Coated Particles Research Articles

Multi‐scale study of fluidized bed‐chemical vapour deposition process in nuclear fuel coated particle fabrication for high‐temperature gas‐cooled reactor: A review

AbstractFluidized bed‐chemical vapour deposition (FB‐CVD) is a kind of key technology used widely in many application fields, such as semiconductors, nuclear energy, energy storage, and catalysts. In recent years, it has drawn much attention in the preparation of nuclear fuel coated particles (CP). It also has long played a crucial role in the preparation of high‐temperature gas‐cooled reactor (HTGR) fuel pebbles. The multi‐scale study of FB‐CVD technology has paid attention to the industrial fabrication of nuclear fuel particles at a large scale. In this paper, the recent FB‐CVD studies of different application fields are summarized first. Then, the recent works of our group in the field of FB‐CVD process in nuclear fuel particle fabrication are summarized. The FB‐CVD process in nuclear fuel particle fabrication and the multi‐scale study of the FB‐CVD process are overviewed in detail. Molecular dynamics (MD) simulation is used to study the CVD process of preparing the coating layer at the micro‐scale. Computational fluid dynamics–discrete element model (CFD‐DEM) simulation is used to study the high‐density particle fluidization, mixing particle fluidization, and particle coating process at the particle scale. Process simulation is used to study the entire FB‐CVD production line at the macro scale. Finally, the great application potential of the multi‐scale coupling study of the FB‐CVD process in the industrial fabrication of nuclear fuel particles is revealed. This paper is helpful to develop the academic research field of fluidized beds. It also has inspiration and reference significance for the expansion of other industrial applications of FB‐CVD.

Design of taste-masked swellable drug particles using dry-coating technology with mechanical curing

A novel dry coating technique for fine particles that does not require any liquids has been developed. Swellable ordered-mixed drug particles (Swell-OM-spheres, SOS), using a modified starch as the core particle and a drug coating layer have been previously developed. In the present work, SOS particles were further processed to generate 100-μm taste-masking particles using an all dry coating processes. SOS particles were coated with a gastric-soluble powder using a mechanical powder processor. The coated particles (CPs) were subsequently heated while rotating in the same powder processor, completing film formation by a process termed dynamic curing. As a control, conventional film formation (static curing) was performed using a drying oven. The CPs obtained by these two curing processes had distinct appearances, but exhibited equivalent dissolution suppression effects in a medium at pH 6.8 (the pH of the oral cavity). The suppression effect was further improved by adding a plasticizer to the coating powder, even though a lower heating temperature was required. Orally disintegrating tablets incorporating these CPs exhibited excellent taste-masking performance, i.e., suppressing taste in saliva while accelerating dissolution in gastric juice. The dissolution behavior indicated that the CPs can provide an ON/OFF switching function in drug release.

Anisotropy measurement of pyrolytic carbon layers of coated particles

Equipment at the National Research Center Kurchatov Institute intended for the anisotropy determination of pyrolytic carbon layers in coated particles (CPs) of the GT-MGR reactor is tested and calibrated. The dependence of the anisotropy coefficient on the size of the measurement region is investigated. The results of measuring the optical anisotropy factor (OPTAF) for an aluminum mirror, rutile crystal, and available CP samples with the known characteristics measured previously using ORNL equipment (United States) are presented. In addition, measurements of CP samples prepared at VNIINM are performed. A strong dependence of the data on the preparation quality of metallographic sections is found. Our investigations allow us to make the conclusion on the working capacity of the existing equipment for measuring the anisotropy of pyrolytic carbon CP coatings using the equipment at the Kurchatov Institute with the relative error of about 1%. It is shown that the elimination of the errors caused by the stochastic fluctuations in a measuring path by mathematical processing of the signal allows us to decrease the relative error of OPTAF measurements to ~0.3%.

Assessment of coated-particle failure with the IMGA device

The coating layers of modern TRISO (tri-structural isotropic) high-temperature reactor fuel provide a barrier against the release of fission products both in normal operation and off-normal conditions. Experimental simulations of D-LOFC (depressurization and loss of forced circulation) accidents up to 1600°C have demonstrated a very low failure fraction of coated particles (CPs). In most instances, not a single particle out of the up to 15,000 CPs contained in a fuel pebble suffered mechanical failure, and all observed fission product release was due to diffusion through intact coating layers. When fuel is irradiated at temperatures well above 1000°C or when accident tests are pushed to more extreme temperatures, single CPs may fail. To selectively examine the defective particles, fuel elements must be deconsolidated and the large number of CPs contained in them analysed in an automated manner. For this purpose, an irradiated-microsphere gamma analyser (IMGA) has been developed at JRC-ITU. In this device, particles are first singularized and then assessed by gamma spectroscopy. The activity ratio of selected fission product pairs with different mobility can be used to assess whether or not a particle has suffered mechanical failure and to separate defective from intact CPs.

Effects of phosphorous-doping and high temperature annealing on CVD grown 3C-SiC

The integrity and property behavior of the SiC layer of the Tri-isotropic (TRISO) coated particle (CP) for high temperature reactors (HTR) are very important as the SiC layer is the main barrier for gaseous and metallic fission product release. This study describes the work done on un-irradiated SiC samples prepared with varying phosphorus levels to simulate the presence of phosphorus due to transmutation. 30Si transmutes to phosphorous (31P) and other transmutation products during irradiation, which may affect the integrity of the SiC layer. The P-doping levels of the SiC samples used in this study cover the range from 1.1×1015 to 1.2×1019atom/cm3 and are therefore relevant to the PBMR operating conditions. Annealing from 1000°C to 2100°C was performed to study the possible changes in nanostructures and various properties due to temperature. Characterization results by X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), are reported in this article. As grain boundary diffusion is identified as a possible mechanism by which 110mAg, one of the fission activation products, might be released through intact SiC layer, grain size measurements is also included in this study. Temperature is evidently one of the factors/parameters amongst others known to influence the grain size of SiC and therefore it is important to investigate the effect of high temperature annealing on the SiC grain size. The ASTM E112 method as well as electron back scatter diffraction (EBSD) was used to determine the grain size of various commercial SiC samples and the SiC layer in experimental PBMR Coated Particles (CPs) after annealing at temperatures ranging from 1600°C to 2100°C. The HRTEM micrograph of the decomposition of SiC at 2100°C are shown and discussed. Nanotubes were not identified during the TEM and HRTEM analysis although graphitic structures were identified. The preliminary conclusion reached is that the P-content at these experimental levels (1.1×1015 to 1.2×1019atom/cm3) does not have a significant influence on the nanostructure of SiC at high temperatures without irradiation.

The influence of annealing temperature on the strength of TRISO coated particles

The integrity of the Pebble Bed Modular Reactor (PBMR) fuel, and specifically the SiC layer system of the Tristructural Isotropic (TRISO) coated particle (CP), namely inner pyrolytic carbon, silicon carbide and outer pyrolytic carbon (I-PyC–SiC–O-PyC), determines the containment of fission products. The PBMR fuel consists of TRISO coated particles (CPs) embedded in a graphite matrix. One of the characterization techniques investigated by PBMR is the determination of strength of CPs. It is a well known metallurgical fact that temperature, amongst many other parameters, may influence the strength of a material. A recently developed method for measuring the strength of the TRISO coated particles was used and is briefly described in this article. The advantages of this method are demonstrated by the comparison of strength measurements of five experimental PBMR CP batches as a function of annealing temperature. Significant modification of strength after annealing was measured with increased temperature within the range 1000–2100 °C. The interesting feature of decreasing standard deviation of the strength with increasing temperature will also be discussed with a possible explanation. A significant difference in coated particle strength is also demonstrated for two CP batches with layer thickness on the extremities of the SiC layer thickness specification. The effect of long duration annealing on these strength values will also be demonstrated by comparing results from 1 h to 100 h annealing periods of coated particles at a temperature of 1600 °C.

Analysis of Pebble-Fueled Zone Modeling Influence on High-Temperature Reactor Core Calculations

The pebble bed gas-cooled reactor is one of the most promising concepts among the Generation III+ and Generation IV reactors. Currently, the pebble bed modular reactor (PBMR) design, both U and Pu and minor actinide fueled, is being developed. Modeling the arrangement of coated particles (CPs) inside a spherical region like a pebble seems to be an important issue in the frame of calculations. To use the (relatively) old Monte Carlo codes without any correction, some approximations are often introduced. Recent Monte Carlo codes like MCNP5 and some new original subroutines that we have developed allow the possibility of obtaining more detailed and more physically correct geometrical descriptions of this kind of system. Some studies on modeling pebbles and pebble bed cores have already been carried out by other researchers, but these works are substantially limited to AVR-type UO2-fueled pebbles. However, the impact of approximated models on fuel mass, reactivity, and reactor life prediction has not yet been investigated for new PBMR-type pebbles.At the same time, an assessment of introducing a stochastic CP arrangement is not so widespread. Analyzing two PBMR pebbles, one Pu- and the other U-fueled, this paper focuses on quantifying errors due to the different approximations generally used to describe the CP lattice inside a high-temperature reactor pebble bed core, as far as mass of fuel, reactivity, and burnup simulation are concerned. This aim was reached also through a new feature implemented in the MCNP5 code, i.e., capability to treat (pseudo) stochastic geometries. Later, we compared the initial mass of fuel, keff, and isotopic evolution versus burnup of some approximated pebble models with the reference model, built by means of this new MCNP5 feature.

Coated particle fuel to improve safety, design, economics in water-cooled and gas-cooled reactors

In the context of Inert Matrix Fuels (IMF), coated particles (CP)s may play an important role to reduce the problems connected to the plutonium (Pu) and minor actinides stockpiles. In addition, the coatings of the fuel kernels made of different low and high-density ceramic layers allow a safe containment of fission products in operational, accidental and disposal conditions. Pu-based CPs embedded in a ceramic matrix (e.g. graphite, like in HTR fuel elements) can be judged as a special cercer type. There is a wide variety of different coatings and kernel compositions that has not yet been explored beyond the HTR development. Thus it can be expected that the CP technology can be applied to other reactor systems, too. For getting full benefit of the fission product retention capability of the CPs, not only the particle design but also the core geometry as well as power densities etc. have to be adopted to each specific requirement.