Research On The Deposition Of Graphite Dust In Temperature Gradient Layer Of HTGR

The motion of graphite dust in HTGR has an important influence on the safe operation of the nuclear reactor.Flowing with the helium gas,the graphite dust would deposite on the surfaces of different parts in the primary loop,which would influence the thermal hydraulic performance of these parts and cause the difficulty on the maintenance and repair.There are many heat transfer area in the primary loop of HTGR,in which the motion and deposition of graphite dust would be affected intensively by thermophoretic force because of temperature gradient.In order to research the deposition of HTGR's graphite dust in the temperature boundary layer,an experiment and two numerical simulations have been conducted,whose results can offer help for the control and settlement of graphite dust issue.The results of graphite dust's deposition experiment in temperature gradient layer of pipe show that the total deposition efficiency and the efficiency in the first pipe increase with the increasing inlet temperature and decreasing Reynold number,the deposition efficiency in the second pipe decrease with the increasing Reynold number.Because the heat exchanger between the gas and surface of pipe concentrates in the first pipe,the deposition in first pipe is caused by the thermophoresis flow;while for the second pipe,the deposition is mainly caused by the flow of fluid because there is not obvious temperature gradient layer in second pipe.The simulation results of graphite dust's deposition in recuperator show that deposition in plain fin model is mainly thermophoresis deposition,deposition efficiency decreases the increasing particle size and inlet velocity;while that in the staggered fin model is mainly inertial deposition and deposition efficiency increases with the model complexity,particle size and inlet velocity.The portion of thermophoresis deposition in plain fin model is more than 90% while that in staggered fin model is about 5%.The comprehensive consideration of performance of fluid flow and heat transfer and the deposition of graphite dust in recuperator indicates that the recuperator with a modest fin length should be adopted for practical application.The simulation results of graphite dust's deposition in pebble bed show that deposition in pebble bed are mainly inertial deposition and turbulent deposition,thermophoresis force has inhibitory effect on deposition of graphite dust because the temperature of fuel element is higher than helium's and the inhibitory effect is strongest in the body-centered cubic structure.Because of the simplicity of the channel in simple cubic,the deposition efficiency is lower obviously.With the increase of pebble bed layer and the particle size,the particle deposition efficiency increases but this tendency is not evident in simple cubic structure.Graphite dust trajectory also has vortex structure because of the eddy structure of fluid velocity.Graphite dust deposition efficiency is the highest in body centered cubic structure and lowest in simple cubic structure.