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[en] The PBR is one type of High Temperature Reactors, which allows high temperature work while preventing the fuel from melting (bringing huge safety margins to the reactor) and high electricity efficiency. The design is also highly scalable; a plant could be designed to be as large or small as needed, and can even be made mobile, allowing it to be used onboard a ship. In a PBR, small particles of nuclear fuel, embedded in a moderating graphite pebble, are dropped into the reactor as needed. At the bottom, the pebbles can be removed simply by opening a small hatch and letting gravity pull them down. To cool the reactor and create electricity, helium gas is pumped through the reactor to pull heat out which is then run through generators. One of the most difficult problems to deal with has been the possible appearance of local temperature hotspots within the pebble bed heating to the point of melting the graphite moderators surrounding the fuel. Obviously, constructing a reactor and experimenting to investigate this possibility is out of the question. Instead, nuclear engineers have been attempting to simulate a PBR with various CFD codes. The thermo-dynamic analysis to simulate realistic conditions in a pebble bed are described and the results are shown. (orig.)
[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] High temperature reactor is one of the core technologies of Generation IV reactors for its inherent safety features, economical competitiveness and broad application prospects. To ensure nuclear safety, ECRO has implemented strict and effective inspection over HTR-PM during the construction. This paper gives a brief introduction on the technical features of HTR-PM, and analyses the keystone and difficulties of its construction. Then a summary of inspection activities is given, which can be beneficial for the inspection over subsequent HTR reactors. (authors)
[en] Complexity in PBMR – Pebble Bed Modular Reactor – design has brought nuclear engineers to use Monte-Carlo based nuclear codes to analyze it. In this work we tried to improve the ability of the MCNP4c code in the analysis of such reactors. The improvement was reached through the updating of the cross-section library. Our main goal was to implement a new multi-temperature ENDFVII based library into MCNP4c and study its effects on PBM reactor analysis through the benchmarking process.
[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] Pebble-bed reactor analysis code COBBLE was developed by coupling deterministic neutronic calculation code CITATION and depletion code ORIGEN2 for the purpose of searching for equilibrium state of pebble-bed reactor with specified recirculation scheme. The spectra of all zones were calculated to modify the cross sections at different places of the reactor core. The equilibrium cycle was searched by an iteration procedure. A simplified version of pebble-bed modular reactor (PBMR) model with 6-pass random recirculation scheme was referred to as a benchmark test. Power and burnup distributions were calculated by COBBLE and compared with those obtained by PBRE code based on Monte Carlo neutronic transport program. Results show that COBBLE is suitable to analyze the equilibrium cycle of pebble-bed reactor. (authors)
[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: •Thermo Hydraulic Porous Program is made to simulate block fuel elements and pebble bed reactors. •Results of the comparisons show full agreement as regards pressurized/depressurized Cooldown states. •Good agreement was revealed as orienting a 2D benchmark to PBMR-400. •Good agreement also was revealed as orienting a 2D benchmark to GT-MHR. -- Abstract: The current study aims at introducing a 2D and fast-running code for the issues pertinent to design, analysis and safety in modular high temperature reactors. While the porous media approach is only applied to pebble bed type, the analysis in this paper covers both pebble bed and prismatic reactor. A time-dependent mass equation along with energy conservation equation for the cooling gas and a time-dependent energy conservation equation for the solid was solved. Appropriate series of constitutive equations (e.g. heat transfer coefficient, effective heat conductivity of solid, heat transfer coefficient, pressure drop etc.) has been recruited as well. In addition a finite-volume method is employed for spatial discretization. The SIMPLET algorithm has been used to solve the velocity and pressure linked to the momentum equation. The method of SIMPLET for natural convection is lot more advantageous over the SIMPLE method and will improve the results. Our developed code utilizes advantages of both Zehner and Schlünder and Kasperek and Vortmeyer models which lead to better results. In addition, in Thermo Hydraulic Porous Program (THPP), the Rhie-Chow technique is also used to correct the velocity components while dealing with the discretization problem of the pressure gradient. In the codes developed so far, staggered grids is usually used in computations. However, here we have adopted most of the advantages of Rhie-Chow technique in precision and computational cost. Making use of some simplified assumptions made by the benchmark definition, the core has been modeled in form of 2D-geometry. The calculations below deal with the loss of cooling accidents with or without depressurization. Having compared 2D results of THPP, the well-established thermal-hydraulics codes of THERMIX (Banaschek, 1983) and TH3D (Hossain, 2008) to simulate pebble bed and block fuel elements, it becomes clear that regarding the transient behavior during a depressurized loss of coolant accident, there exists a good agreement. Besides, there were detected more considerable differences between the results of the two codes regarding the pressurized loss of cooling accident. The program code shall be generally applicable for modular High Temperature Reactors (HTRs) e.g. pebble fuel and block fuel elements.
[en] Pebble bed reactors enable the circulation of pebble fuel elements when the reactors are in operation. This unique design helps to optimize the burnup and power distribution, reduces the excessive reactivity of the reactor, and provides a mean to identify and segregate damaged fuel elements during operation. The movement of the pebbles in the core, or the kinematics of the pebble bed, significantly affect the above features and is not fully understood. We designed and built a detection system that can measure 3-axis acceleration, 3-axis angular velocity, 3-axis rotation angles, and vibration and temperature of multiple pebbles anywhere in the pebble bed. This system uses pebble-shaped detectors that can flow with other pebbles and does not disturb the pebble movement. We used new technologies to enable instant response, precise measurement, and simultaneous collection of data from a large number of detectors. Our tests show that the detection system has a negligible zero drift and the accuracy is better than the designed value. The residence time of the pebbles in a moving pebble bed was also measured using the system. (authors)