 720 K. SAWA et al. Hoshi, Director of Department of Chemistry and Fuel Research, for their encouragement of this study. The authors also appreciate the members of HTTR Reactor Development Division and Fuel Irradiation and Analysis Laboratory for useful comments on this study. -REFERENCES- ( 1 Saito, S., et al.: JAERI-1332, (1994). 2 ) Fukuda, K., et al.: JAERI-M , (1989), [in Japanese]. ( 3 ) Verfondern, V., Martin, R.C., Moormann, R.: 2721, (1993). ( 4 ) Kovacs, W.J., Bongartz, K., Goodin, D.T.: Nucl. Technol., 68, 344 (1985). ( 5 Verfondern, V., Nabielek, H.: Jul-Spez-298, (1985). 6 ) Miller, G.K., Wadsworth, D.C.: Nucl. Technol., 110, 396 (1995). ( 7 ) Baldwin, C.A., et al.: ORNL/M-2850, (1993). ( 8 Bongartz, K.: Jul-1686, (1980). 9 ) Minato, K.,et al.: Nucl. Technol., 106, 342 (1994). (10) Utoguchi,T., et al.: "Zairyo Rikigaku", Shokabo, (1982), [in Japanese]. ( 11) Kovacs, W.J.: GA-A 16807, (1983). (12) Proksh, E., et al.: J. Nucl. Mater., 107, 280 (1982). (13) Lindemer, T.B.: J. Am. Ceram. Soc., 60, 409 (1977). (14) Kaae, J.L.: Nucl. Technol., 35, 359 (1977). ( 15) Schenk, W., Pitzer, D., Nabielek, H.: Jul/-2234, (1988). (16) Verfondern, K., Dunn, T.D., Bolin, J.M.: Jul-2548, (1991). (17) Bongartz, K., et al.: J. Nucl. Mater., 62, 123, (1976). ( 18) Minato, K., Kikuchi, H., Sawa, K., Tobita, T., Fukuda, K.: JA E RI- Tech , (1996). (19) Kania, M.J., Fukuda, K.: ORNL/TM-11346, (1989). (20) Sawa, K., Fukuda, K., Acharya, R.: JAERI-Tech , (1995), [in Japanese]. JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY