Tritium transport simulations in PbLi breeder blankets

1. Tritium transport simulations in PbLi breeder blankets Hongjie Zhang UCLA Ph.D. Student

2. Introduction • Tritium transport, and permeation in fusion blankets are important • To contribute achieving tritium self-sufficiency (for given tritium generation rate) • To accurately characterize tritium inventory and losses (for safety concerns) • Issues • Tritium behavior in LM blanket involves complicated phenomena consisting of spatial and time dependent tritium generation profile, tritium permeation, thermo-fluid, nuclear heating, and chemical reactions. • Prediction of tritium transport inside the blanket requires knowledge of MHD for accurate estimations • Low tritium solubility in PbLi leads to high permeation • If chemical reactions are involved, the mathematical description of which may be complex • Being able to treat 3D complicated geometries • Large He concentrations in liquid metal may result in bubble formation • He concentration can modify heat/mass/electrical transfer interfacial exchange coefficients between the liquid metal and the structural material. • Bubbles could act as an effective T sink, affecting T overall inventory and making it difficult for extraction

3. Scope/Objective • Develop 3D computational models to characterize diffusive, convective and temperature effects on tritium transport in PbLi blankets • Integrate the mass transfer model with the thermal-fluid analysis to account for the velocity (ordinary and MHD flow) and temperature profiles • Account for the tritium generation rate profile and nuclear heating rate profile. • Include complex blanket geometry into analysis domain • Evaluate tritium transport phenomena in PbLi accompanying helium(He) nucleated bubbles and develop relevant transport models to account for He effects • Applications: • Obtain Tritium Concentration profile, Tritium permeation flux, and other parameters of interest for prototypical PbLi Blanket designs (DCLL/HCLL). • Optimize permeator design parameters for tritium extraction. • Assess effect of helium bubbles on permeator extraction efficiency

4. PbLi + T Gas Molecule Gas atom Solid PbLi Relevant Tritium Transport Mechanisms and Issues

5. Pb-17Li mass transport • B.C. • Notes • T transport model CT,S2 C1 Solid QPb-17Li He CT,S1 Mathematical transport models(Temperature and convection effects) • Velocity u (MHD flow) is obtained from HIMAG/Stream • Solubility and diffusivity database are derived from experiments • T generation rate (Qc) is calculated by Neutronics code • U: Turbulent velocity • Turbulent diffusion coefficient is determined by turbulent viscosity and turbulent Schmidt number • Convection-Diffusion in PbLi • Diffusion in Solid • Convection-Diffusion in He coolant • At PbLi/Solid and gas/Solid interfaces: • Continuity of flux • Discontinuity of concentration

6. DCLL Isometric View Z - poloidal T concentration in FS T concentration in PbLi On the plane z=1.57m T concentration Velocity PbLi Outlet FS X - radial Y - toroidal FCI PbLi Inlet He Outlet PbLi 1.66m He Inlet Tritium concentration profile in PbLi and FS structure (DCLL TBM geometry, turbulent PbLi flow without MHD effect) Accounting nuclear heating and T generation profiles

7. 2D Geometry with constant T generation rate(0.035m height, 1m length, 5mm FS thickness) PbLi + T Tritium concentration in PbLi: T concentration vs. y at x=0.8m Velocity distribution vs. y at x=0.8m Tritium permeation flux through the wall Note: Side layer velocity profile y x parabolic velocity profile Velocity profiles affect tritium concentration and permeation characteristics(Parabolic, Side layer, and Ha layer velocity profile) • Same mass flow rates, Constant T generation rate • For parabolic velocity profile, T concentration is higher near wall, however, even closer to the walls, the concentration falls down due to permeation • For the Side layer velocity, T concentration drops at the highest velocity region of the “M” shape velocity profile. • M-Shape MHD velocity profiles reduce tritium permeation

8. Notes: T concentration along center line Initial results of Tritium Concentration impacted by a 3D MHD flow(3 U-bent duct flow with conducting walls connected through inlet/outlet with manifolds) C D A B Velocity Profile • Higher T concentration near the outlet of up-flow ducts • MHD M-shape velocity profile alternate T concentration profile in radial direction, T reductions are observed(red circles A and B). • T concentration is higher near front walls (red square C) due to the high T generation and low velocity close to the front walls, however, even closer to the walls (D), the concentration falls down again due to permeation T concentration in PbLi T Production rate

9. Summary and Next Steps • Summary • 3D computational models are initially developed to predict tritium transport in PbLi liquid breeders • Account the effects of convection and the accompanying velocity profile and temperature profile in a complicated geometry • The low tritium concentration layer close to the permeating walls ( due to M-shape side-layer velocity profile or flat-shape Ha-layer velocity profile) has shown a reduced permeation rate. • Next • Evaluate He Bubble effects • Bubble nucleation and interfacial nucleation • Tritium transport between bubble and LM • Applications to DCLL/HCLL with the latest available MHD velocity profiles