Pellet Radius Research Articles

Characterization of solid fission products in 13.7% FIMA MOX fuel using electron microscopy techniques

This work utilizes electron microscopy-based techniques to examine the radial behavior of solid fission products in plutonium (Pu) bearing mixed oxide (MOX) fuel irradiated to a burnup of 13.7% fissions per initial metal atom (FIMA). Metallic precipitates primarily consist of five fission products: ruthenium (Ru), rhodium (Rh), technetium (Tc), molybdenum (Mo), and palladium (Pd). The five metal precipitates (FMPs) examined in this work have low concentrations of Pd and Mo, with no major compositional differences along the fuel radius. A secondary Pd–Te metallic phase forms in cooler regions of the pellet, likely due to the diffusion of gaseous species away from the central void. X-ray chemical maps indicate that the Pd–Te phase can nucleate on the surface of FMPs before precipitating into separate particles. These particles were also found to alloy with iron (Fe) near the surface of the fuel pellet due to interdiffusion with the stainless-steel cladding. The insoluble perovskite oxide phase was found to form near the central void and at intermediate radial positions, but not at the fuel edge. These findings suggest that solid fission product phases form at varying counts and compositions along the fuel pellet radius, and thus should be considered when describing the thermal behavior of the fuel.

U3Si2-SiC fuel performance analysis in BISON during normal operation

U3Si2-SiC is an accident-tolerant fuel concept currently under development by Westinghouse that has potential to improve economic performance and safety of light water reactors. Since U3Si2 fuel suffers from poorer hydrothermal corrosion performance relative to UO2, it is paramount in an U3Si2-SiC fuel rod to lower the stress in the cladding to prevent any inward leakage of coolant. This motivates a detailed fuel-clad analysis of U3Si2-SiC during normal operation. The SiC cladding is modeled with a layer of SiCf/SiC composite followed by a layer of monolithic SiC. The dependence of thermomechanical properties for each SiC layer on temperature and neutron fluence is considered in the BISON fuel performance code. Particularly, pseudo-ductile deformation and anisotropic swelling models of SiCf/SiC composite are updated. While SiC thermal conductivity is commonly known to decrease as a consequence of irradiation damage, its additional degradation by mechanical damage is also modeled. Owing to limited availability of U3Si2 irradiation behavior models and experimental data under power reactor conditions, several assumptions are made to perform the initial detailed assessment of the fuel system. The results show that the fuel rod designs with larger pellet radius, thinner SiC cladding and larger initial gap width (thus larger cladding inner radius) can be beneficial to lower cladding stress during power operation and reactor shutdown.

Postirradiation examinations of low burnup U3Si2 fuel for light water reactor applications

The United States Department of Energy Advanced Fuels Campaign in collaboration with industry is working to develop enhanced accident tolerant fuels for light water reactors. The initial irradiations of these concepts were performed in the Idaho National Laboratory Advanced Test Reactor using drop-in style irradiations. These irradiations are collectively referred to as Accident Tolerant Fuels 1 (ATF-1). The postirradiation examination of ATF-1 has begun and some concepts have now completed initial examination. This work is focused on postirradiation examination data of U3Si2 fuels at low burnup (i.e., < 20 GWd/tHM) for application in Light Water Reactors (LWRs). The U3Si2 pellets show limited cracking in comparison to the expected behavior of UO2 at same power level. In addition, gamma spectrometry data did not reveal migration of fission products. Minor homogeneous hardening along the pellet radius due to accumulation of fission products and radiation damage was measured by microindentation. The fission gas release and swelling remain very low. Formation of fission gas bubbles resolvable with optical microscopy occurs from the pellets center outward to approximately 60% of the fuel pellet radius. The overall data suggest a good performance of this accident tolerant fuel candidate at low burnup.

Pore-scale simulation of flow and turbulence characteristics in three-dimensional randomly packed beds

Fluid flow and turbulence characteristics were numerically simulated at the pore scale in actual packed beds under different operating conditions. Real geometrical structures of randomly packed beds were modeled using the discrete element software LIGGGHTS (LAMMPS Improved for General Granular and Granular Heat Transfer Simulations). The numerical model was validated against experimental data. Good agreements were achieved between the predicted results and measurements. The influences of different parameters, such as the gas velocity and particle, size were studied and discussed in detail. The results of a macroscopic analysis of the mean parameters show that inside the packed beds, both the mean porosity and flow parameters had fluctuate variations. As the pellet radius increased, the fluctuations in the mean velocity and pressure increased. The results of a local detail analysis demonstrate that complicated turbulent eddy structures exist in the pores of the packed beds. Eddies with large sizes were primarily distributed at the entrance of the main channel and near the spherical wall, and eddies with high intensities were mainly distributed near the above spherical wall and on the right side of the main channel.

Restructuring in high burnup UO2 studied using modern electron microscopy

Modern electron microscopy techniques were used to conduct a thorough study of an irradiated urania fuel pellet microstructure to attempt at an understanding of high burnup structure formation in this material. The fuel was irradiated at low power to high burnups in a light water reactor, proving ideal for this purpose. Examination of grain size and orientation with strict spatial selectivity across the fuel pellet radius allowed for capturing the progression of the restructuring process, from its onset to full completion. Based on this information, the polygonization mechanism was shown to be responsible for restructuring, involving formation of low-angle grain boundaries with their initiation occurring at the original high-angle grain boundaries of the as-fabricated pellet and at the gas bubble-matrix interfaces. The low-angle character of boundaries between the subdivided grains disappeared in the fully developed high burnup structure, likely due to creep deformation in the pellet.

Silver-Exchanged Mordenite for Capture of Water Vapor in Off-Gas Streams: A Study of Adsorption Kinetics

Reduced silver-exchanged mordenite (Ag0Z) has been recognized as the benchmark adsorbent for radioactive iodine removal from the nuclear fuel reprocessing off-gases. It has also been shown to have a considerable adsorption capacity for water vapor, which is a major component (tritiated water) in the off-gases of spent nuclear fuel reprocessing facilities. Therefore, understanding the kinetics of water vapor adsorption on Ag0Z is necessary for a better design of off-gas treatment systems. The adsorption kinetics were studied through adsorption experiments of water vapor on Ag0Z pellets and analyses of the kinetic data with adsorption models that describe processes of mass transfer and inter/intracrystalline diffusion. Uptake curves of water vapor on Ag0Z pellets were obtained with a continuous-flow adsorption system at temperatures of 25, 40, 60, 100, 150, and 200 °C and dew points from −53.6 °C to 12.1 °C. The diffusion-controlling factors were determined experimentally and analytically. It was found that the diffusion process of water vapor in Ag0Z pellets was controlled by macropore diffusion. Gas film mass-transfer resistance also contributed to the adsorption process of the 0.9 mm Ag0Z pellets, but it could be minimized with a high gas velocity and small pellet radius. Kinetic models including macropore diffusion (MD), linear driving force (LDF) and shrinking core (SC) were used to fit the uptake curves. The macropore diffusivity for water vapor adsorption on Ag0Z pellets was determined using the three models. It was found that the LDF and SC models could well describe the kinetic process, while the fitting with the MD model was not quite as good due to the existence of external mass-transfer resistance for the 0.9 mm Ag0Z pellets.

Repartition of the uranium isotopes within the Belgian UOX spent fuel

ABSTRACTIn the framework of the Euratom FIRST-Nuclides program, SCK•CEN conducted leaching experiments on a PWR UOX fuel, taken from the Belgian Tihange 1 reactor, with an average rod burnup of 50 MWd/kgHM in order to determine the rapid release of some of the most critical radionuclides. Experiments were conducted with an intact and a declad spent fuel segment in contact with a reference carbonate solution. During one year, samplings were performed to determine the release of the most relevant radionuclides. The samples taken after six months of leaching were analysed by means of Thermal Ionisation Mass Spectrometry (TIMS). These results were compared to the total inventory measured with radiochemical burnup analyses, as well as with the calculated inventory profile along the pellet radius. The comparison gives information about the origin of the dissolved uranium.

Numerical Analysis of Complicated Heat and Mass Transfer inside a Wustite Pellet during Reducing to Sponge Iron by H2 and CO Gaseous Mixture

Heat and mass transfer through a wustite pellet during converting to sponge iron was investigated. Pellet was reduced by a gaseous mixture containing CO and H2. The grain model was considered to simulate gas-solid reaction rate. A finite volume method (FVM) was implemented for solving the governing equations. The heat transfer mechanism around the pellet includes radiation and convection and within the pellet, effective heat transfer is considered as a blend of particles conduction and intraparticle radiation. Heat and mass distribution along the radius of pellet for two cases of reducing gases composed of pure H2 and pure CO was investigated. Local fractional reduction through the pellet was plotted to examine the heat and mass transfer behavior within the pellet and find their relevance with reduction degree. Afterwards, the impacts of pertinent parameters including gas ratio, pellet size and porosity were studied.

The experimental determination of the 238U(n,γ) and total fission reaction rates along the pellet radius of the IPEN/MB-01 reactor

This work presents a new experimental approach to determine experimentally the reaction rate along the pellet radius of a research reactor facility. The methodology employs concentric hollow cylindrical lead collimators together with gamma-ray spectrometry in a special kind of HPGe detector for the discrimination of very low gamma energy emitted by 239Np and 99Mo. The 238U neutron capture and total fission rates were inferred, respectively, from the 106.2keV gamma emitted by 239Np and from the 140.51keV gamma-ray emitted by 99Mo. These lower gamma-ray energies allow the thickness of the lead collimator to be small enough so that the correction factors applied to the procedure were minimized. This kind somewhat challenging experiment was successfully performed at the IPEN/MB-01 research reactor facility. The experiments are claimed to be well-defined, and they are suitable for a benchmark. The measured values of the total fission rates are mainly due to the thermal fissions in 235U since in the IPEN/MB-01 reactor nearly 85% of the total fissions occur in the thermal neutron energy region. The theoretical analyses were performed using MCNP-5 together with the ENDF/B-VII.0 library. The analyses reveal a very good agreement between the calculated and experimental results for the 238U epithermal neutron capture reaction rates. However, the same can not be said for the thermal reaction rates which show discrepancies both in magnitude as well as in the shape of the attenuation of the reaction rates inside of the fuel pellet. The suspected reason for these discrepancies is the shape of the 235U cross sections below 0.3eV which might be different from that adopted in the ENDF/B-VII.0 library.

Accident-Tolerant-Fuel Performance Analysis of APMT Steel Clad/UO2 Fuel and APMT Steel Clad/UN-U3Si5 Fuel Concepts

While Zircaloy-based claddings have been the workhorse for the nuclear power industry for decades, they have also demonstrated problems, particularly regarding accident scenarios. Work has been performed to assess the viability of stainless steel–based cladding in traditional light water reactors. This paper assesses the reactivity penalty of moving to stainless steel cladding using Monteburns, while attempting to minimize this penalty by increasing the fuel pellet radius and decreasing the cladding thickness. Fuel performance simulations using BISON have also been performed to quantify gains or losses in structural integrity when moving to thinner, stainless steel claddings. Thermal and irradiation creep, along with fission gas swelling, thermal swelling, and fuel relocation, are accounted for in the models for both Zircaloy and stainless steel claddings. Additional models for the lower-oxidation stainless steel APMT are also invoked where available, with irradiation data for HT9 used as a fallback in the absence of appropriate models. In this study the isotopic vectors within each natural element are varied to assess potential reactivity gains if advanced enrichment capabilities were levied toward cladding technologies. Recommendations on cladding thicknesses for a robust cladding as well as the constitutive components of a less penalizing composition are provided.

Reduction of commercial hematite pellet in isothermal fixed bed—experiments and numerical modelling

In the present work, fixed bed reduction experiments were conducted at 1173 K over a range of H2/CO ratios from 0.8 to 2.0 and subsequently modelled numerically. The model consists of two one-dimensional, isothermal and time dependent models. The gas–solid reactions were kinetically modelled using a modified shrinking core approach, and the equations were solved using the commercial software COMSOL Multiphysics®. The simulation results agree with thermal gravity experimental data with an average difference of 2.5%. A sensitivity analysis was conducted using the numerical model to establish the optimum operational conditions. The effects of the reducing gas ratio and flow rate, pellet radius and porosity, and the total bed height on the overall degree of reduction were also investigated.

Enhanced Aspergillus ficuum phytase production in fed-batch and continuous fermentations in the presence of talcum microparticles.

This study aimed to enhance Aspergillus ficuum phytase production in fed-batch and continuous fermentations with addition of talcum microparticles. Phytase activity almost doubled in fed-batch and continuous fermentations by addition of 15g/l of talcum compared to the control. Effect of talcum on fungal morphology was also shown that addition of talcum provided smaller fungal pellets and more homogenized fermentation broth compared to the control. Average fungal pellet radius decreased from 500 to 100µm by addition of 15g/l of talcum in the bioreactors. Also, 15g/l talcum addition increased phytase productivity and optimum dilution rate in the continuous fermentations from 0.293 to 0.621U/ml/h and from 0.09 to 0.1/h, respectively, compared to control.

Microparticle-enhanced Aspergillus ficuum phytase production and evaluation of fungal morphology in submerged fermentation.

Phytase can be used in animal's diets to increase the absorption of several divalent ions, amino acids and proteins and to decrease the excessive phosphorus release in manure to prevent negative effects on the environment. This study aimed to enhance the current submerged fungal phytase productions with a novel fermentation technique by evaluating the effect of the various microparticles on Aspergillus ficuum phytase production. It was observed that microparticles prevented bulk fungal pellet growth, decreased average fungal pellet size and significantly increased phytase activity in the submerged fermentation. Microbial structure imaging results showed that the average fungal pellet radius decreased from 800 to 500 and 200 µm by addition of 15 g/L aluminum oxide and talcum, respectively, in shake-flask fermentation. Also, addition of 15 g/L of talcum and aluminum oxide increased phytase activity to 2.01 and 2.93 U/ml, respectively, compared to control (1.02 U/ml) in shake-flask fermentation. Additionally, phytase activity reached 6.49 U/ml within 96 h of fermentation with the addition of 15 g/L of talcum, whereas the maximum phytase activity was only 3.45 U/ml at 120 h of fermentation for the control in the 1-L working volume bioreactors. In conclusion, microparticles significantly increased fungal phytase activity and production yield compared to control fermentation.

Study of L-H transition triggered by pellet injection based on a power threshold model

Simulations of ITER plasma during an L-H transition triggered by pellet injection are carried out using the 1.5D BALDUR integrated predictive modeling code. In these simulations, plasma core transport is predicted using a combination of the Multimode (MMM95) turbulent transport model and neoclassical transport (NCLASS) model. The pellet ablation behavior is described using the neutral gas shielding pellet model with the grad-B drift effect also included. Because of the increase of the plasma density and the reduction of the temperature, the plasma self-heating powers (ohmic heating and alpha heating) increase, whereas the plasma energy loss (through radiation) decreases. As a result, the total power across the separatrix increases. On the other hand, the L-H power threshold decreases. With all the changes of heating, an L-H transition can be induced even though the auxiliary heating is not enough for the L-H transition before pellet injection. It is also found that the L-H transition triggered by pellet injection depends sensitively on the pellet radius, but only moderately on the pellet velocity.

Experimental and theoretical investigation of wood pellet shrinkage during pyrolysis

Gasification of wood pellets is confronted with the problem of adhesion of fuel in the pyrolysis zone. In order to investigate the cause and regularities of fuel adhesion, experimental and theoretical studies of wood pellets thermal deformations were carried out. Experimental studies were performed by measuring thermal deformation of a wood pellet in an electrical reactor from 300 to 1 000 °C temperature in an inert atmosphere. The studies showed that the pellet radius decreased by 10% from 300 to 500 °C temperature, and by 20% from 500 to 1 000 °C temperature. It was also established that the wood pellet expands to 15% from 300 to 830 °C temperature at the beginning of fuel thermal deformation. The paper also presents the theoretical studies of wood pellets thermal deformations. The theoretical calculations were performed by Matlab using a one-dimen­sional pyrolysis model of wood pellets that includes the experimental data and the observed expansion of wood pellets. Simulation results were compared with the experimental ones. Modelling and experimental results showed that the thermal deformation of wood pellets values was different by 2–5%.

Study of the induration phenomena in single pellet to traveling grate furnace

The process for pelletizing iron ore fines is an important operation unit for producing high quality of raw materials for the subsequent reduction processes such as blast furnace or direct reduction. The process essentially involves production of green pellets and induration on a traveling grate furnace to promote inner partial melt and agglomeration that confers adequate physical and metallurgical properties. This work focuses on the phenomena that occur in the firing step aiming the construction of a mathematical model that describes each phase and chemical species. The model was formulated based on transport equations able to predict the evolution of the temperature profile inside the pellet for each zone on the induration furnace. It was taken into account coupled phenomena of momentum, energy and mass transfer between gas and particles within the agglomerates. The finite volume method was used to discretize the transport equations of momentum, mass and energy describing the behavior of a pellet in an industrial traveling grate furnace. Model results are shown for the temperature profile along the pellet radius during the residence time inside the furnace. In this context, in the present work i a tool was developed to optimize the thermal profile in the induration furnace and hence control the mechanical strength of the agglomerate.

Mathematical model of the direct reduction of dust composite pellets containing zinc and iron

Direct reduction of dust composite pellets containing zinc and iron was examined by simulating the conditions of actual production process of a rotary hearth furnace (RHF) in laboratory. A mathematical model was constructed to study the reduction kinetics of iron oxides and ZnO in the dust composite pellets. It was validated by comparing the calculated values with experimental results. The effects of furnace temperature, pellet radius, and pellet porosity on the reduction were investigated by the model. It is shown that furnace temperature has obvious influence on both of the reduction of iron oxides and ZnO, but the influence of pellet radius and porosity is much smaller. Model calculations suggest that both of the reduction of iron oxides and ZnO are under mixed control with interface reactions and Boudouard reaction in the early stage, but only with interface reactions in the later stage.

Pellet Size Distribution Using Circular Hough Transform in Simulink

This paper presents fast and effective technique for determining the size of pellets from real-time video stream. The circular Hough transforms had been used by several researches in iris detection for face recognition, automatic ball recognition and detecting fingertips position. Here we used circular Hough transform to determine size of pellets in steel plant. This system consists of five steps i.e., take input image, convert into grey scale image, normalize grey scale image, detect edges & perform circular hough transform and do the size analysis. Initially the real-time RGB image is being read from the webcam, as the image is obtained it is converted to grey scale image following that the light normalization of the image is done, after all the circular hough technique is used to detect the pellets and to determine the size of pellets. Since detection and recognition in noisy and cluttered images is challenging problem in computer vision. Some of the pellets are overlapping each other and some itself noise that make the recognition process challenging. So to get rid from all such difficulties the technique used is circular hough transform. This technique helps in determining the centres of the pellets and to measure the radius of pellets and to mark the pellets which are selected. The system is so fast that any change in the input image causes change in the output.

Enhanced enzyme production with the pelleted form of D. squalens in laboratory bioreactors using added natural lignin inducer

White-rot fungi are extensively used in various submerged biotechnology processes to produce ligninolytic enzymes. Transfer of the process from the laboratory to the industrial level requires optimization of the cultivation conditions on the laboratory scale. An interesting area of optimization is pellet growth since this morphological form solves problems such as the decreased oxygen concentration, limited heat, and nutrient transport, which usually occur in dispersed mycelium cultures. Many submerged fermentations with basidiomycetes in pellet form were done with Phanerochaete, Trametes, and Bjerkandera species, among others. In our study, another promising basidiomycete, D. squalens, was used for ligninolytic enzyme production. With the addition of wood particles (sawdust) as a natural inducer and optimization of mixing and aeration conditions in laboratory stirred tank (STR) and bubble column (BCR) reactors on pellet growth and morphology, the secretion of laccase and the manganese-dependent peroxidase into the medium was substantially enhanced. The maximum mean pellet radius was achieved after 10days in the BCR (5.1mm) where pellets were fluffy and 5days in the STR (3.5mm) where they were round and smooth. The maximum Lac activity (1,882Ul(-1)) was obtained after 12days in the STR, while maximum MnP activity (449.8Ul(-1)) occurred after 18days in the BCR. The pellet size and morphology depended on the agitation and aeration conditions and consequently influenced a particular enzyme synthesis. The enzyme activities were high and comparable with the activities found for other investigations in reactors with basidiomycetes in the form of pellets.

An analysis for compressible flows in a packed bed with metathesis gas–solid reaction

Taking into consideration the mass exchange between gas and solid, and the density change of gas mixture due to reaction, a model is proposed for compressible flows in a packed bed with isothermal metathesis gas–solid reaction aA(g) + bB(s) = c C(g) + dD(s), When a ≠ c, the ratio c/a greatly affects the flows of gas mixture, but for a = c, this coupled model reduces to the non-coupled model. One-dimensional numerical solutions show that the velocity profiles obtained from coupled model greatly differ from that from the non-coupled model. The radius of pellets can change the velocity variation trend. The concentration profiles obtained from this coupled model also differ from that from the non-coupled model. A coupled model accounting for only mass exchange between gas and solid, but neglecting the density change of gas mixture due to reaction will produce an extra term, which results in a great deviation of the velocity and concentration from those from the more comprehensive coupled model.