Outer Pyrolytic Carbon Layer Research Articles

Processing of fully ceramic microencapsulated fuels with a small amount of additives by hot-pressing

The effects of processing parameters on the properties of SiC matrices and fully ceramic microencapsulated (FCM) fuel pellets were investigated with a small amount of additives by hot pressing. The increases in the applied pressure as well as the sintering time were beneficial in improving the thermal conductivity of the SiC matrix and FCM pellets. Tristructural isotropic (TRISO) particles without an outer pyrolytic carbon layer are not suitable for processing FCM pellets because of the corrosion of the chemical-vapor-deposited SiC layer by the additive-derived liquid phase during sintering. The FCM pellet sintered with 2.71 wt% AlN–Y2O3–Sc2O3–MgO additives at 1870 °C for 4 h under 30 MPa in argon exhibited an excellent thermal conductivity (61.7 Wm−1 K−1 at a TRISO particle content of 43.3 %), owing to the (i) reduced amount of additives, (ii) high sintered density (residual porosity ∼ 0.4 %), and (iii) highly thermally conductive SiC matrix (105.0 Wm−1 K−1).

Modeling fission product diffusion in TRISO fuel particles with BISON

The diffusion of fission products in intact TRISO particles depends on particle geometry, fission product source rates, time, temperature, and temperature-dependent diffusion coefficients. Simulating this diffusion process requires models for source rates and diffusion coefficients, plus a computation of the temperature field if not prescribed. In addition, the simulation quality depends on the discretization of the geometry, appropriate time stepping, and the accuracy of the solution method.In this paper, we explore the simulation of fission product diffusion in TRISO fuel particles using the finite element method via the fuel performance code BISON. Recent material model development has occurred in BISON for each material present in tri-structural isotropic (TRISO) fuel particles: the buffer, inner pyrolytic carbon, silicon carbide, and outer pyrolytic carbon layers, as well as the fuel kernel. Also, new mesh generation and fission product release fraction capabilities have been added.Diffusion capabilities are shown to converge to the correct solution via formal verification tests. A large number of code benchmarking problems are also given, with good results, showing that BISON’s computed release fractions closely match those of other software tools. Finally, a significant validation effort is detailed in which fission product release, measured as part of the AGR-1 capsule experiments, is compared to BISON outputs. BISON outputs compare favorably to the experimental data and to PARFUME results.

A flexible carbon nanotube-pyrolytic carbon sandwich paper with a stable structure and high heat-dissipation capacity

A thin layer of pyrolytic carbon (PyC) was deposited on a carbon nanotube paper (CNP) by chemical vapor deposition from methane to improve its structural stability while maintaining its excellent mechanical and thermal conductivity performance. The resulting material consisted of layers of PyC carbon on the outer surfaces, a layer of bare CNTs in the center, and layers of CNTs infiltrated with PyC between the outer and inner layers. The resulting sandwich material could be cut into any size without breaking the surrounding material. The CNTs in the center layer were stretched after the CNP was bent 500 times around a cylindrical die, leading to an increase of tensile strength from 8.58 to 11.41 MPa. The outer PyC layers and the PyC-infiltrated layers retained their integrity. The thermal diffusivity and heat-dissipation capacity of the sandwich paper remain almost the same before and after bending, which is attributed to the undamaged CNTs and PyC layers. This paper has potential applications as a lightweight and flexible heat-dissipation material at high temperatures.

Microstructure and thermal conductivity of fully ceramic microencapsulated fuel fabricated by spark plasma sintering

AbstractFully ceramic microencapsulated pellet (FCM), consisting of tristructural isotropic (TRISO) particles embedded in silicon carbide (SiC) matrix, was fabricated using spark plasma sintering. The parameters affecting the densification of SiC matrix were first investigated, and then FCM pellets were prepared using TRISO particles with/without outer pyrolytic carbon (OPyC) layer. Effects of thermal exposure on the TRISO particles during SPS were evaluated. In addition, the thermal condcutvitities of FCM pellet, as well as the SiC matrix, were measured using laser flash. It was revealed that the TRISO particles with OPyC layers significantly lower the thermal conductivity of FCM pellet. Based on Maxwell‐Eucken model, the predicted effective thermal conductivities of TRISO particles with/without OPyC layers were 14.4 W/m K and 25.2 W/m K, respectively. Finite elements simulation indicated that the SiC layer in TRISO particle plays a dominant role on the thermal conductivity of FCM. The presence of OPyC layers would generate gaps/porous SiC near the interface and resist the heat flows, leading to a lower thermal conductivity of FCM.

Fission product release and survivability of UN-kernel LWR TRISO fuel

A thermomechanical assessment of the LWR application of TRISO fuel with UN kernels was performed. Fission product release under operational and transient temperature conditions was determined by extrapolation from fission product recoil calculations and limited data from irradiated UN pellets. Both fission recoil and diffusive release were considered and internal particle pressures computed for both 650 and 800μm diameter kernels as a function of buffer layer thickness. These pressures were used in conjunction with a finite element program to compute the radial and tangential stresses generated within a TRISO particle undergoing burnup. Creep and swelling of the inner and outer pyrolytic carbon layers were included in the analyses. A measure of reliability of the TRISO particle was obtained by computing the probability of survival of the SiC barrier layer and the maximum tensile stress generated in the pyrolytic carbon layers from internal pressure and thermomechanics of the layers. These reliability estimates were obtained as functions of the kernel diameter, buffer layer thickness, and pyrolytic carbon layer thickness. The value of the probability of survival at the end of irradiation was inversely proportional to the maximum pressure.

Encapsulation of TRISO particle fuel in durable soda-lime-silicate glasses

Tri-Structural Isotropic (TRISO) coated particle-fuel is a key component in designs for future high temperature nuclear reactors. This study investigated the suitability of three soda lime silicate glass compositions, for the encapsulation of simulant TRISO particle fuel. A cold press and sinter (CPS) methodology was employed to produce TRISO particle–glass composites. Composites produced were determined to have an aqueous durability, fracture toughness and Vickers’ hardness comparable to glasses currently employed for the disposal of high level nuclear wastes. Sintering at 700°C for 30min was found to remove all interconnected porosity from the composite bodies and oxidation of the outer pyrolytic carbon layer during sintering was prevented by processing under a 5% H2/N2 atmosphere. However, the outer pyrolytic carbon layer was not effectively wetted by the encapsulating glass matrix. The aqueous durability of the TRISO particle–glass composites was investigated using PCT and MCC-1 tests combined with geochemical modelling. It was found that durability was dependent on silicate and calcium solution saturation. This study provides significant advancements in the preparation of TRISO particle encapsulant waste forms. The potential for the use of non-borosilicate sintered glass composites for TRISO particle encapsulation has been confirmed, although further refinements are required.

SEM analysis of the microstructure of the layers in triple‐coated isotropic (TRISO) particles

AbstractOne of the new inherently safe designs in nuclear reactors is the pebble bed modular reactor (PBMR). In this high temperature reactor the fuel is in the form of small multilayered spheres called triple‐coated isotropic (TRISO) particles. These coated particles consist of a UO2 kernel surrounded by four layers, which are all chemical vapour deposition (CVD) deposited. The layers are a buffer graphite layer next to the kernel followed by an inner pyrolytic carbon layer (IPyC), a SiC layer and finally an outer pyrolytic carbon layer (OPyC). This investigation focuses on the microstructure of the particles specifically in relation to their functions. Because the coated particles operate in a very harsh environment (e.g. high temperatures with repeated rapid cooling and heating‐up times, high flux neutron irradiation, containment of the radioactive fission products) the microstructure and composition of the layers are extremely important to preserve the integrity of the particles. In this investigation the microstructures of the different coated layers are investigated by high‐resolution SEM. Copyright © 2010 John Wiley & Sons, Ltd.

Results of Tests to Demonstrate a 6-in.-Diameter Coater for Production of TRISO-Coated Particles for Advanced Gas Reactor Experiments

The next generation nuclear plant (NGNP)/advanced gas reactor (AGR) fuel development and qualification program includes a series of irradiation experiments in Idaho National Laboratory’s advanced test reactor. Tristructural isotropic (TRISO)-coated particles for the first AGR experiment, AGR-1, were produced at Oak Ridge National Laboratory (ORNL) in a 2-in.(5-cm)-diameter coater. A requirement of the NGNP/AGR program is to produce coated particles for later experiments in coaters more representative of industrial scale. Toward this end, tests have been performed by Babcock and Wilcox (Lynchburg, VA) in a 6-in.(15-cm)-diameter coater. These tests have led to successful fabrication of particles for the second AGR experiment, AGR-2. While a thorough study of how coating parameters affect particle properties was not the goal of these tests, the test data obtained provide insight into process parameter/coated particle property relationships. Most relationships for the 6-in.-diameter coater followed trends found with the ORNL 2-in. coater, in spite of differences in coater design and bed hydrodynamics. For example, the key coating parameters affecting pyrocarbon anisotropy were coater temperature, coating gas fraction, total gas flow rate, and kernel charge size. Anisotropy of the outer pyrolytic carbon layer also strongly correlates with coater differential pressure. In an effort to reduce the total particle fabrication run time, silicon carbide (SiC) was deposited with methyltrichlorosilane (MTS) concentrations up to 3 mol %. Using only hydrogen as the fluidizing gas, the high concentration MTS tests resulted in particles with lower than desired SiC densities. However, when hydrogen was partially replaced with argon, high SiC densities were achieved with the high MTS gas fraction.

Fuel Particle Failure Probabilities and Material Strengths Determined from Crush Test Data

The prototypical nuclear-fuel of the New Production Modular High-Temperature Gas-Cooled Reactor (NP-MHTGR) consists of spherical TRISO-coated particles suspended in graphite cylinders. The coating layers surrounding the fuel kernels consist of pyrolytic carbon layers and a silicon carbide (SiC) layer. These coating layers act as a pressure vessel that retains fission product gases. The structural integrity of this pressure vessel relies on the strength of the SiC layer. If the SiC fractures because of the internal pressure loading, then the particle fails. Results obtained from a series of crush tests on unirradiated fuel particles were used to estimate the strength of the SiC layer for internal pressure loading. The SiC strength for a particle was defined in terms of the maximum stress level in the SiC layer at which the particle failed. The transformations between the test results, which involve compressive crushing loads, and the internal pressure loadings experienced in NP-MHTGR reactor conditions were made utilizing the Weibull statistical theory. The test results were also used to estimate failure probabilities for fuel particles that are lacking an outer pyrolytic carbon layer (OPyC). The transformation from the crush test failure data to equivalent SiC strengths under an internal pressure load allowed formore » a direct comparison in strengths between fully coated particles and particles that lack the OPyC layer. In the latter case, the OPyC has typically been removed through a burn-back process. Results showed that strengths for fully coated particles are higher than for burned-back particles. Whether this is attributable to an actual loss of strength because of the burn-back process or is an artifact of the testing process is a subject for further study. Other effects on particle strength measured by making similar comparison were particle compacting and particle flaws (large gold spots). These generally did not have a significant effect on SiC strengths.« less

Analytical solution for stresses in TRISO-coated particles

A closed form solution is presented for stresses in a three-layer spherical pressure vessel representative of the TRISO-coated particles in a New Production Modular High Temperature Gas-Cooled Reactor. The inner and outer layers are pyrolytic carbons that undergo creep and anisotropic swelling behavior during neutron irradiation of the particle, while the central layer is a silicon carbide that is modeled as an isotropic elastic medium. The three-layer pressure vessel is loaded by an internal fission gas pressure that builds up during irradiation and by a constant external ambient pressure. The three-layer solution is also adapted to two-layer shells where either the inner or outer pyrolytic carbon layer is absent. The closed form solution is well suited to determining particle failure probabilities using the Monte Carlo method because of its ease of programming and speed of execution versus finite difference or finite element method solutions.

The oxidation behavior of ZrC coating and powder studied by laser Raman spectroscopy and X-ray diffraction

Zirconium carbide is one of the most refractory materials with low diffusion rate and good nuclear properties. Application as a high-temperature nuclear material is promising, and we have been developing a ZrC-coated particle fuel for high-temperature gas-cooled reactors [I]. In the ZrC-coated fuel. the ZrC layer is covered by a high-density pyrolytic carbon (PyC) layer. For the quality inspection of the ZrC layer. one has to remove the PyC layer without damaging the former. This is readily done by plasma oxidation [2]. but we need confirmation that the ZrC coating is really unaffected by the oxygen plasma. Knowledge on the oxidation behavior in air is also essential for the nuclear applications regarding the accident analyses. Bartlett et al. [3] have reported the oxidation kinetics of ZrC between 723 and X53 K in oxygen. The oxidation at temperatures ranging from 1130-2160 K has also been studied (41. In those studies, the formation of monoclinic ZrO, was shown. However, the character of oxide as a function of oxidation condition has not been studied in detail. In the following. we will report the effect of plasma oxidation as well as air oxidation of ZrC coating and powder. The laser Raman spectroscopy and the X-ray diffraction were applied to the characterization of oxidation products. It was found that (I) ZrC is stable in the low-pressure oxygen plasma. (2) crystal modification of ZrO, in the air oxidation is affected by the powder particle size, and (3) the laser Raman spectroscopy compared with the X-ray diffraction provides a better method to detect the oxide formation at very early stages, while the X-ray diffraction proved to give a clearer picture of crystal modifications of ZrO,. particularly when the two modifications. monoclinic and trtragonal, were coexisting. The ZrC coating used in this study was prepared by chemical vapor deposition on alumina kernels 151. The ZrC powders had two different particle sizes. 2 pm (powder A) and 74 nm (powder B). The oxidation conditions are summarized in table 1. Plasma oxidation was carried out in an RF-coupled plasma with oxygen at 0.05 Torr at room temperature. The outer PyC layer over the ZrC was easily removed at the early stage of the plasma oxidation of about 8 min. The complete removal of the PyC was judged from the emission measurement [2]. Air oxidation was made in a muffle furnace. The sample was kept at a given temperature for 60 min. The Raman spectra were obtained with a JASCO NR-100 spectrometer using the 514.5 nm line of an

Properties Influencing High-Temperature Gas-Cooled Reactor Coated Fuel Particle Performance

Properties affecting the irradiation performance of outer pyrolytic carbon (PyC) layers on Triso- and Biso-coated fuel particles were studied. Irradiation temperatures were 1000 to 1500/sup 0/C (1273 to 1773 K). Fast-neutron fluences reached 12.4 x 10/sup 25/ n/m/sup 2/ (E greater than 29 fJ)/sub HTGR/, which is 55 percent beyond the large high-temperature gas-cooled reactor peak design exposure of 8.0 x 10/sup 25/ n/m/sup 2/. Coatings with densities between 1.85 and 1.95 Mg/m/sup 3/ and mean optical anisotropy values of equal to or less than 1.03 (BAF/sub 0/ units) exhibited the best irradiation performance on Triso particles. For Biso particles, it is necessary to deposit the outer layer at coating rates between 3 and 8 ..mu..m/min and with densities equal to or greater than 1.84 Mg/m/sup 3/ to produce coatings impermeable to fission gases after irradiation. Data from fuel rod tests show that it is important to limit the degree of surface-connected porosity of the outer PyC layer and the amount of binder phase in the matrix to prevent coating failures resulting from coating-matrix interactions.