Overview of Fusion Nuclear Technology (FNT)

1. Overview of Principles, Concepts, and Key Issues of Fusion Nuclear TechnologyMohamed AbdouProfessor of Engineering andDirector of Fusion Science and Technology CenterUniversity of California Los AngelesSeminar Presented to KAERI and KBSI, Korea, April 2004

2. Overview of Fusion Nuclear Technology (FNT) Seminar Outline Principles, Concepts, and Key Issues (this presentation/handout) • FNT components and functions • Tritium Breeding • Solid Breeders - Design Concepts and Materials - Tritium Release, Extraction - Issues and R&D • Structural Materials • Liquid Breeders - Concepts: Materials, Configurations - Li/V - LiPb Dual Coolant - Molten Salts - Issues • Tritium Supply and Need for Blanket Testing in ITER • Possible Areas for US-Korea Collaboration Other Presentations / Handouts - Fabrication Technology - ITER Test Blanket Module (TBM) - ITER Testing : Engineering Scaling

3. Incentives for Developing Fusion • Fusion powers the Sun and the stars • It is now within reach for use on Earth • In the fusion process lighter elements are “fused” together, making heavier elements and producing prodigious amounts of energy • Fusion offers very attractive features: • Sustainable energy source (for DT cycle; provided that Breeding Blankets are successfully developed) • No emission of Greenhouse or other polluting gases • No risk of a severe accident • No long-lived radioactive waste • Fusion energy can be used to produce electricity and hydrogen, and for desalination

4. The Deuterium-Tritium (D-T) Cycle • World Program is focused on the D-T cycle (easiest to ignite): D + T → n + α+ 17.58 MeV • The fusion energy (17.58 MeV per reaction) appears as Kinetic Energy of neutrons (14.06 MeV) and alphas (3.52 MeV) • Tritium does not exist in nature! Decay half-life is 12.3 years (Tritium must be generated inside the fusion system to have a sustainable fuel cycle) • The only possibility to adequately breed tritium is through neutron interactions with lithium • Lithium, in some form, must be used in the fusion system

5. Fusion Nuclear Technology (FNT) Fusion Power & Fuel Cycle Technology FNT Components from the edge of the Plasma to TF Coils(Reactor “Core”) 1. Blanket Components 2. Plasma Interactive and High Heat Flux Components a. divertor, limiter b. rf antennas, launchers, wave guides, etc. 3. Vacuum Vessel & Shield Components Other Components affected by the Nuclear Environment 4. Tritium Processing Systems 5. Instrumentation and Control Systems 6. Remote Maintenance Components 7. Heat Transport and Power Conversion Systems

6. ARIES-AT

7. Blanket Radiation Plasma Neutrons Shield Vacuum vessel First Wall Coolant for energy conversion Magnets Tritium breeding zone

8. Blanket (including first wall) • Blanket Functions: • Power Extraction • Convert kinetic energy of neutrons and secondary gamma-rays into heat • Absorb plasma radiation on the first wall • Extract the heat (at high temperature, for energy conversion) • Tritium Breeding • Tritium breeding, extraction, and control • Must have lithium in some form for tritium breeding • Physical Boundary for the Plasma • Physical boundary surrounding the plasma, inside the vacuum vessel • Provide access for plasma heating, fueling • Must be compatible with plasma operation • Innovative blanket concepts can improve plasma stability and confinement • Radiation Shielding of the Vacuum Vessel

9. Blanket Materials • Tritium Breeding Material (Lithium in some form) Liquid: Li, LiPb (83Pb 17Li), lithium-containing molten salts Solid: Li2O, Li4SiO4, Li2TiO3, Li2ZrO3 • Neutron Multiplier (for most blanket concepts) Beryllium (Be, Be12Ti) Lead (in LiPb) • Coolant – Li, LiPb – Molten Salt – Helium – Water • Structural Material • Ferritic Steel (accepted worldwide as the reference for DEMO) • Long-term: Vanadium alloy (compatible only with Li), and SiC/SiC • MHD insulators (for concepts with self-cooled liquid metals) • Thermal insulators (only in some concepts with dual coolants) • Tritium Permeation Barriers (in some concepts) • Neutron Attenuators and Reflectors

10. Notes on FNT: • The Vacuum Vessel is outside the Blanket (/Shield). It is in a low-radiation field. • Vacuum Vessel Development for DEMO should be in good shape from ITER experience. • The Key Issues are for Blanket / PFC. • Note that the first wall is an integral part of the blanket (ideas for a separate first wall were discarded in the 1980’s). The term “Blanket” now implicitly includes first wall. • Since the Blanket is inside of the vacuum vessel, many failures (e.g. coolant leak from module) require immediate shutdown and repair/replacement. Adaptation from ARIES-AT Design

11. Heat and Radiation Loads on First Wall • Neutron Wall Load≡ Pnw Pnw = Fusion Neutron Power Incident on the First Wall per unit area = JwEo Jw = fusion neutron (uncollided) current on the first wall Eo = Energy per fusion neutron = 14.06 MeV • Typical Neutron Wall Load ≡ 1-5 MW/m2 At 1 MW/m2: Jw = 4.43 x 1017 n · m-2 · s-1 • Note the neutron flux at the first wall (0-14 MeV) is about an order of magnitude higher than Jw • Surface heat flux at the first wall This is the plasma radiation load. It is a fraction of the α-power qw = 0.25 Pnw · fα where f is the fraction of the α-power reaching the first wall (note that the balance, 1 – f, goes to the divertor)

12. Poloidal Variation of Neutron Wall Load • Neutron wall load has profile along the poloidal direction (due to combination of toroidal and poloidal geometries) • Peak to average is typically about 1.4 outboard Inboard (equatorial plane, outboard, in 0º)

14. Natural lithium contains 7.42% 6Li and 92.58% 7Li. The 7Li(n;n’a)t reaction is a threshold reaction and requires an incident neutron energy in excess of 2.8 MeV. Tritium Breeding 6Li (n,a) t 7Li (n;n’a) t

15. Tritium Self-Sufficiency • TBR ≡ Tritium Breeding Ratio = = Rate of tritium production (primarily in the blanket) = Rate of tritium consumption (burnt in plasma) Tritium self-sufficiency condition: Λa > Λr Λr = Required tritium breeding ratio Λr is 1 + G, where G is the margin required to: a) compensate for losses and radioactive decay between production and use, b) supply inventory for start-up of other fusion systems, and c) provide a hold-up inventory, which accounts for the time delay between production and use as well as reserve storage. Λris dependent on many system parameters and features such as plasma edge recycling, tritium fractional burnup in the plasma, tritium inventories, doubling time, efficiency/capacity/reliability of the tritium processing system, etc. Λa = Achievable breeding ratio Λa is a function of FW thickness, amount of structure in the blanket, presence of stabilizing shell materials, PFC coating/tile/materials, material and geometry for divertor, plasma heating, fueling and penetration.

16. Neutron Multipliers Examples of Neutron Multipliers Beryllium, Lead • Almost all concepts need a neutron multiplier to achieve adequate tritium breeding. • (Possible exceptions: concepts with Li and Li2O) • Desired characteristics: • Large (n, 2n) cross-section with low threshold • Small absorption cross-sections • Candidates: • Beryllium is the best (large n, 2n with low threshold, low absorption) • Be12Ti may have the advantage of less tritium retention • Pb is less effective except in LiPb • Beryllium results in large energy multiplication, but resources are limited. 9Be (n,2n) Pb (n,2n)

17. Fuel Cycle Dynamics The D-T fuel cycle includes many components whose operation parameters and their uncertainties impact the required TBR Fueling Fuel management Fuel inline storage Tritium shipment/permanent storage • Examples of key parameters: • ß: Tritium fraction burn-up • Ti: mean T residence time in each component • Tritium inventory in each component • Doubling time • Days of tritium reserves • Extraction inefficiency in plasma exhaust processing Plasma exhaust processing Impurity separation Plasma Isotope separation system FW coolant processing Impurity processing Plasma FacingComponent Coolant tritium recovery system Tritium waste treatment (TWT) PFC Coolant Water stream and air processing Solid waste Blanket Coolant processing waste Breeder Blanket Blanket tritium recovery system Only for solid breeder or liquid breeder design using separate coolant Only for liquid breeder as coolant design

18. Achievable TBR is Very Sensitive to FW Thickness ITER FW Panel Cross Section TBR drops by up to 15% with 2 cm thick FS FW [L. El-Guebaly, Fusion Engr & Design, 2003]

19. TBR is Very Sensitive to Structure Content in Blanket • Impact of structure content on TBR depends on breeder and structural material used • V has the least impact on breeding • Up to 30% reduction in TBR could result from using 20% structure in the blanket Note: Net TBR is substantially lower (~30-40%) than local TBR

20. Blanket Concepts(many concepts proposed worldwide) • Solid Breeder Concepts • Always separately cooled • Solid Breeder: Lithium Ceramic (Li2O, Li4SiO4, Li2TiO3, Li2ZrO3) • Coolant: Helium or Water • Liquid Breeder Concepts Liquid breeder can be: a) Liquid metal (high conductivity, low Pr): Li, or 83Pb 17Li b) Molten salt (low conductivity, high Pr): Flibe (LiF)n· (BeF2), Flinabe (LiF-BeF2-NaF) B.1. Self-Cooled • Liquid breeder is circulated at high enough speed to also serve as coolant B.2. Separately Cooled • A separate coolant is used (e.g., helium) • The breeder is circulated only at low speed for tritium extraction B.3. Dual Coolant • FW and structure are cooled with separate coolant (He) • Breeding zone is self-cooled

21. A Helium-Cooled Li-Ceramic Breeder Concept: Example • Material Functions • Beryllium (pebble bed) for neutron multiplication • Ceramic breeder (Li4SiO4, Li2TiO3, Li2O, etc.) for tritium breeding • Helium purge (low pressure) to remove tritium through the “interconnected porosity” in ceramic breeder • High pressure Helium cooling in structure (ferritic steel) Several configurations exist (e.g. wall parallel or “head on” breeder/Be arrangements)

22. Neutron Multiplier Be, Be12Ti (<2mm) Optional W coating for FW protection Tritium Breeder Li2TiO3, Li2O (<2mm) Coolant water (25MPa, 280/510oC) First Wall (RAFS, F82H) Surface Heat Flux:1 MW/m2 Neutron Wall Load: 5 MW/m2(1.5×1015n/cm2s) JA Water-Cooled Solid Breeder Blanket

23. Helium-cooled stiffening grid Breeder unit FW channel Helium-Cooled Pebble Breeder Concept for EU

24. Radial-poloidal plate Grooves for helium coolant Helium Radial-toroidal plate Stiffening plate provides the mechanical strength to the structural box Cut view

25. Breeder Unit for EU Helium-Cooled Pebble Bed Concept

26. Li(n, 4He)T Mechanisms of tritium transport 1) Intragranular diffusion 2) Grain boundary diffusion 3) Surface Adsorption/desorption 4) Pore diffusion 5) Purge flow convection Breeder pebble Purge gas composition: He + 0.1% H2 Tritium release composition: T2, HT, T2O, HTO (solid/gas interface where adsorption/desorption occurs) Mechanisms of tritium transport (for solid breeders)

27. Diffusion model: Some mathematical formulas Generation rate Activation energy First -order tritium release rate estimated: Surface concentration (atoms/m2) Desorption rate constant Desorption energy

28. MISTRAL (Model for Investigative Studies of Tritium Release in Lithium Ceramics)- a code developed at UCLA To understand and predict tritium release characteristics Gas phase • Transport mechanisms included: • grain diffusion • grain boundary diffusions • adsorption from the bulk and from the pores to the surface • desorption to the pores • diffusion through the pores • Features • includes details of the ceramic microstructure • includes coverage dependence of the activation energy of surface processes (adsorption/ desorption) Solid phase Phenomenological cartoon

29. “Temperature Window” for Solid Breeders • The operating temperature of the solid breeder is limited to an acceptable “temperature window”: Tmin– Tmax • Tmin, lower temperature limit, is based on acceptable tritium transport characteristics (typically bulk diffusion). Tritium diffusion is slow at lower temperatures and leads to unacceptable tritium inventory retained in the solid breeder • Tmax, maximum temperature limit, to avoid sintering (thermal and radiation-induced sintering) which could inhibit tritium release; also to avoid mass transfer (e.g., LiOT vaporization) • The limitations on allowable temperature window, combined with the low thermal conductivity, place limits on allowable power density and achievable TBR

30.  =Porosity,  = pebble sphericity =1 for spherical pebble N = moles/s R = ideal gas constant T = temperature f = helium gas viscosity Ab= gas flow cross-sectional area P0= inlet pressure L = flow path dp = particle diameter single size bed binary bed Effect of helium purge flow rate on pressure drop and tritium permeation Porosity, 

31. Properties are for 100% TD Li2O Li4SiO4 Li2TiO3 Li2ZrO3 Lithium Density(g/cm3) 0.94 0.51 0.43 0.38 Diameter (mm) ~1.0 0.2~0.7 0.7~0.85 0.9~1.5 Thermal Expansion @ 500 ° C (DL/L0%) 1.25 1.15 0.8 0.5 Higher design margin Thermal Conductivity @ 500 ° C (W/m/ ° C) 4.7 2.4 1.8 0.75 Relatively narrow T window Min.-Max. Temp. for Tritium Release (°C) 397-795 325-925 Up to 900 400-1400 Swelling @500 ° C (DV/V0%) 7.0 1.7 - < 0.7 Reactivity w/H20 High Little Less Less Pore for tritium release Grain Size (μm) 50 5-15 1-4 0.5-2 Density (%TD) 80-85 ~98 87~89 93~96 Crush Load (N) - ~ 10 24-33 68-79 Residence time @400 °C (h) 10 2 2 1 Which solid breeder ceramic is better? Parameters: Lithium density Tritium residence time Thermal-physical properties Mechanical properties Temperature window Transmutation nuclides (activation products) Reactivity Fabrication Irradiation effects (e.g, swelling) • Notes: • Li2O is highly hygroscopic: 2Li2O + H2O → 2LiOH (ΔH = 128.9 kJ/mole); LiOH is highly corrosive • Li2O has been observed to swell under irradiation • Li2O is the only ceramic that may achieve the desired TBR without a neutron multiplier (but not assured)

32. Absorb impurities from tritium stream at ambient temperature 1% H2 Remove the remaining impurities and the hydrogen isotopes at a cryogenic temperature molecular sieve bed Tritium form: HT and HTO When the CMSB is saturated with Q2 (hydrogen isotopes) it is taken off line for regeneration and its companion bed can be put into service. A CMSB is regenerated by warming. The Q2 desorbs and is sent to a Pd/Ag permeator. The “bleed” stream is sent to a shift catalyst bed where reactions such as steam reforming and water gas shift can be used to move hydrogen isotopes from impurities such as CQ4 and Q2O to the form of Q2 Tritium Extraction for Solid Breeder Blankets

33. Solid Breeder Concepts: Key Advantages and Disadvantages Advantages • Non-mobile breeder permits, in principle, selection of a coolant that avoids problems related to safety, corrosion, MHD Disadvantages • Low thermal conductivity, k, of solid breeder ceramics • Intrinsically low even at 100% of theoretical density (~ 1-3 W · m-1 · c-1 for ternary ceramics) • k is lower at the 20-40% porosity required for effective tritium release • Further reduction in k under irradiation • Low k, combined with the allowable operating “temperature window” for solid breeders, results in: • Limitations on power density, especially behind first wall and next to the neutron multiplier (limits on wall load and surface heat flux) • Limits on achievable tritium breeding ratio (beryllium must always be used; still TBR is limited) because of increase in structure-to-breeder ratio • A number of key issues that are yet to be resolved (all liquid and solid breeder concepts have feasibility issues)

34. Configurations and Interactions among breeder/Be/coolant/structure are very important and often represent the most critical feasibility issues. • Configuration (e.g. wall parallel or “head on” breeder/Be arrangements) affects TBR and performance • Tritium breeding and release • - Max. allowable temp. (radiation-induced sintering in solid breeder inhibits tritium release; mass transfer, e.g. LiOT formation) • - Min. allowable Temp. (tritium inventory, tritium diffusion • - Temp. window (Tmax-Tmin) limits and ke for breeder determine breeder/structure ratio and TBR • Thermomechanics interactions of breeder/Be/coolant/structure involve many feasibility issues (cracking of breeder, formation of gaps leading to big reduction in interface conductance and excessive temperatures) Thermal creep trains of Li2TiO3 pebble bed at different stress levels and temperatures

35. Solid Breeder Blanket Issues • Tritium self-sufficiency • Breeder/Multiplier/structure interactive effects under nuclear heating and irradiation • Tritium inventory, recovery and control; development of tritium permeation barriers • Effective thermal conductivity, interface thermal conductance, thermal control • Allowable operating temperature window for breeder • Failure modes, effects, and rates • Mass transfer • Temperature limits for structural materials and coolants • Mechanical loads caused by major plasma disruption • Response to off-normal conditions

36. Major R&D Tasks for Solid Breeder Blanket • Solid breeder material development, characterization, and fabrication • Multiplier material development, characterization, and fabrication • Tritium inventory in beryllium; swelling in beryllium irradiated at temperature, including effects of form and porosity • Breeder and Multiplier Pebble Bed Characterization • Pebble bed thermo-physical and mechanical properties, thermomechanic interactions • Blanket Thermal Behavior • Neutronics and tritium breeding • Tritium Permeation and Processing • Nuclear Design and Analysis (Modeling Development) • Advanced In-Situ Tritium Recovery (Fission Tests) • Fusion Test Modules Design Fabrication and Testing • Material and Structural Response

37. Structural Materials • Key issues include thermal stress capacity, coolant compatibility, waste disposal, and radiation damage effects • The 3 leading candidates are ferritic/martensitic steel, V alloys and SiC/SiC (based on safety, waste disposal, and performance considerations) • The ferritic/martensitic steel is the reference structural material for DEMO • Commercial alloys (Ti alloys, Ni base superalloys, refractory alloys, etc.) have been shown to be unacceptable for fusion for various technical reasons

40. Comparison of fission and fusion structural materials requirements • Fusion has obtained enormous benefits from pioneering radiation effects research performed for fission reactors • Although the fusion materials environment is very hostile, there is confidence that satisfactory radiation-resistant reduced activation materials can be developed if a suitable fusion irradiation test facility is available

42. Fission (PWR) Fusion structure Coal Tritium in fusion

43. Liquid Breeders • Many liquid breeder concepts exist, all of which have key feasibility issues. Selection can not prudently be made before additional R&D results become available. • Type of Liquid Breeder: Two different classes of materials with markedly different issues. • Liquid Metal: Li, 83Pb 17Li High conductivity, low Pr number Dominant issues: MHD, chemical reactivity for Li, tritium permeation for LiPb • Molten Salt: Flibe (LiF)n· (BeF2), Flinabe (LiF-BeF2-NaF) Low conductivity, high Pr number Dominant Issues: Melting point, chemistry, tritium control

44. Liquid Breeder Blanket Concepts • Self-Cooled • Liquid breeder circulated at high speed to serve as coolant • Concepts: Li/V, Flibe/advanced ferritic, flinabe/FS • Separately Cooled • A separate coolant, typically helium, is used. The breeder is circulated at low speed for tritium extraction. • Concepts: LiPb/He/FS, Li/He/FS • Dual Coolant • First Wall (highest heat flux region) and structure are cooled with a separate coolant (helium). The idea is to keep the temperature of the structure (ferritic steel) below 550ºC, and the interface temperature below 480ºC. • The liquid breeder is self-cooled; i.e., in the breeder region, the liquid serves as breeder and coolant. The temperature of the breeder can be kept higher than the structure temperature through design, leading to higher thermal efficiency.

45. Physical Properties of Molten Natural Li (temperature in degrees Kelvin) Valid for T = 455-1500 K Melting Temperature: 454 K (181ºC) Density [1] r (kg/m3) = 278.5 - 0.04657 · T + 274.6 (1-T/3500)0.467 Specific heat [1; see also 2] CP (J/kg-K) = 4754 - 0.925 · T + 2.91 x 10-4 · T2 Thermal conductivity [1] Kth (W/m-K) = 22.28 + 0.0500 · T - 1.243 x 10-5 · T2 Electrical resistivity [1] re (nW-m) = -64.9 + 1.064 · T - 1.035 x 10-3 T2 + 5.33 x 10-7 T3 - 9.23 x 10-12 T4 Surface tension [1] g (N/m) = 0.398 - 0.147 x 10-3 · T Dynamic viscosity [1] note: h = ru where u = kinematic viscosity (m2/s) ln h (Pa - s) = -4.164 - 0.6374 ln T + 292.1/T Vapor pressure [1] ln P (Pa) = 26.89 - 18880/T - 0.4942 ln T References: [1] R.W. Ohse (Ed.) Handbook of Thermodynamic and Transport Properties of Alkali Metals, Intern. Union of Pure and Applied Chemistry Chemical Data Series No. 30. Oxford: Blackwell Scientific Publ., 1985, pp. 987. [2] C.B. Alcock, M.W. Chase, V.P. Itkin, J. Phys. Chem. Ref. Data 23 (1994) 385.

46. Physical Properties of Pb-17Li Melting Temperature: TM = 507 K (234ºC) Density [1] r (kg/m3) = 10.45 x 103 (1 - 161 x 10-6 T) 508-625 K Specific heat [1] CP [J/kg-K] = 195 - 9.116 x 10-3 T 508-800 K Thermal Conductivity [1] Kth (W/m-K) = 1.95 + 0.0195 T 508-625 K Electrical resistivity [1] re (nW-m) = 10.23 + 0.00426 T 508-933 K Surface tension [2,3] g(N/m) =0.52 - 0.11 x 10-3 T 520-1000 K Dynamic viscosity [1] h (Pa - s) = 0.187 x 10-3 exp [1400./T] 521-900 K Vapor pressure [2-4] P (Pa) = 1.5 x 1010 exp (-22900/T) 550-1000 K References: [1] B. Schulz, Fusion Eng. Design 14 (1991) 199. [2] H.E.J. Schins, Liquid Metals for Heat Pipes, Properties, Plots and Data Sheets, JRC-Ispra (1967) [3] R.E. Buxbaum, J. Less-Common Metals 97 (1984) 27. [4] H. Feuerstein et al., Fusion Eng. Design 17 (1991) 203.

47. Physical Properties of Molten Flibe (LiF)n · (BeF2) Melting temperature [1] TM(K) = 636 K (363ºC) n=0.88 (TM=653 K for n=1) TM(K) = 732 K (459ºC) n=2 Density [2] r (kg/m3) =2349 – 0.424 · T n = 1 930-1130 K r (kg/m3) =2413 – 0.488 · T n = 2 800-1080 K Specific heat [3] CP (J/kg-K) ≈ 2380 n=2 600-1200 K ? Thermal conductivity [3] Kth (W/m-K) = 1.0 n=2 600-1200 K ? Electrical resistivity [2] re (W-m) = 0.960 x 10-4 exp (3982/T) n=1 680-790 K re (W-m) = 3.030 x 10-4 exp (2364/T) n-2 750-920 K Surface tension [2,4] g (N/m) = 0.2978 - 0.12 x 10-3 · T n = 1 830-1070 K g (N/m) = 0.2958 - 0.12 x 10-3 · T n = 2 770-1070 K Dynamic viscosity [2] h(Pa - s) = 6.27 x 10-6 exp (7780/T) n = 1 680-840 K h(Pa - s) = 5.94 x 10-5 exp (4605/T) n = 2 740-860 K Vapor pressure [3] P (Pa) = 1.5 x 1011 exp (-24200/T) n = 2 770-970 K References: [1] K.A. Romberger, J. Braunstein, R.E. Thoma, J. Phys. Chem. 76 (1972) 1154. [2] G.J. Janz, Thermodynamic and Transport Properties for Molten Salts: Correlation equations for critically evaluated density, surface tension, electrical conductance, and viscosity data, J. Phys. Chem. Ref. Data 17, Supplement 2 (1988) 1. [3] S. Cantor et al., Physical Properties of Molten-Salt Reactor Fuel, Coolant and Flush-Salts, ORNL-TM-2316 (August 1968). [4] K. Yajima, H. Moriyama, J. Oishi, Y. Tominaga, J. Phys. Chem. 86 (1982) 4193.

48. Liquid Breeders Summary of some physical property data

49. Some key physical property data for Flinabe are not yet available • (melting temperature measurements for promising compositions are in progress. Measurement at Sandia in early 2004 shows ~ 300ºC) • Physical property data for Flibe are available from the MSR over a limited temperature range

50. Flows of electrically conducting coolants will experience complicated magnetohydrodynamic (MHD) effects What is magnetohydrodynamics (MHD)? • Motion of a conductor in a magnetic field produces an EMF that can induce current in the liquid. This must be added to Ohm’s law: • Any induced current in the liquid results in an additional body force in the liquid that usually opposes the motion. This body force must be included in the Navier-Stokes equation of motion: • For liquid metal coolant, this body force can have dramatic impact on the flow: e.g. enormous MHD drag, highly distorted velocity profiles, non-uniform flow distribution, modified or suppressed turbulent fluctuations