Research On Bypass Flows In Vertical Gaps Between Side Reflectors In HTR-PM

HTR-PM(High Temperature Gas-cooled Reactor Pebble-bed Module)is considered as one of the candidates for the 4th generation reactors.Due to its inherent safety,high generating efficiency,potential heat application and modular build characteristic,HTR-PM has received worldwide attention in the field of nuclear energy.Large amount of graphite blocks serve as the structural material in the reactor core of HTR-PM.Meanwhile,the paths of main coolant flow and bypass flows are also built by graphite blocks.Due to the loose body structure,while most of helium flows through pebble bed,a small part of coolant may flow through narrow gaps between the graphite blocks and become bypass flows of HTR-PM.Bypass flows will reduce the amount of effective coolant flow of reactor core,thus they will influence the temperature and velocity distributions in HTR-PM.Moreover,bypass flows may lead to a higher radiation level of primary circuit and affect the safety of HTR-PM.Therefore,the evaluation of bypass flows are a key part of HTR-PM thermos-hydraulics analysis.In HTR-PM,the narrow gaps have enormous amount and varied structure,and connected with each other and with the pebble-bed at the same time.The complex connecting relations of narrow gaps make it difficult to study bypass flows of HTR-PM with experiment.Most of previous studies were conducted by numerical methodology.With the progress of computational capabilities,we can build a three dimensional Computational Fluid Dynamic(CFD)model based on the actual structures and study the flow phenomena of bypass flows with CFD method.In this thesis,ANSYS Fluent was utilized to study bypass flows in vertical gaps between side reflectors.At first,CFD models to for helium flow in pebble-bed and narrow gaps were validated.Then,we built the CFD model of bypass flows in vertical gaps in side reflector.According to the simulation results,we understood the bypass flow phenomena in HTR-PM and analyzed the influence of bypass flows to mass flow and temperature distribution in HTR-PM.The main works of this thesis are summarized as follows:(1)The non-equilibrium porous model was used to simulate helium flow in pebble-bed and the equivalent for helium flow in bottom reflector and hot plenum.Thecomparison results of CFD simulation and THERMIX showed that relative tolerances of key parameters were less than 4%.(2)The experiment,which is conducted by Northwest University,South Africa,about helium flow in mini-scale gap was simulated in CFD method.The comparison results of CFD simulation and experiment indicated the importance of boundary layer.In addition,the RNG k-? turbulence model with enhanced wall treatment or SST k-?turbulence model is more appropriate to simulate the helium flow in mini-scale gap.(3)The model of bypass flows in vertical gaps between side reflector graphite blocks in HTR-PM was built.We studied helium flow phenomena of the main flow,which flowed through cold plenum,top reflector,pebble-bed,bottom reflector,hot plenum successively.The bypass flows and cross flows were also studied.To the vertical gap with width size 1.6mm,the mass flow proportion of bypass is 0.96% at cold plenum gap inlet,0.63% at side reflector gap inlet and 1.94% at side reflector gap outlet.