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Blanket Materials and Systems

Description:

Blanket systems including an integrated first wall facing the plasma are complex, multi-function, multi-material components that capture neutrons emitted from the burning plasma to both produce tritium via nuclear reactions with lithium, and extract the energy for efficient power conversion. Associated with the blanket are coolant and tritium processing systems, all of which have scientific and technological issues in need of resolution. Proposals that address these issues in areas such as:

  • thermofluid and thermomechanical simulation of coolant flows and structural responses under surface and volumetric heat loads;
  • mass transport (corrosion and tritium) modeling development and simulations;
  • ceramic breeder and beryllium pebbles material fabrication, characterization, and thermomechanics;
  • SiC or alternate insulators for electric current and thermal heat;
  • tritium permeation barriers and permeator windows, corrosion barriers, etc.;
  • chemistry and impurity control in coolants (helium, liquid metals, etc.);
  • Flow and other diagnostic sensors compatible with fusion environment; or any blanket and tritium system relevant development issue.

Several areas of particular interest are described in more detail below.

There is a strong need to understand and predict in greater detail both the corrosion, transport and redoposition of materials, and the generation, bubble formation, transport and permeation of tritium in the fusion relevant coolant and breeder material Pb-15.7Li alloy. Both numerical predictive tools and increased database from experimental studies are needed to better characterize the corrosion and tritium transport behavior in Pb-Li alloy under fusion relevant conditions that include operation at 400-700C and the presences of strong magnetic fields in contact with various materials such as ferritic steels, silicon-carbide, and other proposed tritium or corrosion barrier or permeator materials for tritium extraction.

The pebble-bed solid breeder configuration introduces several operational limits: thermomechanical uncertainties caused by pebble-bed wall interaction, potential sintering and subsequent macro-cracking, and a low pebble-bed thermal conductivity – all of which result in small characteristic bed dimensions and limit windows of operation. A new form of solid breeder morphology is required that holds the promise for increased breeding ratios – dictated by increased breeder material density; long term structural reliability; and enhanced operational control – compared to packed beds. Grant applications are sought for new solid breeder material concepts that include: (1) increased breeder material densities (~80%); (2) higher thermal conductivities (provided by a fully interconnected structure, as opposed to point contacts between pebbles); (3) better thermal contact, such as reliable bonded contact, with cooling structures (instead of point contacts between pebbles and wall); (4) the absence of major geometry changes between beginning-of-life and end-of life (such as sintering in pebble beds) in the presence of high neutron fluence; and (5) structural integrity in freestanding and selfsupporting structures with significant thermo-mechanical flexibility.

Flow channel inserts (FCIs) act as magnetohydrodynamic and thermal insulators in ferritic steel channels containing, for example, a slowly flowing tritium breeder such as molten Pb-15.7Li alloy. The insert geometry is approximately box-channel-shaped in straight channels, with more complex shapes possible, for insertion in manifolds and other complex-geometry elements in the flow path. Although SiC/SiC composite is a candidate FCI material, its use would differ from its potential application as a structural material in that high thermal and electrical conductivity would not be desirable. In fact, the electrical conductivity should be low, with a target maximum around 1 to 50 ?-1m-1. In addition, the strength requirements for a SiC/SiC FCI are reduced compared to the composite’s application as a structural material, because the primary stresses and pressure loads will be very low. On the other hand, the insert must be able to withstand thermal stresses from through-surface temperature differences in the range of 150-300K, over a thickness of 3 to 15 mm depending on designs. Grant applications are sought to develop manufacturing techniques for radiation resistant, low thermal/electrical conductivity SiC/SiC composites or other suitable, compatible materials that would make for effective FCIs. One approach that has been envisioned is the use of a final “sealing” layer of SiC matrix material, which would be near theoretical density and cover any porosity or exposed fibers in the main body of the insert. Two-dimensional weaves are also thought to be satisfactory, as well as an effective way to reduce electrical conductivity normal to the interface between the insert and the Pb-15.7Li (the more important of the directions). In addition, grant applications are sought to develop experimental techniques for determining: (1) the compatibility between the SiC/SiC composite and such breeder materials as Pb-15.7Li alloy, and (2) the insert integrity under cyclic thermal loading and other in-service conditions.

One of the missions of the ITER project is the integrated testing of fusion blanket modules in a true integrated fusion environment. This ITER fusion  environment includes radiation and magnetic fields, along with surface and volumetric heating, under pulsed and/or steady-state plasma operation. The testing of first wall/blanket components will be performed in ITER by inserting “test blanket modules” (TBMs) that will be complicated systems of different functional materials (breeder, multiplier, coolant, structure, insulator, etc.) in various configurations with many responses and interacting phenomena (e.g., thermomechanical, thermofluid, nuclear). As part of the design and validation process an overall simulation of a “virtual” TBM, integrating all of the individual computational modeling simulations at the system level, is essential to define meaningful experiments. Such a simulation would be inherently multi-scale and multi-physics and will require careful code and algorithm design. Therefore, grant applications are sought to develop a TBM and general power reactor relevant simulation code that can provide detailed predictions of: (1) fluid flow and thermal hydraulic characteristics; (2) the thermal response of all materials (structure, breeder, multiplier, coolant, insulator, etc); (3) structural responses such as stress and deformation magnitudes with respect to different loadings, including both steadystate surface heat flux and dynamic loadings; (4) mass transfer characteristics including both corrosion and tritium transport phenomena, and (5) other important performance characteristics of the TBM or blanket system. The overall code framework/structure must effectively link all of the simulation components of the virtual TBM and serve as an efficient, useful, and user-friendly tool that is extendable from ITER to demonstration power reactor conditions.

Questions – contact Ed Stevens, edward.stevens@science.doe.gov

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