Analysis on the amount of graphite dust deposited on steam generator of HTGR Wei Peng 1, Tian-qi Zhang 1, Su-yuan Yu 2 * 1 Institute of Nuclear and new Energy Technology (INET), Tsinghua University, China 2 Center for Combustion Energy, Tsinghua University, China

Background Physical Model Simulation model Result Conclusion Outline 2

Background 3 Gen-IV aims: more efficiency and more safety VHTGR: Very High Temperature Gas-cooled Reactor Gen III/III+ units represent the current level of BAT (Best Available Techniques).

Background 4 Carbonaceous dust is suspected to be a hidden problem for HTGR. It comes from abrasion between pebbles/ pebbles and the fuel handling pipe. Dusts finally deposit on the primary surface and influence the surface’s feature

5 Combined with FP (fission products) released by fuel elements  Radioactive source of HTR in depressurization accidents Hamper operation and maintenance in normal operation.  steam generator is an important component for the dust deposition. 5

Physical Model 6  Heat transfer component between the hot helium and the cool water→ great temperature gradient→ thermophoretic deposition  Spiral heat exchange tube→complex flow field→ turbulent deposition

7 the amount of graphite dust coated on steam generator is the dynamic equilibrium between deposition of graphite dust and resuspension of deposited dust.

Deposition model 8  turbulent deposition  thermophoretic deposition Model of Fan, F. G., & Ahmadi, G. Model of Brock-Talbot The total deposition mass depends on the deposition velocity

Resuspension model 9  The revised Rock’n’Roll model (Reeks and Hall, 2001) by Biasi et al. (2001)  Particle Resuspension: caused by the coupled effects of aerodynamic lift and drag; particles detach from the substrate when it receives enough energy from the turbulent flow. f a is the adhesive force which is assumed to be a log-normal distribution: F is the aerodynamic force couple acting on the particle F L is the lift force and F D is the drag force on the particle

Conditions 10 NameUnit Value Inlet velocity of hot heliumm/s27.6 Inlet temperature of hot helium K PressureMPa3 Diameter of graphite dust mm Conductivity of graphiteW/m.K25 Density of graphitekg/m Diameter of graphite dust

Velocity and temperature distribution 11 The heat flux of the tube in steam generator can be divided into four parts: heat flux distribution friction velocity distribution Distribution of temperature gradient

Collect bag Vacuum tank Link to the sonic nozzle Deposition  the thermophoretic deposition velocity is larger than turbulent deposition velocity. 12 thermophoretic deposition turbulent deposition total deposition velocity Average deposition velocity is m/s

Collect bag Vacuum tank Link to the sonic nozzle Resuspension  The resuspension rate is associated with the time: in the beginning stage, the resuspension rate is high, it decreases rapidly as the time increasing, and then it decline slower.  the resuspension rate after a long sufficient time is used to analyze the graphite dust behavior in present study, ​​ is of about s resuspension rate

Vacuum tank Distribution of dust in the steam generator The result indicated that the amount of the graphite dust loading on the tube surface will tend to 6,760mg/m the amount of the graphite dust on the tube surface depends on the rate of the deposition and resuspension.

Collect bag Vacuum tank Link to the sonic nozzle Conclusion The deposition and resuspension of graphite dust in the steam generator was studied numerically : (1)The turbulent deposition and thermophoretic deposition are the two mechanisms that make the graphite dust deposit on the heat transfer tube surface in SG, and thermophoretic deposition is the main depositional mechanism; (2)The preliminary calculations result showed that the amount of the graphite dust loading on the tube surface will tend to 6,760mg/m 2. 15

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