Tritium transport simulations in Pb Li breeder blankets

Tritium transport simulations in Pb. Li breeder blankets Hongjie Zhang UCLA Ph. D. Student

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

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

Relevant Tritium Transport Mechanisms and Issues Mechanisms Issues Solution/Diffusion/Convection(MHD velocity profile) of atomic tritium within the Pb. Li Requires MHD velocity profile and temperature dependent properties for accurate estimation Tritium transfer across Pb. Li/solid interface Low tritium solubility in Pb. Li lead to high permeation 1. Pb. Li + T(L) <-> Solid + T(s) or 2. Pb. Li + T(L) <-> T 2(g) <-> Wall + T(s) Diffusion of atomic tritium through the structure Dissolution-recombination at the solid/gas interface Convection-diffusion of T 2, in the He coolant Bubbles could act as an effective T sink, affecting T overall inventory and making it difficult for extraction Need to account He bubble effect Pb. Li + T Gas Solid Pb. Li Gas Molecule atom

Mathematical transport models (Temperature and convection effects) l T transport model 1. Convection-Diffusion in Pb. Li 2. Diffusion in Solid 3. Convection-Diffusion in He coolant l B. C. Pb-17 Li mass transport l Notes 1. Velocity u (MHD flow) is obtained from HIMAG/Stream 2. Solubility and diffusivity database are derived from experiments 3. T generation rate (Qc) is calculated by Neutronics code 4. U: Turbulent velocity 5. Turbulent diffusion coefficient is determined by turbulent viscosity and turbulent Schmidt number CT, S 2 C 1 QPb-17 Li Solid CT, S 1 He At Pb. Li/Solid and gas/Solid interfaces: 1. Continuity of flux 2. Discontinuity of concentration

Tritium concentration profile in Pb. Li and FS structure (DCLL TBM geometry, turbulent Pb. Li flow without MHD effect) T concentration in Pb. Li DCLL Isometric View Pb. Li Outlet T concentration in FS FS FCI He Outlet Pb. Li 1. 66 m Pb. Li Inlet Z - poloidal On the plane z=1. 57 m T concentration He Inlet X - radial Y - toroidal Accounting nuclear heating and T generation profiles Velocity

Velocity profiles affect tritium concentration and permeation characteristics (Parabolic, Side layer, and Ha layer velocity profile) Tritium concentration in Pb. Li: 2 D Geometry with constant T generation rate (0. 035 m height, 1 m length, 5 mm FS thickness) Side layer velocity profile y Pb. Li + T parabolic velocity profile x Velocity distribution vs. y at x=0. 8 m Tritium permeation flux through the wall T concentration vs. y at x=0. 8 m Note: • • 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

Initial results of Tritium Concentration impacted by a 3 D MHD flow (3 U-bent duct flow with conducting walls connected through inlet/outlet with manifolds) T concentration along center line C D A B Velocity Profile Notes: • • T concentration in Pb. Li T Production rate • 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

Summary and Next Steps ❖ Summary l 3 D computational models are initially developed to predict tritium transport in Pb. Li liquid breeders § l ❖ 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 l l Evaluate He Bubble effects l Bubble nucleation and interfacial nucleation l Tritium transport between bubble and LM Applications to DCLL/HCLL with the latest available MHD velocity profiles