1. Top-Level Technical Issues for FNST #3 MHD Thermofluid Phenomena and Impact on Transport Processes in Electrically Conducting Liquid Coolants/Breeders #7 Fluid-materials interactions including interfacial phenomena, chemistry compatibility, surface erosion and corrosion Presented by Sergey Smolentsev, UCLA FNST MEEETING August 18-20, 2009 Rice Room, 6764 Boelter Hall, UCLA

2. MHD and heat/mass transfer considerations are primary drivers of any liquid metal blanket design The motion of electrically conducting breeder/coolant in the strong, plasma- confining, magnetic field induces electric currents, which in turn interact with the magnetic field, resulting in Lorentz forces that modify the original flow. The study of this behavior, which is similar in many ways to magnetized plasma physics, is known as magnetohydrodynamics (MHD). MHD forces in fusion blankets are typically 4 to 5 orders of magnitude larger than inertial and viscous forces, changing the fluid dynamics in remarkable ways. MHD forces are non-local, flow in one location can be controlled by current closure in boundary layers or structure in another location. These unique MHD coolant/breeder flows are non-linearly coupled to other transport phenomena (heat/mass transfer) – blanket performance and design requires an in- depth understanding of all these phenomena. A.Buoyancy forces associated with neutron heating cause intensive thermal convection. B.MHD turbulence in blanket flows takes a special quasi-two-dimensional form. C.Strong effect of turbulence on temperature in liquid and solid. D.Typical MHD effect is formation of special “M-type” velocity profiles. A B D C Laminar flow Turbulent flow #3 MHD Thermofluid Phenomena

3. Examples of MHD Issues impacting fusion blankets High MHD pressure drop in conducting ducts possibly exceeds the material limits and necessitates the use of insulating breaks between the fluid and structure. Lorentz forces resulting from field gradients and geometric complexities lead to formation of highly unstable internal shear layers or even reversed flows. Strong energy dissipation via Joule heating competes with turbulence production in the shear layers leading to a new turbulence phenomena known as quasi-two-dimensional turbulence Electromagnetic flow coupling controls flow distribution between parallel channels and results in higher MHD pressure drop. Interactions of MHD with buoyancy effects resulting from fusion nuclear heating drive convection cells and modify heat and mass transport in ways similar to turbulence. MHD is a complex, multi-physics, multi-scale, non-local set of phenomena  No commercial MHD CFD codes. Application of the existing MHD codes is still limited to either simple geometries or low magnetic fields Blanket conditions are also complex and can only be partially simulated outside of a fusion device  Experiments are limited to surrogate materials. Limited magnet space. Magnetic filed is typically ~2 T (10 T in IB blanket!) No prototypic volumetric heating Underlying physics of many MHD flows is still not understood. Material databases for fusion- relevant conditions are incomplete  Coupling between MHD flows and heat and mass transfer requires new mathematical models and boundary conditions Challenges in Resolving MHD issues #3 MHD Thermofluid Phenomena

4. Where we are on MHD Research for Fusion For decades, blankets were designed using very simple models (slug flow, core flow approximation, etc.) and limited experimental data. These blankets were never built or tested. Development of insulator coatings capable of meeting the crack tolerances has not been successful. New ideas about insulation are currently evolving Recent blanket studies have shown that the MHD phenomena in blankets are more complicated (e.g., turbulence, coupling with heat and mass transfer, etc.). Recent trends: insulation with flow channel inserts, capturing 3D effects, real geometry, strong multi-component magnetic fields, complete physical models. Modeling: either high Ha~10 4 but canonical geometry or complex geometry but moderate Ha~10 2 -10 3.. Experiment: prototypical geometries but moderate Ha (~10 3 ) and reduced dimensions (~1/4). Objectives and required R&D Scientific objectives: understanding non-linear transport phenomena associated with MHD, heat and mass transfer in flowing liquid breeders in the presence of a strong time- and space- varying magnetic fields and nuclear heating. Engineering objectives: predicting impact of blanket performance, improvement of existing and development of new blanket designs that lead to high thermal efficiency while meeting material and safety limits in normal and off-normal conditions. We need: (A) effective 3D CFD codes suited for complex 3D blanket geometry (flows with FCI, manifolds, etc.) strong magnetic fields (Ha~10 4 ), prototypic flow velocities (Re~10 5 ) and volumetric heating (Gr~10 12 ); (B) new prototypic experimental MHD facilities at higher field and, (C) experiments in a real fusion environment to study synergistic phenomena. #3 MHD Thermofluid Phenomena

5. The interfacial processes (e.g. corrosion, tritium permeation) are tightly coupled with the breeder/coolant flow. The underlying physics is not well understood, limiting further progress towards high-efficiency breeder blankets When liquid breeder/coolant flows through a magnetic field, the induced electric currents are closed through the conducting structure affecting the flow itself. On the other hand, there exists the interplay between the flowing liquid and the physical-chemical processes at the solid-liquid interface, e.g., corrosion/erosion, tritium permeation, interfacial slip and thermal leakages. These interrelated processes make up a group of phenomena called “fluid-materials interactions,” which have a strong impact on blanket operation and, thus, are among the most important blanket feasibility issues. Macrostructure of the washed samples after contact with the PbLi flow From: F. Muktepavela et al. EXPERIMENTAL STUDIES OF THE STRONG MAGNETIC FIELD ACTION ON THE CORROSION OF RAFM STEELS IN Pb17Li MELT FLOWS, PAMIR 7, 2008 Strong experimental evidence of significant effect of the applied magnetic field on corrosion rate. Corrosion rate for samples with and without a magnetic field B=0B=1.8 T #7 MHD Fluid-Materials Interaction

6. Issues: electromagnetic, thermal, chemical interaction  Effect of MHD flows on temperature distribution in the surrounding solid structure –How do MHD flows affect the temperature distribution and associated thermal stresses in the surrounding solid structure, including the ferritic wall and the flow insert ? What is the effect of MHD flows on the interfacial heat fluxes and temperatures?  Tailoring flow channel insert properties (for DCLL) –How to design the flow channel insert and tailor its properties (electrical and thermal conductivity) to reach high exit temperatures, while reducing the MHD pressure drop, minimizing heat leakages, and meeting the material limitations?  Effect of the interfacial slip on the blanket flows –What is the interfacial slip between the flowing liquid metal and silicon carbide and how strong is its impact on the reduction of the MHD pressure drop and heat transfer enhancement? How to engineer super-hydrophobic surfaces leading to MHD drag reduction?  Corrosion processes at the liquid-solid interface –How do the MHD flows (flow regime, velocity profile, etc.) and the magnetic field itself affect corrosion/deposition processes at the liquid-solid interface?  Tritium permeation –What is the effect of MHD flows, including those in the thin gaps, on tritium permeation in the blanket? #7 MHD Fluid-Materials Interaction

7. Where we are: at the very beginning Objectives and required R&D Scientific objectives: to develop proper boundary conditions and transport models that take into account interfacial processes and MHD interactions. To incorporate them into the CFD codes. Engineering objectives: reduction of MHD pressure drop and heat leakages by tailoring the flow insert properties, evaluation of material limitations associated with thermal stresses and corrosion/deposition, and minimization of tritium permeation. We need an extensive experimental and theoretical program to improve our fundamental knowledge of interfacial physics in the presence of a magnetic field. Challenges: lack of fundamental knowledge There is a need for proper boundary conditions that take into account mass and heat transport across the solid-liquid interface. The physics of corrosion/deposition, tritium permeation, and interfacial slip processes is not well understood. The existing transport models are incomplete lacking MHD interactions. Experimental and theoretical studies aiming at characterization of interfacial phenomena have been started to replace conservative approximations (examples: PbLi-Fe temperature is limited to 470  C - very conservative assumption! Tritium permeation – gains much attention but still no solution!). The progress is limited because of lack of the physical knowledge. #7 MHD Fluid-Materials Interaction