Discrete Simulation Of Pebble-bed Salt-cooled Reactor Core

Pebble-bed Fluoride-salt-cooled high-temperature reactor(PB-FHR)is a new kind of Generation IV nuclear reactor.It combines the clean salt coolant technology from the molten salt reactor and the improving fuels technology from gas-cooled reactors to improve safety margins and commercial availability.Different from the coupling characteristics in gas-solid system,pebble flow and coolant flow are highly correlated owing to low density ratio and the diameter of pebble fuel is of the order of centimeters.It is very time-consuming to generate an initial pebble packing under low gravity environment by traditional Discrete Element Method(DEM).Due to the large size of pebble,it makes the fluid grid too coarse to capture the complicated flow pattern.Based on the actual coupling dynamic characteristics,we propose the corresponding algorithm and form a three-step procedure to analyze the movement of pebbles,including the generation of pebble bed,extraction of topological information of packing and simulation pebble-fluid coupling process.The first step is to construct an initial pebble packing.The net buoyancy of pebbles is about one tenth of gravity since the density of pebbles is close to that of coolant,which results in long time to simulate the process of falling spheres by DEM approach.On the other hand,the effect of pebble bed with different packing fraction on the design of the neutronics and Thermal-Hydraulics should be considered in reactor safety analysis.To resolve these problems,we propose an efficient algorithm to produce random packing for mono-sized spheres in cylindrical containers.It can adjust the average overlapping between pebbles and adjust the packing fraction in a wide range from loose to dense packing efficiently.After getting the initial packing,the only index of average packing fraction is difficult to evaluate the packing comprehensively.Some other indexes on the particle scale are usually required,so we propose a tessellation algorithm based on Voronoi-Delaunay approach on single particle scale.The algorithm can compute the coordinate number of arbitrary particle,detect the topology of pore,compute the three-dimensional distribution of porosity and extract the outline of packing,etc.Now the envelope surface of packing is our research emphasis.In order to validate tessellation algorithm,the outline of the packing generated by vibration is extracted.The regular texture found in the outline reveals that crystallization of the particles next to the wall contribute to the packing densification by vibrating process.The ultimate goal is to simulate the coupled motion between pebble and fluid in irregular fluid domain.In the traditional framework of DEM-CFD,the control volume should be much larger than the particle size(i.e.,3–5 times)to meet the prerequisite that for which the drag relation is derived.However,too coarse fluid grids are not proper for the complex flow field.To solve this problem,a two-grid approach is proposed to calculate inter-phase momentum transfer between pebbles and coolant without the constraint on the shape and size of fluid meshes.The solid velocity,fluid velocity,fluid pressure and void fraction are mapped between hexahedral coarse particle grid and fine fluid grid.Then the total interphase force can be calculated independently to speed up computation.Three cases of gas-solid system are used to validate the algorithm.To test the effectiveness of the algorithm,a scaled facility is launched to generate typically complex fluidsolid environment.Water was chosen as liquid to simulate the molten salt coolant,and polypropylene balls were used to simulate the pebble fuels.Results show that the pebble flow pattern captured from experiment agrees well with the simulation from two-grid approach,which suggests the applicability of the two-grid approach for the later PB-FHR core design.