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[en] Highlights: • Effect of density difference on packing structure and flow pattern are studied. • Equivalent density and voidage are analyzed to describe spatial distribution. • Light particles tend to be pushed to side walls by heavy ones during circulation. • Higher density ratio may lead to more obvious squeezing phenomenon. • Different velocity patterns in lower and upper parts of the bed are observed. - Abstract: Based on the discrete element method, this work simulates the mixing flow of particles with different density in packed bed. The aim of this work is to explore the effect of density difference on the spatial distribution and flow characteristics of particles. After a premixed stacking, the particles start to discharge at fixed number rates being extracted from the bottom and then reinserted into the top of the bed to maintain the circulating flow. The results reveal that although different density can affect the local equivalent density, it has little influence on the uniformity and compactness of the whole bed with overall average voidage of about 0.39. The wall effects on particle distribution and voidage are observed near the wall. In addition, during the unloading time (time after the start of discharge), the proportion of light particles near the wall is always higher than 0.5 and keeps increasing. The increasing proportion and much higher number fraction of light particles compared with heavy one indicate that the heavy particles have the tendency to push the light particles to the side during the circulation process. Besides, the higher the ratio is, the more obvious this phenomenon is. Last but not least, both the axial and radial velocity profiles are analyzed, which show different patterns between the bottom and upper part of the packed bed.
[en] Highlights: • This is the first Method of characteristics code for pebble bed reactor. • Special geometry modeling is developed for pebble bed reactor. • Special algorithm for ray tracing is optimized pebble bed reactor. - Abstract: The method of characteristics which is an effective method for neutron transport calculation with high flexibility in complex geometry has been applied to high temperature gas-cooled pebble-bed reactor geometry. The MOCP (the Method of Characteristics for Pebble-bed high temperature gas-cooled reactor) code was developed in Institute of Nuclear and New Energy Technology, Tsinghua University to achieve this goal. The first stone standing ahead is the geometry modeling for pebble-bed district. In this work, a solid modeling method called constructive solid geometry (CSG) is introduced to model the pebble-bed district. A universal ray tracing technique is developed to arrange tracks in 3-D cylinder geometry. Furthermore, tree search method is implemented into MOCP to improve the ray tracing efficiency. Automatic meshing is implemented to improve the geometry modeling efficiency. A redundant tree was constructed to mesh the interspace among pebbles. The geometry modeling capability of MOCP is verified by fast converging single group benchmarks.
[en] Molten salt pebble bed reactor is one of the sixth-generation IV reactor types. To investigate the mechanical behavior of the fuel pebbles in the core, a visualization experiment facility of pebble bed (VEFPB) is designed. To obtain a uniform flow field of the core and analyze the influence of the flow field on the structure of the pebble bed, computational fluid dynamics software Fluent is used to simulate the flow field distribution of the core of VEFPB. The simulation results show that the disturbance at the bottom of the pebble bed is proportional to the flow velocity of the inlet pipe, and the flow velocity close to the inlet side is more significant than that in other parts; the design of the cylinder bottom plate with holes of different sizes can effectively reduce the flow velocity and the disturbance at the bottom of the pebble bed. In addition, according to the velocity contours of the core of VEFPB, it is observed that the flow field distribution of the core is considerably uniform except at the bottom of the pebble bed. This ensures the stability of the pebble bed and verifies the rationality of the design of VEFPB. This study provides the technical support and reference for the flow field analysis of the core of molten salt pebble bed reactor.
[en] In this work, discrete numerical simulations are used to study the influence of container’s geometry on probability of jamming J. The height of the pebble-bed h and the angle of the conical hopper α are among our varied parameters. By increasing the angle of the conical hopper, the probability of jamming remains almost constant till 40deg, then decreases when α≥60deg. Which is due to the influence of angle on the granular flow pattern and the fact that 60deg is almost the critical angle of repose of the flow. The results have shown that height has a crucial effect on jamming probability when h is smaller than a critical value, flow becomes more susceptible to jamming if we increase h, when h exceeds this critical value, J approaches unity. To understand the underlying physical mechanism, we computed the average total stress near the aperture during the discharge and measured it for various h. The trace of stress tensor tr[σij] increases when h increases. Based on these observations, one can conclude that the stress has an imperative effect on the probability of jamming and deserves some leeway of research. (author)
[en] HTGR-10 MWth is one type of HTGRs pebble bed reactor. This type of reactors has negative reactivity for their inherent safety functions. In addition, criticality parameters are important factors to find out that a nuclear reactor is in a critical condition so that the reactor can operate. Criticality on the active core of a reactor is strongly influenced by the active core height, the level of fuel enrichment and the reactor core geometry, and others. The goal of this research is to obtain the necessary neutron parameters so that the reactor can operate optimally. The methodology used begins with modeling the TRISO-coated kernels with an SC lattice (simple Cubic lattice), followed by reactor geometry modeling. The fuel and pebble moderators on the reactor core were modeled in the lattice of Body Centered Cubic (BCC) with the ratio of fuel pebble and moderator of 57:43. The program package used in the calculation is the MCNP6 code. From the calculation results, it is obtained that the initial active core height for enrichment of 17 % is 125 cm, for enrichment 14 % as high as 141 cm, for enrichment 12 % is 161 cm and for enrichment 10 % is 196.1 cm, while for enrichment 8 % the core height is beyond the available limit. (author)
[en] Highlights: • PANGU code is developed for pebble-bed reactor physics and fuel cycle simulation. • PANGU employs a new two-step calculation scheme utilizing pre-tabulated constants. • PANGU code is advantaged in simulating multi-type particles and pebbles. • PANGU implements an iteration approach for fast calculating equilibrium cycles. • Numerical tests demonstrate PANGU code’s applicability in pebble-bed HTGRs. - Abstract: PANGU is a modern computer code for pebble-bed High Temperature Gas-cooled Reactor (HTGR) physics analyses and fuel cycle simulations, developed by Institute of Nuclear and New Energy Technology (INET), Tsinghua University. This paper gives a comprehensive review on PANGU code’s features including its calculation scheme, methodologies, models and capabilities. Numerical tests are presented for typical pebble bed reactor cases, to verify PANGU code’s applicability. It demonstrates that PANGU code is promising for physical designs of traditional and new-conceptual pebble-bed HTGRs.
[en] There has been a renewed interest to develop new nuclear power systems for commercial deployment. Of the design categories receiving significant attention is molten salts reactors (MSRs). There are many MSR design configurations under development, this paper focuses on the thermal-spectrum, pebble-bed, salt-cooled concept, often referred to as the Fluoride High-temperature Reactor (FHR). Constructing a geometrically accurate model of a pebble bed reactor with double heterogeneity is addressed before any further analysis is conducted. Within the FHR design concept geometric and material parameters can vary considerably. The purpose of the paper is to examine two fundamental design parameters; the level of moderation and the coolant salt selection. Results indicate that additional moderation beyond the graphite in the pebbles is beneficial and alternative salts to 27LiF-BeF2, despite inferior results, do generate an adequately high multiplication factor. (author)
[en] Highlights: • Pebble flow in two-region reactor is studied on effects of density & loading ratio. • Pebble has invariable central boundary & stable discharge ratio (=loading ratio). • L-H-L makes central region reduced, stagnant zone larger & total retention time longer. • H-L-H slows middle-pebble flow, enlarges central area & increases side-pebble density. • H-L-H leads to shorter total retention time and smaller stagnant zone. - Abstract: The pebble flow of a two-region-designed dynamic reactor core is simulated by discrete element method. The aims are to verify the feasibility of the two-region-designed reactor and explore the influence of loading ratio and pebble density on flow pattern. Results show that after a period of recirculation flow, the pebble bed can reach equilibrium states with invariable central boundary and stable discharging number ratio of pebbles in the middle to the side regions, which is consistent with the loading ratio of them. The mixing region at different heights and the dispersion of pebbles are analyzed. The mixing zone between the two regions is constrained within reasonable and acceptable ranges. The loading ratio has no influence on the retention rate. But it could significantly affect the two-region configuration, i.e. a larger loading ratio corresponds to a larger central region. Besides, both the shape of the central region and the stagnant zone could be affected by pebble density. Compared to the single-density condition, increasing the middle pebble density (the L-H-L condition) can accelerate the pebble flow and reduce the size of central region. Meanwhile the stagnant zone may be larger and the total retention time may be longer, which are not beneficial to the core safety. On the other hand, although the flow of middle pebbles may be much slower and the central area size may be larger by increasing the side pebble density (the H-L-H condition), all pebbles will flow out of the bed in shorter time, leading to smaller stagnant zone and shorter total retention time. Finally, the vertical flow can be greatly affected by pebble density distribution, and the axial velocity profiles show different patterns between the bottom and the upper part of the packed bed.
[en] Highlights: • LB-IB-DEM simulation of pebble bed recirculation in HTGR is conducted. • Velocity, FFT and correlation analyses are used to explore gas-pebble two phase flows in HTGR. • Slow gas-pebble flows in HTGR have intermittent and periodic mass flow pattern. • Gas-pebble interaction follows a simultaneous and linearly-dependent two-phase flow pattern. • Helium-pebble flow in real HTGR is predicted to be dominated by pebble recirculation. - Abstract: The pebble bed is one type of the core of the high temperature gas-cooled reactor (HTGR), which is regarded as the candidate of the generation IV advanced reactor. It is important to explore the gas-pebble flow characteristics and the pebble recirculation under the helium atmosphere. In this work, we presented a lattice Boltzmann (LB) method – immersed boundary (IB) method – discrete element method (DEM) coupled approach to simulate a test facility of pebble bed under the recirculation mode of operation. After model validation by an experiment of sphere sedimentation, the process of pebble recirculated at five constant rates are simulated. The correlations of gas motion and pebble motion in the upper and lower half beds are analyzed to uncover the inter-phase relationships for such intermittent pebble flows. Based on the systematic analyses of the two-phase flows, including the mean field and r.m.s field, the historical variation, inter-correlation, and the spectrum and phase space representations, we found sufficient evidences for the characteristics of intermittency, simultaneity, periodicity, and linear dependence for the inter-phase interaction of gas-pebble flows.
[en] The development plan of Experimental Power Reactor (RDE) in Indonesia is non-commercial and leads to the technology type of Pebble Bed Reactor (PBR) - High Temperature Gas Cooled Reactor (HTGR). The fuel used for PBR reactors is kernel dispersed in spherical fuel elements. The matrix used in PBR nuclear fuel is graphite which functions as a neutron moderator, fuel protective material and heat conductor. Domestication of the domestic fuel matrix needs to be conducted to improve national independence. Therefore, it is necessary to do research on the potential of local graphite to be used as RDE fuel matrix. This study focused on the identification and characterization of local and commercial graphite. The results are compared with the literature, how far it is fulfilling nuclear grade graphite for PBR fuel matrix. Characterization of graphite includes phase analysis with XRD, micro-structure with SEM, surface area/porosity, impurities determination with AAS, ICP-OES and NAA, equivalent boron content, carbon content, density, particle size distribution and ash content. The characterization results show that the carbon content obtained was 87.0 ± 4.2 % for local graphite and 100 % for commercial graphite. Meanwhile, for the purposes of nuclear graphite it requires a carbon content of >99 %. The impurity content in local and commercial graphite still does not meet the RDE fuel matrix standard. The results of XRD analysis show that the local graphite phase is the same as the commercial graphite phase, namely the 2H graphite hexagonal crystal system with the lattice group of P 63/mmc. Particle size distribution and surface area of local graphite are higher compared to nuclear graphite literature. The ash content of commercial graphite was 0.236 ± 0.029 and local graphite was 9.587 ± 0.010 %. The results of this study indicate that the local graphite from the flotation still requires a further refinement process to obtain local graphite that can be used as a fuel matrix for RDE. (author)