Analysis On Moisture Transfer Behavior In Carbon Material In HTGR

High-temperature gas-cooled reactor(HTGR)is the first kind of power plant with fourth-generation features in the world,and a large amount of graphite and carbon material is adopted as structural material and fuel matrix material.As a kind of porous material,the carbon material contain a certain amount of moisture and other impurities.In order to reduce the corrosion of internal material in high-temperature reactor core of HTGR,the initial core or post-accident core must be strictly dehumidified.The dehumidifying process is mainly determined by the moisture transfer in carbon material.With dehumidification of reactor core as the background,Considering the low moisture content of material and high temperature of dehumidification,the research on the mechanism and feature of moisture transfer in porous media was conducted from theoretical and experimental aspects.The pores parameters of carbon material IG-110(graphite)and BC(carbon material with boron carbide)are acquired by mercury intrusion method and SEM analysis.According to the analysis,the moisture transfer mechanism includes molecular diffusion and fluid convection.The molecular diffusion is driven by molecular concentration gradient,and the convection i.e.seepage in porous media,is driven by pressure gradient.Based on the mechanism analysis,the theoretical models,including diffusion model and diffusion-seepage model,are established to describe the moisture transfer in porous media.A comparison of moisture transfer mechanism and models in different situation is drawn.The mathematical models are applied to numerical simulation of moisture transfer in cylinder specimens of IG-110 and BC.The simulation result shows that during dehumidification process the moisture content of specimens decreases exponentially,i.e.moisture content decreases fast at first,and then slowly.The decreasing of moisture in diffusion-seepage model is faster than that in diffusion model,and the moisture decreasing of IG-110 is faster than taht of BC.Higher the temperature is,higher the porosity is,or lower the system pressure is,faster the moisture transfer is.The mathematical models are also applied to numerical simulation of dehumidification of reactor core in HTR-PM.The dynamic moisture distribution andconstraint factors are analysed.In order to optimize the dehumidifying condition,drop the system pressure from 7 MPa to 0.1 MPa,so the moisture diffusion and seepage can be both greatly improved,and the dehumidifying efficiency increases significantly.The approximate analytic solution of moisture diffusion model is derived,so the relational expression between moisture content and time can be simply calculated with the characteristic length of geometry object.The analytic solution result is compared with the numerical simulation result for several geometry object and dehumidifying problem of HTR-PM.More representative the characteristic length is for the overall geometry structure,better the fit of analytic solution and numerical result is.In the aspect of experiment,the moisture absorption experiment of IG-110 and BC was conducted.The experimental result shows that with the same temperature and humidity condition in air,the moisture adsorbance of BC is higher than that of IG-110,and the absorption of IG-110 is faster.Besides,the moisture adsorbance distribution present a feature of scattering.With a certain temperature condition,higher the humidity is,higher the moisture adsorbance is.With a certain absolute humidity condition,higher the temperature is,lower the adsorbance is.In addition,the experimental result was fitted with simulation result to derive the effective moisture diffusion coefficient of BC.The dehumidification experiment of IG-110 and BC was also conducted.The experimental result shows that the equilibrium moisture content can be significantly reduced by improving temperature.But the vacuum condition can not provide an obvious promotion to drying efficiency.At last,the moisture absorption problem differs from dehumidification problem of porous media in several aspects,so they can not be treated simply as a couple of reversed process.