Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel

Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel Comprehensive nuclear materials 3 08 advanced concepts in TRISO fuel

3.08 Advanced Concepts in TRISO Fuel K Minato and T Ogawa Japan Atomic Energy Agency, Tokai-mura, Ibaraki, Japan ß 2012 Elsevier Ltd All rights reserved 3.08.1 Introduction 216 3.08.2 3.08.2.1 3.08.2.2 3.08.2.3 3.08.2.4 3.08.2.4.1 3.08.2.4.2 3.08.2.4.3 3.08.2.4.4 3.08.2.4.5 3.08.3 3.08.3.1 3.08.3.2 3.08.3.2.1 3.08.3.2.2 3.08.4 3.08.4.1 3.08.4.2 3.08.4.3 3.08.4.3.1 3.08.4.3.2 3.08.5 3.08.5.1 3.08.5.2 3.08.6 References ZrC-Coated Particle Fuel Designs of ZrC-Coated Particle Fuel Fabrication of ZrC-Coated Particle Fuel Characterization Techniques for ZrC-Coated Particle Fuel Performance of ZrC-Coated Particle Fuel Irradiation performance Resistance to chemical attack by fission products High-temperature stability Retention of fission products Behavior under oxidizing conditions ZrC-Containing TRISO-Coated Particle Fuel Designs of ZrC-Containing TRISO-Coated Particle Fuel Performance of ZrC-Containing TRISO-Coated Particle Fuel Irradiation performance Retention of fission products SiC-Containing TRISO-Coated Particle Fuel Designs of SiC-Containing TRISO-Coated Particle Fuel Fabrication of SiC-Containing TRISO-Coated Particle Fuel Performance of SiC-Containing TRISO-Coated Particle Fuel Irradiation performance Behavior under simulated conditions TiN-Coated Particle Fuel Designs of TiN-Coated Particle Fuel Fabrication of TiN-Coated Particle Fuel Outlook 216 216 217 219 220 220 221 222 225 227 227 227 229 229 230 231 231 232 233 233 233 234 234 234 235 235 Abbreviations DB-MHR DBa(ZrC) DCe(ZrC) DCs(SiC) DCs(ZrC) DRu(ZrC) DSr(ZrC) GA GAC Deep-burn modular helium reactor Diffusion coefficient for Ba in the ZrC coating layer Diffusion coefficient for Ce in the ZrC coating layer Diffusion coefficient for Cs in the SiC coating layer Diffusion coefficient for Cs in the ZrC coating layer Diffusion coefficient for Ru in the ZrC coating layer Diffusion coefficient for Sr in the ZrC coating layer General Atomics General Atomic Company GFR HFIR HTGR ICP-AES IPyC JAEA JAERI JMTR JRR-2 LANL LASL LMFBR MTS OPyC Gas fast reactor High Flux Isotope Reactor High-temperature gas-cooled reactor Inductively coupled plasma-atomic emission spectrometry Inner dense PyC Japan Atomic Energy Agency Japan Atomic Energy Research Institute Japan Materials Testing Reactor Japan Research Reactor-2 Los Alamos National Laboratory Los Alamos Scientific Laboratory Liquid metal fast breeder reactor Methyltrichlorosilane Outer dense PyC 215 216 ORR PyC R/B VHTR Advanced Concepts in TRISO Fuel Oak Ridge Research Reactor Pyrolytic carbon Release-to-birth ratio Very-high-temperature reactor 3.08.1 Introduction The TRISO-coated fuel particle consists of a microspherical fuel kernel and coating layers of porous pyrolytic carbon (PyC), inner dense PyC (IPyC), silicon carbide (SiC), and outer dense PyC (OPyC) The chemical form of the fuel kernel can be oxide, carbide, or a mixture of the two The function of these coating layers is to retain fission products within the particle The porous PyC coating layer, called the buffer layer, attenuates fission recoils and provides void volume for gaseous fission products and carbon monoxide in the cases of oxide and oxycarbide fuels The IPyC coating layer acts as a containment to gases during irradiation and protects the fuel kernel from the reaction with the coating gases during the SiC coating process The SiC coating layer provides mechanical strength for the particle and acts as a barrier to the diffusion of metallic fission products, which diffuse easily through the IPyC layer The OPyC coating layer protects the SiC coating layer mechanically The recent interest in the coated particle fuel concept includes its application outside the past experience of the high-temperature gas-cooled reactor (HTGR)1: Very-high-temperature reactor (VHTR) with a gas outlet temperature of 1273 K for supplying both the electricity and the process heat for hydrogen production, as proposed in the Generation-IV International Forum.2 Actinide burning in deep-burn modular helium reactor (DB-MHR) with fuel kernels consisting of high concentrations of transuranium elements.3,4 Advanced gas fast reactor (GFR) with nitride fuel, which aims at a more improved performance compared with the conventional liquid metal fast breeder reactor (LMFBR) and/or the efficient actinide burning.1,5 Although SiC has excellent properties, it gradually loses its mechanical integrity at very high temperatures, especially >1973 K.6–8 The annealing temperature during fuel element fabrication is limited to 2073 K for h Higher temperatures should result in a porous structure due to the b-SiC to a-SiC transformation and the thermal dissociation of SiC,9 which leads to an extensive release of fission products from the TRISO-coated fuel particles The fuel temperatures were limited to well below 1973 K during the design-basis accidents in HTGR designs.10–12 Chemical interaction of the SiC coating layer with fission products is one of the possible performance limitations of the TRISO-coated fuel particles The fuel performance of TRISO is described in Chapter 3.07, TRISO-Coated Particle Fuel Performance The fission product of palladium is known to react with the SiC layer Corrosion of the SiC layer could lead to fracture of the coating layers or provide a localized fast diffusion path, which degrades the fission-product retention capability within the particle Since the fission yield for palladium from 239Pu is about tenfold that from 235U,13 a careful particle fuel design should be made in the actinide burning The PyC layer develops gas permeability with increasing fast neutron doses Intactness of the IPyC layer is crucial in keeping the integrity of the SiC layer in the oxide fuel When the IPyC layer fails, or develops gas permeability, the SiC layer will react with CO gas to form volatile SiO.14 The PyC layer will also develop anisotropy above 2173 K, which is deleterious to its irradiation behavior To improve the high-temperature performance of the TRISO-coated fuel particles, a new material other than SiC is needed Zirconium carbide is a candidate, and ZrC-coated particles, where the SiC layer was replaced by a ZrC layer, have been tested New configurations of the coating layers, with a layer containing SiC or ZrC added to the TRISO coating, have been proposed and tested to improve the chemical stability of the TRISO-coated particles For the application to the fast reactor fuel, TiN coating layers have been proposed and tested instead of PyC layers The following sections summarize the designs and the research and development of the advanced concepts in TRISO fuel: (1) ZrC-coated particle fuel, (2) ZrC-containing TRISO-coated particle fuel, (3) SiC-containing TRISO-coated particle fuel, and (4) TiN-coated particle fuel 3.08.2 ZrC-Coated Particle Fuel 3.08.2.1 Designs of ZrC-Coated Particle Fuel Zirconium carbide (ZrC) is known as a refractory and chemically stable compound, which melts eutectically with carbon at 3123 K The properties of Advanced Concepts in TRISO Fuel ZrC are summarized in Chapter 2.13, Properties and Characteristics of ZrC To improve the hightemperature stability, the resistance to chemical attack by fission products, and the retention of fission products, the ZrC coating layer is a candidate that can replace the SiC coating layer of the TRISO-coated fuel particle; the resulting particle is termed a ZrC-TRISO-coated fuel particle The apparent drawback of the ZrC-TRISO coating may be that ZrC does not withstand the oxidation in such an accident as a massive air-ingress accident though it is highly hypothetical in the modern HTGR designs Historically, several coating designs have been tested in the United States15,16: (1) ZrC-TRISOcoated particles, (2) ZrC-TRISO type coated particles without OPyC layer, (3) ZrC-coated particles with ZrC-doped OPyC layer, and (4) ZrCcoated particles with graded C–ZrC layer(s) In the graded C–ZrC layer, the compositions were changed gradually from the pure PyC through the ZrC with excess carbon and into the pure ZrC The graded layer was applied to either the inside or the outside surface of the ZrC layer Propylene was used to produce the pure PyC and to provide the carbon for the graded portion of the codeposited carbon and ZrC ZrC-coated fuel particles are being developed in Japan17 since the early 1970s The ZrC-coated fuel particles at the early stage of the development were characterized by a thick ZrC layer with a composition of C/Zr > 1.0 and by the absence of the OPyC layer A ZrC layer of this kind was called ‘zirconium-carballoy,’ meaning ZrC–C alloy Later, it was found that the retention of metal fission products, especially 90 Sr, by the zirconium-carballoy was rather poor,18 presumably owing to a short circuit through the free carbon phase It was also felt from the irradiation experiences that the presence of the OPyC layer was essential for the mechanical integrity of the coated fuel particle The emphasis was, therefore, placed on the development of ZrC-TRISO-coated particles with the stoichiometric composition of C/Zr ¼ 1.0 Although most of the reported work on the use of ZrC in coated fuel particles has been directed toward the development of a replacement for the SiC barrier layer, ZrC was also tested as a fission product and oxygen getter,19,20 in which ZrC was deposited over the fuel kernel or dispersed throughout the buffer layer These types of the coated particle fuels are described in Section 3.08.3 217 3.08.2.2 Fabrication of ZrC-Coated Particle Fuel The coating layers of ZrC and ZrC–C were produced by chemical vapor deposition, in which the pyrolytic reaction of zirconium halide with hydrocarbon in the presence of hydrogen was used in principle Mainly, two different processes have been developed in supplying zirconium halide to the coater: (1) using ZrCl4 powder and (2) using in situ generation of zirconium halide vapor The chemical vapor deposition of ZrC has been studied using a gas mixture of CH4, H2, ZrCl4, and Ar at Los Alamos Scientific Laboratory (LASL; now Los Alamos National Laboratory, LANL).16,21–23 A key development in the ZrC coating project proved to be the ZrCl4 powder feeder for metering ZrCl4 into the coater ZrCl4 is a solid at room temperature and sublimes at 625 K In this process, hygroscopic ZrCl4 powder was supplied from the powder feeder, whose rate was controlled by the auger speed and metered by the output of the load cell on which the powder feeder was The powder was swept by Ar to the coater base where it was mixed with the other coating gases supplied from a gas manifold The ZrCl4 powder in the gas stream was vaporized in the coater base before entering the coating chamber.22 Figure shows an apparatus for ZrC deposition by the process using ZrCl4.22 The effects of varying CH4 and H2 concentrations and particle bed area on the coating rate, the appearance, and the composition of the ZrC were studied using the ZrCl4 powder feeder.22 Increases in CH4 Coater Induction heater ZrCI4 powder Auger Mixing chamber Drive motor Gas manifold Argon Figure Experimental apparatus for ZrC deposition by the process using ZrCl4 Reproduced from Wagner, P.; Wahman, L A.; White, R W.; et al J Nucl Mater 1976, 62, 221–228 218 Advanced Concepts in TRISO Fuel Scrubber Quartz Carbon wool Graphite Induction coil Alumina Zr Br4 Quartz Zr Furnace Ar Ar Br2 + Ar CH4 + H2 Ar and H2 concentrations were found to be effective in increasing the linear coating rate of ZrC Increases in the ratio of CH4 to ZrCl4 in the coating gas resulted in a decreased metallic appearance of the coating and an increase in the C/Zr in the deposit Increases in H2 inhibited these effects The ZrC coating layers were prepared using a gas mixture of C3H6, H2, ZrCl4, and Ar with the same coater and ZrCl4 powder feeder as described earlier.23 In general, ZrC coating layers made with CH4 and C3H6 were similar and were affected similarly by variations in the hydrocarbon and hydrogen concentrations The coating layers produced using C3H6 were more sensitive to changes in hydrogen concentration than those produced using CH4 The coating processes based on the in situ generation of zirconium halide vapor are developed at Japan Atomic Energy Research Institute (JAERI; now Japan Atomic Energy Agency, JAEA) to avoid the handling of highly hygroscopic halide powder.17 Several processes were studied: the chloride process,24 the methylene dichloride process,25 the iodide process,26,27 and the bromide process.28–31 Among these processes, the bromide process proved to be the most convenient and reliable; in this process, ZrBr4 is produced by reacting bromine with zirconium sponge inside the coater ZrBr4 is preferred to ZrCl4 since the reaction of excess chlorine with hydrogen is a potential explosion hazard Figure shows an apparatus for ZrC deposition by the bromide process.32 In this process, bromine, which is liquid at room temperature, was carried by argon onto the heated zirconium sponge beneath the spouting nozzle and reacted to generate ZrBr4 vapor, which was mixed with the other coating gases of CH4 and H2 before entering the chamber Propylene could be used instead of methane The flow rate of ZrBr4 was controlled successfully by controlling the flow rate of Ar passing through liquid bromine at 273 K and maintaining the temperature of zirconium sponge at 873 K Along with the deposition experiments of ZrC by the bromide process, thermochemical analyses were performed to find the optimum deposition condition.31 The multiphase equilibrium in the system was analyzed based on the minimization of the total free energy of the system The analyses predicted that the ZrC monophase region exists in a wide composition range of the feed gas mixture It was concluded from the analyses that the deposition rate could be controlled through the methane flow rate, and the composition of the deposit through the ZrBr4 flow rate in Figure Experimental apparatus for ZrC deposition by the bromide process Reproduced from Ogawa, T.; Ikawa, K Deposition of LTI pyrolytic carbon by a nozzle without water cooling, JAERI-M 9568; Japan Atomic Energy Research Institute, 1981 the presence of excess hydrogen The experimental results agreed with the predicted results By adjusting the deposition condition, stoichiometric ZrC layers were obtained with the bromide process Recently, a new ZrC coater was installed and ZrC coating experiments were carried out at JAEA.33 The ZrC coater was designed with the maximum batch size of 0.2 kg, which is about 10 times larger than the previous one The ZrC coater mainly consists of the gas supply equipment, the coater, and the off-gas combustion equipment The coater is composed of the lower and the upper heaters with in-line configuration Advanced Concepts in TRISO Fuel The lower one is for the reaction of bromine with zirconium sponge and the upper one, for the chemical vapor deposition of ZrC at 1873 K in the maximum The off-gas treatment equipment removes soot, hydrogen bromide, and residual hydrogen 3.08.2.3 Characterization Techniques for ZrC-Coated Particle Fuel It is important to characterize the key basic properties of the coating layers that are critical to the fuel performance Although most of the characterization techniques used for the ordinary TRISO-coated particle fuels can be applied to the ZrC-coated particle fuels, some techniques have to be developed primarily for the ZrC-coated particle fuels In the case of the ordinary TRISO-coated fuel particles, the PyC layers are burnt off to recover the SiC fragments for characterization, such as density, composition, and strength measurements However, it is almost impossible to separate the ZrC from the PyC layers by the same method since ZrC, in contrast with SiC, does not form a protective oxide layer, resulting in oxidation of ZrC to ZrO2 when exposed to air at high temperatures To begin with, a method of obtaining fragments of the ZrC coating layer from the ZrC-coated particles is needed The plasma oxidation method was developed to obtain the ZrC fragments from the coating layers containing the PyC.34 The difference in the oxidation rates between PyC and ZrC is very large In this method, the samples were set in the plasma oxidation apparatus, where low-pressure oxygen was ionized by high frequency induction coupling Plasma reaction was monitored by a color analyzer and an optical power meter The color changed from the pale violet of pure oxygen to pale blue during vigorous oxidation of free carbon, and again to pale violet when the PyC was completely removed and a very thin oxide scale was formed on the ZrC The brightness also changed dramatically during the reaction The obtained ZrC fragments were examined by Raman spectroscopy and X-ray diffraction It was confirmed that the bulk of the ZrC remained unaffected by the plasma oxidation.35 The physical grinding technique was also developed to obtain the ZrC fragments.33 In this technique, quartz powder having the Mohs hardness of was used since the Mohs hardness for ZrC is about 8–9 and that for graphite is The fragments of the combined layers of ZrC and PyC were ground with quartz powder After grinding, the fragments of the ZrC layers without PyC were separated from the rest 219 in liquid tetrabromoethane (C2H2Br4) by the density difference The specific gravity of tetrabromoethane is 2.965 Mg mÀ3 The density of the SiC layers is measured by the sink-float technique or a liquid gradient column, in which a liquid having the same density as the sample is needed The density of the SiC layers is around 3.21 Mg mÀ3, and a liquid mixture of methylene iodide (CH2I2) having a density of 3.325 Mg mÀ3 and benzene (C6H6) having a density of 0.8785 Mg mÀ3, for example, is used for the measurement of density In the case of the ZrC density measurement, no suitable liquid is present since the density of the ZrC layers is around 6.6 Mg mÀ3, and so, other techniques are needed Gas pycnometry, which required at least 100 mg of the samples,33 was developed for the ZrC density measurement The stoichiometry of the ZrC layer affects the thermal conductivity,36 fission product retention,18,37 etc The properties of ZrC are summarized in Chapter 2.13, Properties and Characteristics of ZrC Analysis of the free carbon is important for controlling the quality of the ZrC coating Plasma oxidation with emission monitoring was also applied to the quantitative analysis of the free carbon in ZrC powder.38 The emission was monitored with an optical color analyzer and was calibrated with standard samples of ZrO2 + C mixtures The oxidation rates of the free and the combined carbons are so different that it is possible to estimate the amount of the former from the emission With powdered ZrC of about 10 mg, free carbon of 29 fJ) Reference HRB-7 HRB-8 HRB-12 HRB-15A HRB-15A HRB-16 HRB-16 UC2 UC2 UC4.6O1.1 UC2 UO2 UC2 UCxOy 1498 1498 1523 1328–1403 1348–1398 1353 1433 84.4 84.4 84–86 24.9–28.3 27.2–28.8 20.9 20.3 4.50 Â 1025 5.92 Â 1025 (4.4–6.9) Â 1025 (4.9–6.3) Â 1025 (6.0–6.2) Â 1025 4.1 Â 1025 3.8 Â 1025 [43] [43] [42] [44] [44] [45] [45] Advanced Concepts in TRISO Fuel Table Irradiation tests of ZrC-TRISO-coated fuel particles in Japan Capsule Fuel kernel 78F-4A 80F-4A ICF-26H VOF-8H VOF-14H 88F-3A UO2 UO2 UO2 UO2 UO2 UO2 Temperature (K) 1373 1173 1673–1773 1643 (15 K mmÀ1) 1873 (15 K mmÀ1) 1673–1923 221 Burnup (% FIMA) Fast neutron fluence (mÀ2, E > 29 fJ) Reference 4.0 1.5 1.8 1.6 1.6 4.5 2.2 Â 10 1.2 Â 1025 1.0 Â 1025 1.1) fuel kernels are used.68,69,81 Lanthanide is kept as a stable oxide within the oxide and oxycarbide (O/U > 1.1) fuel kernels This is a typical example of the first method mentioned earlier On the other hand, the compositions of the fuel kernels of oxide, oxycarbide, and carbide showed little effect on the corrosion of the SiC layer by palladium: the corrosion of the SiC layer by palladium has been observed in the TRISO-coated particle fuel with almost all kinds of fuel compositions of UO2, UC2, UCxOy, PuO2Àx, 3ThO2–PuO2Àx, ThO2, and (Th,U) O2.49,50,70,82–84 A new idea was needed to keep the fission product palladium within the fuel kernel Based on the out-of-reactor experiments,85 a concept of the TRISO-coated UO2 and UCO particles gettered with SiC dispersed in the kernels has been suggested to prevent the corrosion of the SiC layer by the fission product palladium.86 The idea is that the formation of a compound UPd3Si3C5 with a high melting temperature of >2225 K would keep palladium in the kernel Although the thermodynamic calculations have been carried out, the fabrication and irradiation tests of particles of this kind have not been performed yet To prevent the corrosion of the SiC layer by fission product palladium based on the second method described above, three types of new combinations of the coating layers have been proposed and tested.87 The idea is to add a layer that traps palladium by chemical reaction inside the SiC layer of the TRISO coating Two kinds of additional layers have been selected: an SiC + PyC layer and an SiC layer The SiC ỵ PyC layer is composed of SiC with free carbon.88 This kind of layer has been studied to improve the capability of fission product retention of the dense PyC layer.89 Figure 12 shows ceramographs of three types of advanced coatings, together with that of the TRISO coating for comparison.87 The type-A coating has an additional layer of SiC ỵ PyC adjacent to the inside of the SiC layer As the SiC ỵ PyC layer has a better capability of fission product retention,89 the thickness of the IPyC layer can be reduced; the IPyC layer should act as a seal to prevent the chemical reaction of the fuel kernel with coating gases during the coating process The type-B coating is similar to the type-A coating, but the dense PyC layer is present between the SiC ỵ PyC and SiC layers The expected role of the intermediate dense PyC layer is to interrupt the radial extension of the corrosion zone from the SiC ỵ PyC layer to the SiC layer The corrosion zone would extend circumferentially in the SiC ỵ PyC 232 Advanced Concepts in TRISO Fuel Den Den se se P yC SiC SiC SiC Den se + Py C Den SiC se P yC Por ous (a) Den Por ous UO 20 µm (b) yC PyC PyC UO 20 µm Den se P se P yC SiC se P yC S Den iC se P yC UO (c) yC SiC Den Por ous PyC +P se PyC Den PyC Den se P yC Por ous PyC 20 µm UO (d) PyC 20 µm Figure 12 Ceramographs of three types of advanced coatings, together with that of the TRISO coating for comparison; (a) type-A coating, (b) type-B coating, (c) type-C coating, and (d) TRISO coating Reproduced from Minato, K.; Fukuda, K.; Ishikawa, A.; et al J Nucl Mater 1997, 246, 215–222 layer when the intermediate dense PyC layer is present The thickness of the IPyC layer can be reduced in the same manner as the type-A coating In the type-C coating, SiC is used for an additional layer The role of the inner SiC layer is to react with fission products, and the intermediate dense PyC layer is expected to interrupt the radial extension of the corrosion from the inner to outer SiC layers The increase in the thickness of the SiC layer of the TRISO coating may be one of the solutions to the corrosion of the SiC layer by fission products However, the thicker coating layers will result in a smaller amount of the fuel material contained in a unit volume With these advanced coatings, no corrosion of the SiC layer by fission products is expected to occur without increasing the total thickness of the coating layers 3.08.4.2 Fabrication of SiC-Containing TRISO-Coated Particle Fuel No new technology was needed and a conventional coating apparatus without modification could be used to fabricate the SiC-containing TRISO-coated particle fuel This point is of great advantage for fuel fabrication Three types of advanced coatings, type-A, type-B, and type-C, were prepared The porous PyC layer was deposited at 1573 K with the pyrolysis of acetylene (C2H2) in the flowing argon, and the dense PyC layer was deposited at 1638 Kwith the pyrolysis of propylene (C3H6) in the flowing argon The SiC layer was chemically vapor deposited at 1873 K using methyltrichlorosilane (CH3SiCl3; MTS) and hydrogen The SiC ỵ PyC layer was deposited at 1873 K with MTS and argon.88 The content of free carbon in the SiC ỵ PyC Advanced Concepts in TRISO Fuel layer was about 40 wt% as examined by electron probe microanalysis For the irradiation experiments, three types of the advanced coatings, together with the TRISO coating, were deposited on the fuel kernels of UO2 with 19.7 wt% enriched uranium.87 SiC + PyC 233 SiC 3.08.4.3 Performance of SiC-Containing TRISO-Coated Particle Fuel 3.08.4.3.1 Irradiation performance The SiC-containing TRISO-coated fuel particles with the advanced coatings of the type-A, type-B, and type-C, together with the TRISO-coated fuel particles, were irradiated in the JRR-2 The sample particles were put individually into the holes of graphite disks, which were piled and then loaded in two irradiation capsules The burnups of the fuels irradiated in two capsules were 3.7 and 7.0% FIMA, respectively, and the irradiation temperature was 1603 K in both the capsules.87 The irradiated coated fuel particles were examined by visual inspection, X-ray microradiography, ceramography, and electron probe microanalysis No anomaly was found by visual inspection and X-ray microradiography The ceramography revealed no crack in the advanced coating layers or in the TRISO coating layers The SiC ỵ PyC layer in the advanced coatings of the type-A and type-B showed good irradiation performance as a coating layer The mechanical integrity of the advanced coatings was confirmed in this irradiation experiment.87 The behavior of the fission product palladium in the coating layers was examined by electron probe microanalysis In the TRISO-coated fuel particles, palladium was distributed along the inner surface of the SiC layer and reacted with SiC On the other hand, in the type-A coating, palladium was distributed along the inner surface of the SiC + PyC layer, and no corrosion was found in the SiC layer The fission product palladium released from the fuel kernel was trapped by the SiC ỵ PyC layer, as expected The same behavior was also found in the type-B coating, as shown in Figure 13.87 In the type-C coating, palladium was found along the inner surface of the inner SiC layer and no corrosion was found in the outer SiC layer The inner SiC layer trapped the fission product palladium by reacting with it.87 The irradiation experiment demonstrated that the advanced coatings had good irradiation performance, and the additional layers of SiC and SiC ỵ PyC trapped palladium effectively to prevent the corrosion of the SiC layer.87 SEI 10 µm Si Pd Figure 13 Electron probe microanalysis of the polished cross-section of a particle fuel with type-B coating after irradiation at 1603 K to 7.0% FIMA; (a) secondary electron image, (b) X-ray image for silicon, and (c) X-ray image for palladium Reproduced from Minato, K.; Fukuda, K.; Ishikawa, A.; et al J Nucl Mater 1997, 246, 215–222 The effect of the intermediate dense PyC layer between the SiC and SiC + PyC layers in the type-A or between the inner and outer SiC layers in the typeC on the extension of the corrosion zone could not be demonstrated in the irradiation experiment since the corrosion depth of the coating layer by palladium was small compared with the thickness of the layer But this point was examined by the out-of-reactor tests 3.08.4.3.2 Behavior under simulated conditions The coated fuel particles having the three types of advanced coatings inside out were heated with the powder of palladium at 1773 K for h in flowing argon 234 Advanced Concepts in TRISO Fuel In the out-of-reactor tests, palladium was designed to diffuse into the coating layers from the outside of the particles The heated coated fuel particles were polished and examined with an optical microscope.87 In the type-B coating, which has the intermediate dense PyC layer between the SiC + PyC and SiC layers, part of the SiC + PyC layer was completely attacked through the coating thickness by palladium, while no corrosion was found in the SiC layer This interruption of the radial extension of the corrosion zone from the SiC + PyC layer to the SiC layer was exactly the expected role of the intermediate dense PyC layer The interruption of the radial extension of the corrosion zone from the outer to inner SiC layers by the presence of the intermediate dense PyC layer was also observed in the type-C coating The intermediate dense PyC layer between two SiC layers functioned effectively, as expected In the out-of-reactor experiment, the SiC + PyC layer was proved to be effective as a barrier to the diffusion of palladium to the SiC layer, and the intermediate dense PyC layer was found to interrupt the radial extension of the corrosion zone from the barrier layer to the SiC layer However, it should be noted that the intermediate dense PyC layer could not interrupt the radial extension of the corrosion zone when the amount of palladium exceeded the capacity of the barrier layer to react with palladium 3.08.5 TiN-Coated Particle Fuel 3.08.5.1 Fuel Designs of TiN-Coated Particle The fast neutron environment of GFR requires a new and different fuel design The TRISO-coated particle fuel for HTGR contains the PyC layers, but they will not withstand the fast neutron environment The HTGR fuel elements, in which the coated particles are dispersed, consist of a light element of carbon, but structural materials made of light elements have to be minimized in the fast spectrum reactor core.1 The chemical form of the fuel kernel should also be considered Among the various fuel types, nitride fuel has good potential The heavy-metal density of the oxide kernel is smaller than those of the carbide and nitride kernels The carbide kernel has better potential than that of the oxide, but the fabrication of carbides of transuranium elements is more difficult due to high vapor pressures of plutonium and americium over the carbide.90 The nitrides of transuranium elements could be fabricated with the prevention of the vaporization loss of the transuranium elements by keeping the nitrogen partial pressure sufficiently high during the fabrication processes.91 The coating has to be selected so as to be compatible with the fast neutron environment, the nitride kernel, and the matrix or structural materials The use of SiC as a coating material would have to be avoided as it reacts with transition metals, such as Ti, Fe, Cr, and Ni,92 unless the matrix or the structural container is made of ceramics with good thermal properties, such as TiN and TiSi2, which are compatible with SiC.1 Titanium nitride with enriched 15N may be a good choice because of its refractoriness (Tm = 3223 K), low neutron cross-sections, and compatibility with steels It is advantageous for the coating that TiN is little miscible with actinide nitrides.93 Two configurations of the coated particles have been proposed One is the duplex coating, where the inner layer is a low-density buffer layer and the outer layer is of high-density fission products container The other configuration is a porous kernel and a thin but strong and dense outer layer to maximize the fraction of fuel in the particle while maintaining the integrity of the miniature pressure vessel.1,5 Ferritic steel may be a good choice for the metal matrix of a pebble-bed type core or the porous metal frit of a particle bed type core TiN is completely stable when in contact with the ferritic steels.1 3.08.5.2 Fabrication of TiN-Coated Particle Fuel A series of experiments were carried out to explore the fluidized bed chemical vapor deposition of the TiN coating layer for GFR particle fuel.5 In the experiments, microspheres of zirconium dioxide (ZrO2)-coated with carbon layers were used The TiN layers were chemically vapor deposited on the particles from titanium tetrachloride (TiCl4), nitrogen (N2), and hydrogen (H2) The deposition conditions of temperature, reactant gas concentration, and reactant gas composition were varied to study the effects on the deposited coating properties Coating thickness, composition, density, crush strength, and microstructure were examined and related to changes in deposition conditions Coating rate, density, strength, appearance, cracking, and crystallinity were influenced by deposition conditions A standard set of deposition parameters was developed.5 Advanced Concepts in TRISO Fuel 3.08.6 Outlook The recent interest in the coated particle fuel concept includes its application outside the past experience of HTGR To improve the high-temperature performance of the TRISO-coated fuel particles, a new material other than SiC is needed Zirconium carbide is a candidate and the ZrC-coated particles have been tested To improve the chemical stability of the TRISO-coated particles, new configurations of the coating layers have been proposed and tested For the application to the fast reactor fuel, TiN coating layers have been proposed and tested instead of PyC layers The laboratory-scale experiments on these advanced fuels form the basis of further discussions of the application and development of advanced concepts in TRISO fuel Although engineering-scale fabrication of the advanced fuels and irradiation tests are needed for the demonstration, fundamental studies are invaluable to develop the advanced concepts Mechanical and thermal properties measurements of the coating layers, and thermochemical analyses of the fuel including fission products and coating layers/additives would be very helpful in developing the concepts and further modeling the fuel behavior References 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 10 11 12 Ogawa, T.; Minato, K.; Sawa, K Advanced coated particle fuels: Experience of ZrC-TRISO fuel 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... remained intact.66 From this point of view, ZrC may not be a retentive layer in air or water ingress accidents 3. 08 .3 ZrC-Containing TRISOCoated Particle Fuel 3. 08 .3. 1 Designs of ZrC-Containing TRISO- Coated... ZrC-coated particle fuel, (2) ZrC-containing TRISO- coated particle fuel, (3) SiC-containing TRISO- coated particle fuel, and (4) TiN-coated particle fuel 3. 08. 2 ZrC-Coated Particle Fuel 3. 08. 2.1 Designs... ZrC layer on UO2 ZrC dispersed in buffer ZrC layer on UO2 ZrC dispersed in buffer ZrC layer on UO2 131 3– 139 8 133 3– 135 8 1 133 –1178 1188 135 3–1485 22.0–29.1 23. 6–24 .3 23. 8–26.6 25.2–26.2 19.0–27.6