Top-Level Technical Issues for FNST

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
3 MHD Thermofluid Phenomena

• 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.

1. Buoyancy forces associated with neutron heating
   cause intensive thermal convection.
2. MHD turbulence in blanket flows takes a special
   quasi-two-dimensional form.
3. Strong effect of turbulence on temperature in
   liquid and solid.
4. Typical MHD effect is formation of special
   M-type velocity profiles.

3
3 MHD Thermofluid Phenomena
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
4
3 MHD Thermofluid Phenomena
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 Ha104 but
  canonical geometry or complex geometry but
  moderate Ha102-103.. Experiment prototypical
  geometries but moderate Ha (103) and reduced
  dimensions (1/4).

Objectives and required RD

• 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 (Ha104),
  prototypic flow velocities (Re105) and
  volumetric heating (Gr1012) (B) new prototypic
  experimental MHD facilities at higher field and,
  (C) experiments in a real fusion environment to
  study synergistic 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
7 MHD Fluid-Materials Interaction

• 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.

Corrosion rate for samples with and without a
magnetic field
Macrostructure of the washed samples after
contact with the PbLi flow

• Strong experimental evidence of
• significant effect of the applied
• magnetic field on corrosion rate.

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
6
7 MHD Fluid-Materials Interaction
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
7 MHD Fluid-Materials Interaction
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.

Where we are at the very beginning

• 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.

Objectives and required RD

• 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.