The High Temperature Gas Cooled Reactor Fuel

Transcription

1 The High Temperature Gas Cooled Reactor Fuel Kazuhiro Sawa 1*, Shouhei Ueta 2 and Tatsuo Iyoku 2 1 Office of Planning, Japan Atomic Energy Research Institute, Kashiwa, Chiba, , Japan 2 Department of HTTR Project, Japan Atomic Energy Research Institute, Oarai, Ibaraki, , Japan This paper provides present status of research and development for the coated fuel particle (CFPs) including the advanced ZrC-CFP. Current HTGR employs so-called TRISO-CFPs with SiC layer. The coating layers are deposited on the kernels in a CVD process using a fluidized coater. In safety design of the HTGR fuels, it is important to retain fission products within CFPs so that their release to primary coolant does not exceed an acceptable level. The behavior of TRISO-CFPs has been investigated through experiments and reactor operation. These data show excellent performance of the TRISO-CFPs when they are correctly fabricated. On the other hand, the crystalline material comprising the SiC layer has a tendency to decompose at high temperature. The transition temperatures of β-sic (as-deposited) to α-sic vary from 1600 to 2200 o C. Then the maximum fuel temperature shall not be exceed 1600 ºC at any anticipated transient to avoid fuel failure, i.e. to permit reutilization of the fuel after anticipated transients. Zirconium carbide (ZrC) is one of the transition metal carbides which are characterized by the high melting point and the thermodynamic stability etc. The CFPs with CVD-ZrC coatings have been investigated including the fabrication processes and characterization techniques developments. KEYWORDS: HTGR, Coated fuel particle, Safety design criteria, Fuel failure, SiC layer, ZrC layer I. Introduction The High Temperature Gas-cooled Reactors (HTGRs), with its inherent safety and high temperature heat supply of about 1000 o C at the exterior of the reactor, can achieve effective utilization of nuclear energy in various fields by stages. For example, HTGRs make it possible to produce hydrogen with its high temperature heat supply. Hydrogen is expected as alternative energy source for oil near future. Therefore, HTGRs are expected to contribute to the reservation of the global environment and to provide a diverse energy supply 1,2). In Japan, the Japan Atomic Energy Research Institute (JAERI) has carried out the research and development of HTGRs since 1960's 3). The construction of High Temperature Engineering Test Reactor (HTTR) of 30 MW thermal power started In December 2000, the HTTR reached full power. The HTTR has started safety demonstration tests to verify inherent safety features of the HTGR. In the HTGRs, two main fuel element concepts are IPyC layer UO 2 kernel Buffer layer SiC layer UO 2 kernel Buffer layer PyC layers SiC layer OPyC layer Fig. 1 TRISO coated fuel particle. Tel , Fax , sawa@oarai.jaeri.go.jp

2 presently in use, the spherical fuel element and the block-type fuel element. In both concepts, the high temperature heat supply and inherent safety features of the HTGRs are mainly achieved using refractory-coated fuel particles (CFPs). Current HTGR employs so-called TRISO-CFPs, where the fuel microsphere (kernel) is coated with the low-density carbon buffer, the inner isotropic high-density carbon (IPyC), the silicon carbide (SiC) and the outer isotropic high-density carbon (OPyC) layers in this order from within as shown in Fig. 1. The function of each coating layer is as follows. (1) Buffer layer The buffer layer provides free volume for stable fission gases and CO gas generated by fission and mitigates stress on other coating layers. The damage to the IPyC layer by the fuel kernel swelling and by recoil of fission fragments is also protected by the buffer layer. (2) IPyC layer The IPyC layer, the second layer, prevents corrosion of the UO 2 kernel in the SiC layer coating. Release of fission products is mitigated by this layer during irradiation. The PyC layer almost completely retains short-lived fission gases. (3) SiC layer The SiC layer is the strongest layer and is the pressure vessel against internal pressure caused by fission gases and CO gas from UO 2 kernel during irradiation. This layer also acts as the primary barrier to the release of metallic fission products, as well as fission gases and iodine. (4) OPyC layer The OPyC layer, the final layer, protects the inner SiC layer from mechanical failure during handling of CFPs. The PyC layers undergo irradiation-induced shrinkage as a result of fast neutron exposure and the OPyC layer places a compressive load on the SiC layer, which counteracts the SiC tensile stress due to internal pressure. Similar to the IPyC layer, this layer retains short-lived fission gases. The recent interest in CFP concept also includes its application outside the present experiences of HTGRs: - VHTR (Very high-temperature gas reactor) with the gas outlet temperature of 1000 o C for supplying both the electricity and the process heat for hydrogen production, as proposed in the Generation-IV International Forum. - Actinide burning in HTGR with fuel kernels consisting of high concentrations of transuranium elements. - Advanced fast reactors with the nitride fuel, which aim the improved performance, compared with the conventional LMFBR and/or the efficient actinide burning. This paper provides present status of research and development for the TRISO-CFP as well as the advanced ZrC-CFP. II. Design Principle 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. In this Chapter, the safety design criteria, which were settled for the HTTR fuel, were described as an example 4). 1. Normal Operation (1) The initial, as-fabricated, failure fraction in the coating layers of the CFPs shall be less than design limit which is determined from the viewpoint of limit of off-site exposure during normal operation. Small fractions of the particles with defective coating layers present during the fabrication process. Among several modes of defective coating layers, a defective SiC coating layer is the most harmful from the standpoint of fission product retention. In the fabrication of the first-loading fuel of the HTTR, as-fabricated failure fraction was limited less than 0.2% in terms of the sum of exposed uranium and SiC defects. The value of 0.2% was determined from the viewpoint of limit of off-site exposure during normal operation 5). (2) The CFPs shall not fail systematically considering failure mechanisms such as Pd-SiC interaction, kernel migration and internal pressure. (a) The CFPs shall be designed so as to avoid, in principle, failure considering irradiation-induced damage and chemical attack through the full service period. The additional failure fraction in the coating layers of the CFPs shall be less than design limit through the full service period. (b) The fuel compacts and the graphite sleeves shall not be broken or cracked considering thermal stress and irradiation-induced damage. The fuel compact and graphite sleeve shall not contact with each other to keep their mechanical integrity. Temperature gradients in CFPs due to extreme operating conditions lead to carbon transport from the hot side to the cold side of the IPyC layer causing a migration of the fuel kernel toward the hot side. The distance of kernel migration shall not exceed the thickness of the first buffer layer plus the second (IPyC) layer to avoid failure of the SiC layer. The interaction of palladium with SiC layer was experimentally investigated by measuring the depth of penetration of the resulting intermetallic compound into the SiC layer. The penetration depth of the Pd/SiC interaction shall not exceed the thickness of the SiC layer, because the fully-penetrating Pd/SiC interaction is thought to lead loss of fission product retention in the SiC coating layer. (3) The CFPs shall be designed so as to avoid, in principle, failure considering irradiation-induced damage and chemical attack through the full service period, that is, the additional failure fraction in the coating layers of the CFPs shall be less than 0.2% through the full service period.

3 2. Anticipated Transients The crystalline material comprising the SiC layer of the TRISO coating has a tendency to decompose at high temperature. Several studies have been performed on the phase transition of SiC at high temperatures. The transition temperatures of β-sic (as-deposited) to α-sic vary from 1600 to 2200 o C. Then the maximum fuel temperature shall not be exceed 1600 ºC at any anticipated transient to avoid fuel failure, i.e., to permit reutilization of the fuel after anticipated transients. 3. Accidents The reactor shall not be seriously damaged and sufficient cooling capacity for residual heat removal shall be maintained. For the HTTR, guidelines of core damage are determined as follows. (1) The fuel shall be maintained in the graphite block or sleeve. When the fuel is kept in the graphite block or sleeve, the fuel can be cooled by auxiliary cooling system and/or vessel cooling system. In the case of oxidization accident such as depressurization accident or water ingress accident, integrity of the bottom plate of the graphite sleeve should be examined to maintain the fuel in the graphite block or sleeve. (2) The structural integrity of the graphite support structures such as support posts shall be maintained. The core of HTTR is composed of the fuel blocks which are piled on the support posts. In the case of oxidization accident, decrease of the support post strength by oxidization also should be examined to maintain core configuration. III. Fabrication and Quality Control The investigation of fundamental characteristics of the CFPs fabrication has taken place since 35 years. Two directions for the fuel element design have been pursued, the block type in Japan and the US, and the spherical fuel element in Germany, China. The fabrication process of the HTTR fuel is introduced as an example. In Japan, high quality and production efficiency of fuel was established through a lot of R&D activities and fabrication experiences of irradiation examination samples spread over about 30 years 7). Finally, it was decided to fabricate the first-loading fuel of the HTTR by the fuel kernel process using the vibration dropping technology, the continuous four-layer coating process and optimization of the compaction conditions 8). Figure 2 depicts a flow diagram of the HTTR fuel production process. 1. Coated fuel particle The UO 2 kernels were fabricated in a gel-precipitation process. After formation of uranyl nitrate solution containing methanol and an additive, spherical droplets are produced by a vibration dropping technique. Following the drying and calcinating, reduction of the calcinated kernels to Uranyl nitrate solution UO 2 particle Coated fuel particle Overcoat particle Fuel compact Fuel rod Fuel assembly Fig. 2 Buffer (C 2 H 2 +Ar) IPyC (C 3 H 6 +Ar) SiC (CH 3 SiCl 3 +H 2 ) OPyC (C 3 H 6 +Ar) Graphite powder Binder Hot pressing Preheating Heating Graphite sleeve Graphite block Burnable poison Fuel fabrication process. UO 2 was carried out. Kernel fabrication was completed by a sintering process to produce dense UO 2 kernels. The coating layers were deposited on the kernels in a CVD process using a fluidized coater. The TRISO-coating process is divided into four coating processes for the porous PyC, IPyC, SiC and OPyC layers. The buffer and high density PyC coating layers were derived from C 2 H 2 and C 3 H 6, respectively, and the SiC layer from CH 3 SiCl 3. The amount of charged particles corresponded to 3 kg uranium per coating batch. At a desired temperature, reactants were put into the coater to produce a coating layer on the particles fluidized in the coater. After a certain time to produce the desired thickness of the layer, the reactant gas supply was replaced by argon. The coater and the CFPs were cooled down, and then the CFPs were removed from the bottom of the coater. All UO 2 kernels and TRISO-CFPs are classified by means of a vibrating table to exclude odd shape particles.

4 The as-manufactured quality of the fuel has been improved by the modification of fabrication conditions and processes. The coating failure during coating process was mainly caused by the strong mechanical shocks to the particles given by violent particle fluidization in the coater and by the unloading procedure of the particles. The coating process was improved by optimizing the mode of the particle fluidization and by developing the process without unloading and loading of the particles at the intermediate coating process. Figure 3 shows measured dimensions and distributions of as-fabricated coated fuel particles. The thickness of the coating layers was measured by optical microscopy. The stresses acting on the coating layers during irradiation depend on irradiation conditions and coating layer thicknesses. The fabrication data showed that the deviations of thickness of the PyC layers and the SiC layer were small, however, the deviation of the buffer layer thickness was relatively large. Since the internal pressure depends on the free volume in the buffer layer, an evaluation was carried out to confirm the coating layer intactness during operation 9). Frequency SiC IPyC OPyC Buffer Thickness (mm) Fig. 3 Measured dimensions and distributions of coating layers of as-fabricated CFPs of the HTTR. 2. Fuel Compact The fuel compacts of the HTTR are produced by warm-pressing of the CFPs with graphite powder. In the first step, the CFPs are overcoated by resinated graphite powder with alcohol. The resinated graphite powder is prepared by mixing electrographite powder, natural graphite powder, and phenol resin as a binder in the ratio 16:64:20, followed by grinding the mixture to powder. The aim of the overcoating is to avoid direct contact with neighboring particles in the fuel compact. The thickness of overcoating layer is about 200 µm, which is determined by the specification for the volume fraction of the CFPs in the fuel compact (30 vol%). Then the overcoated particles are warm-pressed by metal dies to form annular green fuel compacts. The final step of the compaction process is the heat-treatment of the green fuel compacts at 800 in flowing N 2 to carbonize the binder and at 1800 in vacuum to degas the fuel compacts. Ideally, the coating layers should be intact before and during irradiation. In practice, however, small fractions of the particles with defective coating layers have been present during the fabrication process. Among several modes of defective coating layers, a defective SiC coating layer is the most harmful from the standpoint of irradiation performance and metallic fission product retention 9,10). Then the fabrication process was modified to reduce the defective particle fraction during the compaction process before fabrication of the first-loading fuel of the HTTR. The compaction process was improved by optimizing the combination of the pressing temperature and the pressing speed of the overcoated particles to avoid the direct contact with neighboring particles in the fuel compact. As shown in Fig. 4, as-fabricated fuel compacts contained almost no through-coatings failed particles and few SiC-defective particles. Average through-coatings and SiC defective fractions were and respectively. In the beginning of fabrication, unexpected large SiC-failure fractions, about 3-5 particles in a fuel compact, were observed. Then, relations between the measured SiC-failure fractions and fabrication parameters, such as coating layer thickness, overcoating layer thickness, pressing speed, etc., were analyzed during fabrication. Finally, it was concluded that the SiC layer thickness should be thicker than 27 µm to avoid as-fabricated SiC-failure during the compaction process. In addition, odd-shaped overcoated particles were removed before compaction process. After these improvements, a significant SiC-failure was no longer observed during fabrication 11). Frequency 100% 80% 60% 40% 20% 0% Through-coatings failure SiC-failure Number of failed particles in a fuel compact Fig. 4 Number of through-coatings failed particles and SiC-defective particles in an as-fabricated fuel compact. of coating layers of as-fabricated CFPs of the HTTR. 3. Quality Control (1) Inspection Standards The inspection items are determined to confirm specifications which certify nuclear and thermal-hydraulic design, irradiation performance and so on. From the viewpoint of purposes, the inspection items are divided into three categories, namely (1) compulsory, (2) user s

5 requirement or optional and (3) vender s quality control. Table 1 shows major specifications applied in the fabrication of the first-loading fuel of the HTTR 4,11). Table 1 Major specifications of the first-loading-fuel of the HTTR. Fuel kernel Diameter (µm) 600±55 Density (g/cm 3 ) 10.63±0.26 Impurity (ppm EBC *1 ) 3 *1 EBC: Equivalent Boron Content Coating layers Buffer layer thickness (µm) 60±12 IPyC layer thickness (µm) 30±6 SiC layer thickness (µm) OPyC layer thickness (µm) 45±6 Buffer layer density (g/cm 3 ) 1.10±0.10 IPyC layer density (g/cm 3 ) SiC layer density (g/cm 3 ) 3.20 OPyC layer density (g/cm 3 ) OPTAF *2 of IPyC and OPyC layers 1.03 *2 OPTAF: Optical Anisotropy Factor Coated fuel particle Diameter (µm) Sphericity 1.2 Fuel compact Coated fuel particles packing fraction (vol%) 30±3 Impurity (ppm EBC *1 ) 5 Exposed uranium fraction SiC-failure fraction Outer diameter (mm) 26.0±0.1 Inner diameter (mm) 10.0±0.1 Height (mm) 39.0±0.5 Matrix density (g/cm 3 ) 1.70±0.05 Compressive strength (N) 4900 Fuel rod Uranium content (g U) ±5.66 Total length (mm) 577±0.5 Fuel compact stack length (mm) 544 (a) Compulsory The following items are selected mainly to certify irradiation performance of the fuel. The corresponding acceptance (fail/pass) criteria are examples applied to the HTTR fuel. Sphericity of the fuel kernel and CFPs is inspected to prevent locally high stress on the coating layers during irradiation. Sphericity should be less than 1.2 in 95% confidence limit O/U ratio of fuel kernel should be almost 2.0 to assure irradiation performance. O/U ratio is inspected as a form of fuel kernel and the fuel compact. Coating layer density and thickness are important to confirm integrity of CFP during irradiation. OPTAF in high density PyC layer should be less than 1.04 to prevent excessive deformation by fast neutron irradiation. The failure fractions (exposed uranium and SiC-failure fractions) are measured to assure fission products release behavior. Since fission products are retained by the coating layers, the dominant sources of fission product release during normal operation are failed particles and contaminated uranium in the fuel compact matrix. The through-coatings failed particle and uranium contaminated fuel compact matrix determine the fission gas content in the primary coolant at the beginning of operation. On the other hand, since the as-fabricated SiC-failed particle does not have the mechanically strongest coating layer, SiC, the as-fabricated SiC-failed particle is predicted to result in through-coatings failed particle due to internal pressure during operation. It means that the as-fabricated SiC-failure fraction determines the additional through-coatings failure fraction, i.e., fission gas content in the primary coolant during operation. Fuel compact matrix density is inspected to assure irradiation performance. (b) Optional The following items are determined to be specified mainly by user s requirements depending on the design. 235 U enrichment of the fuel kernel is inspected to certify nuclear design. Kernel diameter and density are inspected to certify nuclear design. Impurities in the fuel kernel and in the fuel compact are inspected to certify nuclear design. Some elements should be limited to prevent their reaction with coating layers during irradiation. The diameter of the CFP is inspected to certify nuclear design. Appearance of the CFP and the fuel compact are inspected to confirm irradiation performance. Cross sections of the CFP and the fuel compact are also inspected to examine the coating process. Uranium content in a fuel compact is measured to certify nuclear design. Packing fraction of CFPs in a fuel compact is measured to certify nuclear design. Fuel compact dimension is measured to certify thermal-hydraulic design. (c) Vender s quality control Strength of CFP is measured to confirm validity of the coating process. 235 U enrichment of the fuel compact is inspected by printed serial number on the fuel compact. Properties of graphite powder and binder are examined to assure property of the fuel compact matrix. Strength of the fuel compact is measured to confirm validity of the compaction process.

6 (2) Inspection Methods 235 U enrichment and uranium content in a fuel compact are measured by γ-ray spectro analysis. Diameter and sphericity of fuel kernels and the CFPs are measured by optical particle size analysis. Density of fuel kernel is measured by mercury substitution method. O/U ratio is measured by oxidation and weighing. Impurities are measured by emission spectro analysis etc. Coating layer thickness and density are measured by X-ray radiograph and solvent substitution or sink-float methods, respectively. OPTAF of the PyC layers is measured by polarization photometer. Appearance and ceramographed cross section of the CFPs and the fuel compacts are visually inspected. Strength of the CFPs is measured by point crushing. Density, impurities, grain size, water content in graphite powder and contents, ash, melting point, impurities in binder are inspected. Exposed uranium fraction and SiC-failure fraction are measured by deconsolidation / acid leaching and burn / acid leaching, respectively. Packing fraction of CFPs and matrix density in a fuel compact is measured by weighing and calculation. Dimensions of the fuel compacts are measured by micrometer. Marking on the fuel compact is inspected by visual observation. Compressive strength of the fuel compact is also measured. Uranium content in the fuel rods is confirmed by calculation of measured fuel compacts. Total length of fuel rod is measured. Number of fuel compacts in a fuel rod is checked by assembling record. Stack length of the fuel compacts is checked by calculation. Surface contamination of the fuel rod is measured by smear method. Appearance of the fuel rod and the fuel assembly is visually observed. 235 U enrichment is checked by fuel compacts and graphite sleeve marking. Components in the fuel rod and the fuel assembly are checked by assembling record. Uranium content in the fuel assembly is calculated. Weight of the fuel assembly is also measured. (3) Sampling Rate Since the HTGR core contains enormous number of CFPs (for example, 30 MWt HTTR core contains about 10 9 CFPs), it is impossible to inspect all fuel products in fabrication process. Therefore it is necessary to establish sampling method. Inspections of the HTGR fuel are carried out in a unit of inspection lot to certificate fuel quality. An inspection lot is defined as a product group with uniform quality. The arrangement of the inspection lot is shown in Fig. 5. A raw uranium powder lot is an assemblage of same enriched uranium material which is mixed by solution. A kernel lot and a CFP lot are arranged by mixing several fabrication batches uniformly in each process stage. A compact lot means a product assemblage heat-treated at the same time. The sampling rate is determined by considering the uniformity of inspected data. Three categories are basically classified as (a) small-scattering data, (b) medium-scattering data and (c) large-scattering data. One Fig. 5 Fuel kernel lot TRISO-CFP lot Baking batch Fuel compact lot Uranyl nitrate batch (Enriched uranium batch) Fuel kernel batch Fuel kernel lot Coating batch TRISO CFP lot Overcoating batch Hot pressing batch Arrangement of inspection lots of the HTTR. sample is measured from an inspection lot for the small-scattering data. For the inspection lot with medium-scattering data, three samples are measured and all of them should satisfy criterion. For the large-scattering data, measured data should meet statistically required, basically with 95% confidence, criterion. IV. Fuel Performance 1. Normal Operating Condition Operating experience from HTGRs comprised all aspects rising from fuel fabrication, irradiation testing, and performance modeling to in-reactor chemistry surveillance, fission product release and transport measurement and modeling and reactor component decontamination. As a recent experience, the fuel performance in the HTTR operation is introduced below 12). During the rise-to-power test of the HTTR, which started in September 1999, primary coolant sampling measurements were carried out to measure fission gas concentration 13). The concentrations of fission gas nuclides of 85m Kr, 87 Kr, 88 Kr, 133 Xe, 135 Xe, 135m Xe and 138 Xe were less than 0.1 MBq/m 3. The fractional releases, (R/B)s, of fission gases were calculated based on the measured concentrations. Figure 6 shows (R/B)s of 88 Kr as a function of the reactor

7 power. The (R/B) values are as low as up to 60% of the reactor power, then increase to at full power operation. This result indicates that in lower reactor power, the fission gas release mechanism is recoil from the contaminated uranium in the fuel compact matrix. Beyond 60% of the reactor power, fractional release increases because diffusion release becomes main release mechanism. (R/B) of Kr-88 1E-7 1E-8 1E-9 1E Reactor power (%) Fig. 6 Measured fractional releases during rise-to-power test of the HTTR. The capsule irradiation tests and the post-irradiation tests were also carried out to investigate fuel behavior under higher burnup condition 14). 2. Accident Conditions Estimations of CFP behavior under accident conditions have been carried out to reveal important points for the HTGR design. Core heat-up, oxidization and reactivity-initiated accidents (RIA) are considered to be important because the CFPs might fail under these accident conditions, i.e., high temperature and/or oxidation. In JAERI, a model to predict the ultimate failure of CFPs was proposed 15). The feature of the model allows treatment of the statistical variation of the number of particles and a thermodynamic estimation of the stoichiometry of irradiated UO 2 kernels and the equilibrium CO pressures. Also SiC strength is reduced by the porosity which is caused by thermal decomposition of the layer and is preferentially developed at grain boundaries of the SiC. For oxidation accident condition, a fuel failure model based on thermodynamic analysis, which showed active-to-passive transitions of oxidation of SiC layer, was developed 16). Also, RIA tests were carried out in JAERI 17). The coating failure fraction increased with the amount of energy deposition. In this condition, the coating failure may have been caused by the internal high pressure due to UO 2 evaporation. To clarify the mechanisms of the coating failure, additional tests simulating the reactor accident condition (moderate condition) are needed. VI. Advanced Fuel 18) 1. Limitation of TRISO-CFP With increasing fast neutron dose, the PyC develops gas permeability. Intactness of the IPyC layer is crucial in keeping the integrity of the SiC layer. If the IPyC fails or develops gas permeability, the SiC layer will be lost by forming volatile SiO. SiC is a covalent-bonded material. The dangling bonds at the SiC grain surface keep a rather rigid tetrahedral angle, resulting in high grain-boundary energy, γ gb, reaching to ~1.91γ sv (γ sv : surface energy). Generally speaking, the sintering of discreet particles is driven by minimization of the total free energies by increasing the grain-boundary area and reducing the surface area. The high grain-boundary energy and the low diffusion coefficients of Si and C in SiC do not favor the further modification of microstructure by heat treatments after CVD. In addition, the CVD-SiC contains little impurity which may assist the modification of the grain boundary structure. Thus, the as-deposited microstructure of the CVD-SiC does not change significantly by annealing. The annealing temperature is limited to 1800 o C. Higher temperatures should result in a porous structure due to the β-to-α transformation and the dissociation of SiC. The PyC layers will also develop anisotropy above 1900 C. The anisotropy in PyC is deleterious to their irradiation behavior. The CVD-SiC, thus, would have a spectrum of grain boundary characters from nearly open to closed. Besides, its microstructure cannot be significantly altered from the as-deposited condition. This may explain within-batch and batch-to-batch variations of apparent diffusion coefficients of metal fission products in the SiC coating, which significantly affect the CPF performance above the operating temperatures of HTGR. Among metal fission products, palladium is known to corrode the SiC layer. The TRISO-CFPs were heated in Pd-impregnated graphite powder at 1400 C for 9 days. While the Pd source is inside of the SiC layer, it was at the outside in this simulation. A very rapid nodular penetration was observed. Nodular feature of the reaction may be related to the high grain-boundary energy. Since either the bonding with carbon or the surface oxidation stabilizes the surface of the SiC layer, the grain boundary may give nucleation points for the palladium suicide formation. It is suspected that the corrosion is controlled by the supply of palladium from the fuel kernel to the SiC. Neither the diffusion of palladium through IPyC nor the chemical reaction of palladium with SiC is considered the rate-determining step. The experience on the conventional TRISO-CFP, as summarized above, revealed its limitation as to its further applications: (1) It cannot be used at higher temperatures than the currently designed HTGR. (2) Careful fuel designing and testing programs are needed in its application for burning plutonium and the other transuranium elements, since the fission-yields of

8 palladium from transuranium nuclides are larger by about ten-fold than that from 235 U. (3) Under the fast neutron environments, the PyC layers would be damaged early in their life. Significant improvement in points (1) and (2) can be achieved by replacing SiC with ZrC as described below. 2. ZrC-CFP ZrC is one of the transition metal carbides which are characterized by (1) the good-compatibility with structural metals, (2) the high melting point and the thermodynamic stability, (3) the wear resistance etc. The bonding in the transition metal carbides has a covalent character, but, being more or less metallic, it is not being so directional as those in the truly covalent materials such as diamond and SiC. The grain boundary energy is considered not so high as that in SiC, giving more homogeneous polycrystalline structures in the CVD-ZrC in contrast to those of SiC. CFPs with CVD-ZrC coatings have been developed at the JAERI since early 1970s. Studies include (1) the fabrication processes and characterization techniques developments, (2) the out-of-pile and in-pile performance tests and (3) the post-irradiation heating tests simulating accident conditions. Initially, coated UO 2 particles with the Zirconium Carballoy (ZrC+C alloy) coatings without an outer protective PyC Layer were tested. The results on the Zirconium-Carballoy CPF are summarized in Ref 19). Although they proved to be less retentive of metal fission products, they showed excellent resistance to chemical attacks by fission products. The present design of ZrC-CFPs, which is based on the TRISO-type concept, where ZrC replaces SiC, evolved after these experiences. The ZrC coating is produced by pyrolytic reaction of ZrBr 4, CH 4 and H 4 in a spouted-bed coater at about 1500 o C. Propylene can be used instead of methane. ZrBr 4 is preferred to ZrCl 4, since, in the JAERI process, the zirconium halide is produced inside the coater beneath the spouting nozzle by reacting halogen with the zirconium sponge. The reaction of excess chlorine with hydrogen is a potential explosive danger. Since handling of the zirconium halides is difficult due to their highly hygroscopic nature, JAERI adopts the in-situ generation of ZrBr4. By adjusting the coating condition, one may obtain the stoichiometric ZrC layer. The as-deposited microstructure completely vanishes by annealing at 1800 o C. The microstructure of the as-deposited ZrC is predominantly columnar, but it becomes equiaxed by the annealing. Larger thermal expansivity (CTE) should result in as-deposited tensile stress, due to a mismatch with that of PyC, whose CTE lies between those of SiC and ZrC. In addition, thermal creep of ZrC is considered controlled by carbon self-diffusion at lower temperatures below 0.5Tm (Tm: melting point in Kelvin), as is probably the case in TiC. One should bear in mind these drawbacks of ZrC in designing the ZrC-CFP. The outer PyC layer is necessary to give mechanical protection to the ZrC layer during fabrication and handling. Considering the high chemical stability of ZrC, the IPyC layer may be thinner than the conventional TRISO-CFP. The specification of the latter should be determined with consideration of preventing the contamination by bromine during the chemical vapor deposition of ZrC. In view of the residual stress due to CTE mismatch between ZrC and IPyC, a thinner IPyC layer is considered preferable within the above limitation. The breached or gas-permeable IPyC, however, causes the deterioration of ZrC due to the reaction with CO for prolonged heating at 2000 o C. One notable advantage of the ZrC layer is its virtual immunity to the attacks by fission-product Pd. The irradiation testing at o C and the post-irradiation heating confirmed the immunity of the ZrC layer against the palladium attack. The melting point of ZrC alone is 3420 o C, but it eutecticaily melts with carbon at 2850 o C. The ZrC-CFPs did not fail until ~6000 sec was reached at 2400 o C, while a few percent of the conventional TRISO-CFPs failed already by 2200 o C, and almost 100% instantaneously at 2400 o C 20). It also proved that the ZrC layer can sustain a large strain of ~20% before it loses gas tightness. The retention of metal fission products by the ZrC layer at temperatures above 1600 o C has been studied. Mianto et al., have demonstrated that the fractional release of 137 Cs is below 10-3 at 1800 o C K after 3000h 21). However, its retention of 106 Ru was inferior to SiC, the implication of which has to be analyzed for its application in VHTR. The diffusion coefficient of 106 Ru in ZrC is about the same as that of 137 Cs in SiC. The retention of the other metal fission products such as barium, silver and promethium appears to be better than SiC 22). Fig. 7 Installed ZrC coater in JAERI.

9 The apparent drawback of the ZrC-CFP is that ZrC does not withstand the oxidation in a massive air-ingress accident, although it is highly hypothetical in the modern HTGR designs. For further investigation, JAERI has constructed a new coater of 100g-scale for ZrC coating test as shown in Fig. 7 23). The following investigations will be carried out to establish ZrC-CFP technology. (1) Optimization of deposition condition to obtain stoichiomentric ZrC is needed for commercial-scale (> 3kg batch) coater. (2) ZrC behavior should be examined including grain/crystal growth under high burnup (3) Study on oxidation resistance and countermeasures against ZrC-oxidation should be developed. VII. Conclusions The recent interest in CFP concept includes its application outside the present experience of HTGR: - VHTR with the gas outlet temperature of 1000 o C for supplying both the electricity and the process heat for hydrogen production. - Actinide burning in HTGR with fuel kernels consisting of high concentrations of transuranium elements. - Advanced fast reactors with the nitride fuel, which aim the improved performance, compared with the conventional LMFBR and/or the efficient actinide burning. This paper provided present status of the TRISO-CFP as well as the advanced ZrC-CFP. At first, the safety design criteria for normal operation, anticipated transients and accidents for the TRISO-CFP were introduced. Then fabrication and quality control methods, which are special features of CFPs, were described The fuel performance in normal operation and accident conditions was summarized for TRISO-CFP. In the final chapter of this paper showed present status of ZrC-CFP development. 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