Spherical Fuel Elements Research Articles

Hybrid Surrogate Model-Based Multi-Objective Lightweight Optimization of Spherical Fuel Element Canister

A number of canisters need to be lightweight designed to store the spherical fuel elements (SFE) used in high-temperature gas-cooled reactors (HTGR). The main challenge for engineering is pursuing high-accuracy and high-efficiency optimization simultaneously. Accordingly, a hybrid surrogate model-based multi-objective optimization method with the numerical method for the lightweight and safe design of the SFE canister is proposed. To be specific, the drop analysis model of the SFE canister is firstly established where the finite element method—discrete element method (FEM–DEM) coupled method is integrated to simulate the interaction force between the SFE and canister. Through simulation, the design variables, optimization objectives, and constraints are identified. Then the hybrid radial basis function—response surface method (RBF–RSM) surrogate method is carried out to approximate and simplify the accurate numerical model. A non-dominated sorting genetic algorithm (NSGA-II) is used for resolving this multi-objective model. Optimal design is validated using comprehensive comparison, and the reduction of weight and maximum strain can be up to 2.46% and 44.65%, respectively. High-accuracy simulation with high-efficiency optimization is successfully demonstrated to perform the lightweight design on nuclear facilities.

Development of a hybrid model for fuel performance analysis of spherical fuel elements under normal conditions

The spherical fuel element (SFE) dispersed with many tri-isotropic fuel (TRISO) particles has been primarily employed in the high-temperature gas-cooled reactor. Owing to the complex structure of SFE, behaviors will be evolved for both TRISO particles and the fuel matrix with the increasing burnup. To evaluate the fuel performance of SFE, the homogenization method has been conventionally adopted for SFE scale modeling, and dispersed TRISO particles are analyzed subsequently with fixed temperature boundary conditions. However, owing to the varying properties of TRISO particles under reactor operation, the homogenization models dedicated to certain circumstances can hardly reflect the properties evolution of TRISO with high fidelity, which may lead to uncertainties in analysis results. To this end, a hybrid model, namely the conventional homogenization model informed by the lower scale model online, has been developed in this paper, which may increase modeling fidelity for both SFE and particles. Specifically, property models for the TRISO particle and graphite matrix have been implemented in COMSOL. Subsequently, based on the linking of COMSOL and MATLAB, the hybrid model for SFE analysis was established. Through the hybrid model, the temperature field and stress distribution of TRISO, the evolution of effective properties, and the thermal and mechanical state of the SFE were obtained and analyzed. The results show that within the scope of this paper, the central temperature of TRISO in the innermost region rises with the increasing burnup for the decreasing thermal conductivity of materials, and the peak value is about 1324 K, which is about 60 K higher than that in the outermost region. The difference in the maximum plenum pressure is more than 5 MPa among TRISO particles, which is mainly caused by the variations in CO production. Consequently, the hoop stress of SiC in the outermost region is slightly elevated. The distribution of effective thermal conductivity (ETC) for TRISO is nearly 4.3 W/(m·K) at the beginning of life (BOL), while it was reduced to 3.6 W/(m·K) at the end of life (EOL). There are marginal variations along the radial direction for the ETC of TRISO. Meanwhile, the ETC of SFE decreases from about 21 W/(m·K) for fresh fuel to 13 W/(m·K) for spent fuel. The predicted effective elastic modulus and coefficient of thermal expansion of SFE increase gradually versus time. The former is caused by the densification of pyrolytic carbon, and the latter is mainly determined by the high volume fraction of matrix and the changes of CTE for pyrolytic carbon. In addition, the comparison in the temperature field between the proposed model and the Differential Effective Material Theory (D-EMT) model may further demonstrate the feasibility and effectiveness of the hybrid model.

Optimization analysis of HTR-PM600 fresh fuel transport container under multiple impact loading conditions

Reliable transportation of Spherical Fuel Element (SFE) is vital for the stable operation of a High-Temperature Reactor (HTR) power plant. As a solution, Tsinghua University has developed the HTR-PM600 transport container, for transporting the inner storage canister and the SFEs. However, the cost of fabrication and transportation has been a major obstacle for the implementation of it. Thus, one of the most feasible methods is to make a lightweight design. Accordingly, this present study aims to investigate and optimize the lightweight and safety performance of the HTR-PM600 transport container under multiple impact loading conditions. In this study, the parametric study is conducted to reveals the effect of the thickness of HTR-PM600 transport container components under multiple impact loading conditions. On the basis, the ε-Constraint method coupled with the Taguchi method is utilized to explore the optimal design of HTR-PM600 transport container for multi-objective optimization for lightweight and safety performance, which is verified through a comparison between the initial and optimal design. The optimal design presents a superior safety performance and a 15.7% improvement on lightweight under multiple impact loading conditions.

A SIMILARITY CRITERION FOR SPHERICAL FUEL ELEMENTS FREE FALL VELOCITY IN CYLINDRICAL CHANNELS WITH VISCOUSE LIQUID

The introduction of nuclear high-temperature gas-cooled reactors (HTGR) with an active zone based on spherical fuel elements (SFE) poses the task of determining the velocity of their free fall in cylindrical channels with a viscous liquid. To solve it, the experimental data of other researchers are generalized, and for a certain range of Reynolds numbers the criterion of similarity for the velocity of free fall of spheres in cylindrical channels with water is found. The criterion is formulated on the basis of the Freud number. It is shown that from the dependence of the velocity of falling of the model sphere in a cylindrical vessel with water on the dimensionless diameter of the sphere, it is possible to determine the velocity of falling of the sphere in water, arbitrary.

Release Rates of Short-lived Fission Gases from Modern Spherical Fuel Elements with TRISO-coated Particles

Measurement and prediction of fission product release is important in design, licensing and operation of HTRs. Latest development status is near zero particle manufacturing defects and in-pile failures combined with very low matrix contamination levels. In-pile Release over Birth rate measurements will be described and evaluated. Quantitative predictive analyses of short-lived gas release will be provided for MTR irradiation testing with a focus on the most recent experiment with Chinese fuel from HTR-PM production. For irradiation tests with no single particle failure, models describing particle kernel release are not applicable; instead modeling of matrix contamination release is essential. Good agreement between post calculations and in-line R/B measurements has been achieved.

Scale-up strategy study of coating furnace for TRISO particle fabrication based on numerical simulations

In this paper, the whole system of coating furnace is simulated based on 3D single phase simulation to optimize the FB-CVD (Fluidized Bed-Chemical Vapour Deposition) reactor structure design, which is called the multiple-ring inclined-nozzle spouted bed. The spouted bed with present and enlarged diameter for two times of production, were simulated using a 3D CFD model in a complex geometry, aimed at proposing a quick scale-up strategy of the coating furnace. A new scale-up criterion is proposed and discussed based on single-phase two-component simulation results, which were central-inlet gas concentration distribution at different furnace heights. Four cases of the newly design with enlarged diameter of spouted bed was compared quantitatively, and one was recommended to use firstly in the coating experiments based on the newly proposed scale-up criterion. The present China SFE (Spherical Fuel Element) commercial production line (300000 SFEs per year) will be updated based on the present scale-up research results, aimed at providing the SPE for the HTR-PM600 in the future.

Spatial Uniformity Assessment of Particles Distributed in a Spherical Fuel Element Using a Non-Destructive Approach

In many manufacturing procedures, a large number of identical particles need to be disseminated uniformly into a given space. The uniformity of the spatial distribution of the particles can be critical to the properties of the final products. We proposed an image processing-based non-destructive technique to evaluate the particles’ spatial uniformity in a spherical space imaged with computed tomography. Both graphic (qualitative) and numerical (quantitative) methods were developed to demonstrate the (non-) uniformity of the particles. Simulation results indicated that the technique helped detecting the non-uniformity in the particles’ spatial distribution accurately. We conclude that the proposed technique can be used to test whether a number of particles in a sphere are uniformly distributed statistically and graphically.

Uniformity Assessment of TRISO Fuel Particle Distribution in Spherical HTGR Fuel Element Using Voronoi Tessellation and Delaunay Triangulation

Nonuniform distribution of tri-structural-isotropic (TRISO) fuel particles in a spherical fuel element (SFE) may increase the failure probability of the SFE in the high-temperature gas-cooled reactor, leading to the release of fission products. To evaluate the uniformity of the TRISO particles nondestructively, 3-dimensional cone-beam computed tomography is used to image the SFE, and TRISO particles are segmented. After TRISO particle positions are identified, the Voronoi tessellation and Delaunay triangulation are used to extract several geometric metrics. Results indicate that both the Voronoi volume distribution and the nearest neighbor-distance distribution follow the log-normal distributions, which provide strong evidence that the TRISO particles are approximately randomly uniformly distributed. Further study will be focused on validating the conclusion with more SFE data.

The Electric Current Effect on Electrochemical Deconsolidation of Spherical Fuel Elements

For High-Temperature Gas-Cooled Reactor in China, fuel particles are bonded into spherical fuel elements by a carbonaceous matrix. For the study of fuel failure mechanism from individual fuel particles, an electrochemical deconsolidation apparatus was developed in this study to separate the particles from the carbonaceous matrix by disintegrating the matrix into fine graphite powder. The deconsolidated graphite powder and free particles were characterized by elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and ceramography. The results showed that the morphology, size distribution, and element content of deconsolidated graphite matrix and free particles were notably affected by electric current intensity. The electrochemical deconsolidation mechanism of spherical fuel element was also discussed.

Preparation of ammonium diuranate particles by external gelation process of uranium in INET

In order to satisfy the mass spherical fuel elements (SFE) requirements for the High Temperature gas-cooled Reactor (HTR)–Pebble bed Module (HTR-PM), a modified and optimized fabrication of dense uranium dioxide (UO2) kernel via an acid-deficiency uranium nitrate (ADUN) solution was researched and developed in the institute of nuclear and new energy technology (INET). In this study, external gelation process of uranium (EGU) based on the total gelation of uranium (TGU) in the period of 10MW HTR (HTR-10) was used to prepare the ammonium diuranate (ADU) compound particles, an intermediate for UO2 kernel. The scale of batch production was improved from 500g U/batch at that time to current 3kg U/batch. Improvements in many aspects such as large scale preparation of ADUN, multiple-nozzle casting system and new aging, washing and drying (AWD) system without isopropanol were also presented. Under strict controls and measures, ADU particles with good sphericity and uniform diameter were acquired to ensure the subsequent preparation of dense UO2 kernels for HTR-PM.

Postirradiation Testing of High Temperature Reactor Spherical Fuel Elements Under Accident Conditions

A new furnace for accident condition testing of spherical high temperature reactor fuel elements has been installed and now operates in the hot cells of the Institute for Transuranium Elements (ITU) Karlsruhe. The recent apparatus was constructed on the basis of a former development by Forschungszentrum Jülich (Schenk, Pitzer, and Nabielek, 1988, “Fission Product Release Profiles From Spherical HTR Fuel Elements at Accident Temperatures,” Jülich Report No. 2234), where it was named KüFA, the German acronym for cold finger apparatus. In a preceding publication (Kostecka, Ejton, de Weerd, and Toscano, 2004, “Post-Irradiation Testing of HTR-Fuel Elements Under Accident Conditions, Part 1 and 2,” Second International Topical Meeting on High Temperature Reactor Technology, Beijing, China) the general concept and details of the device were described. The present paper reports on the first operation under hot conditions, and the calibration of the fission gas measurement and of the efficiency of the cold finger, which is used to plate out solid fission products. Finally the results of fission product release and analysis of two heating tests on two fuel elements from the high temperature reactor K6 irradiation experiment (Nabielek, Conrad, Roellig, and Meyers, 1993, “Fuel Irradiation Experiments on HFR-K6 and HFR-B1 With Intermittent Water Vapour Injections,” Technical Committee Meeting on Response of Fuel, Fuel Elements and Gas Cooled Reactor Cores Under Accidental Air or Water Ingress Conditions, Beijing, China, Oct. 25–27) are presented and discussed.

Spherical Fuel Element Concept for Small Reactor Design

The Pacific Northwest National Laboratory (PNNL) is currently developing a novel spherical fuel element concept that offers low fuel temperatures, low stored energy, and long core life. Fuel performance modeling has been conducted using the PNNL-developed Atoms for Peace Reactor (AFPR)-100 as a platform for demonstrating the potential of the fuel element concept. The AFPR-100 is a small [100-MW(electric), 300-MW(thermal)], water-cooled reactor concept that is designed to use established technology, be passively safe, and be proliferation resistant. The fuel performance modeling has demonstrated that this fuel element has a short thermal time constant, has low fuel temperature, provides a barrier for retention of fission products, and will have long-term dimensional stability.A technique for manufacturing these fuel elements was developed. A fabrication demonstration was conducted in cooperation with a commercial vendor to evaluate the feasibility of manufacturing the fuel elements. In order to demonstrate the proposed technique, the proposed spherical elements were produced using existing processes that could be scaled to large batch sizes. Surrogate ZrO2 kernels were substituted for the fuel in this demonstration. Thorough characterization of the fuel elements was performed at various stages in the fabrication process. The metallographic characterization included electron microscopic analysis of coating microstructure, and particular attention was paid to interface regions to search for deleterious reaction zones, debonding, and porosity. Although this demonstration is not complete, early results are promising and will be discussed in this paper.This paper will describe the fuel element, show the results of fuel performance calculations for this element, describe the proposed fabrication process, and discuss the results of a fabrication demonstration to date that has been performed for this concept.

Fuel irradiation of the first batches produced for the Chinese HTR-10

An irradiation test of four spherical fuel elements (SFE) had been performed in the Russian reactor IVV-2M. The elements were sampled randomly from the first and second product batches which were manufactured for the 10 MW high-temperature gas-cooled test reactor (HTR-10). The maximum burnup of the irradiated fuel elements reached 107,000 MWd/tU and the maximum fast neutron fluence was 1.31 × 10 25 m −2. The release-to-birth rate ratio ( R/ B) did not increase significantly during irradiation. However, an in-pile heating-up test of element SFE 7 in Capsule 5 led to a failure of approximately 6% of the coated particles. After the test it was estimated that the fuel temperature had very likely been much higher than the intended 1600 °C.

Modular High-Temperature Helium-Cooled Nuclear Reactor with Spherical Fuel Elements for Electricity and Hydrogen Production

The results of optimizational neutron-physical and thermohydrulic calculations of the core of a modular high-temperature helium-cooled reactor with mobile spherical fuel elements are presented. A special structural feature of such fuel elements is that they contain fuel microelements with multilayered ceramic coatings capable of confining radioactive fission products at high temperatures with deep burnup of nuclear fuel. The thermal power of the reactor is 850 MW(t) with average power density 30 MW/m3 and helium temperature 1000°C at the core exit. This makes it possible to use such reactors to produce hydrogen by a cost-effective high-temperature process using steam conversion of methane and to generate electricity in a one-loop helium turbosystem with efficiency >45%.

Characteristic Features of Heat Transfer and Gas Dynamics in Fuel Assemblies with Spherical Fuel Elements and Radial Dispensing of the Coolant

Certain basic results from the study of the special features of the gas dynamics and heat transfer in fuel assemblies with spherical fuel elements placed in a ring-shaped gap between two perforated cases are presented. The gaseous coolant flows in a radial direction from periphery to center. The special features of the packing of spherical fuel elements in such gaps are studied. The formation of a gas-dynamic structure of the coolant flows inside fuel-element packings and in the dispensing collectors as well as heat transfer and the working capacity of fuel assemblies was investigated. 17 figures, 3 tables, 6 references.

Migration Behavior of Fission Products in and from Spherical High-Temperature Reactor Fuel Elements

Diffusion behavior of some metallic fission products in high-temperature reactor fuel elements, which had been irradiated in an in-pile gas loop (Saphir) installed in the Pegase reactor (France), was studied. Diffusion coefficients of cesium and silver in hightemperature isotropic pyrolytic carbon and graphite matrix under in-pile conditions were obtained by analyzing the concentration profiles of the fission products in the fuel elements, which had been measured by postirradiation examination. Although ruthenium profiles were measured, analysis of the diffusion coefficients could not be carried out because of the virtually flat distributions. By comparing the concentrations of the cesium isotopes in the fuel-free zone of the elements, it was found that TUCs behaved anomalously in the graphite matrix, which was, probably, caused by activation of an undetectable amount of TTCs impurity involved in the matrix. For the extremely high concentration of these fission products, which had been observed near the surface of the element, two causes, the uranium contamination concentrating there and the trapping effect in the defects introduced by fission of the locally concentrated uranium, were considered, although these high concentrations of the fission products were neglected in the analysis. Furthermore, Transport behavior of the fission products through the gas gapmore » from the fuel element to the graphite tube containing the elements was studied by measuring the concentration profiles in the tube. It was concluded that ruthenium transport occurred by direct fission recoil from the surface uranium contamination, whereas that of cesium, by desorption from the surface.« less

Fission Product Release from Coated Particles Embedded in Spherical Fuel Elements for High-Temperature Reactors

The results of irradiation experiments on spherical fuel elements in the Studsvik R2 and the Juelich FR12 reactors are used to derive release data for both gaseous and metallic fission products. Fission gas release follows the well-known dependence on the square root of half-life showing that diffusion is the controlling release process for both intact and failed particles. The release of /sup 110m/Ag was generally about two orders of magnitude higher than that of /sup 137/Cs. Profile measurements indicate good retention of strontium. Results of the concentration profile and in-pile release measurements allow verification of data from out-of-pile experiments.