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The dual-coolant lead-lithium, or DCLL, blanket concept is of strong interest in the US fusion technology program. In the DCLL blanket, the flow channel insert (FCI) is a critical component. FCIs must have low electrical and thermal conductivity and be compatible with lead–lithium eutectic alloy (Pb–17Li) at elevated temperatures. FCIs must retain structural integrity and desirable properties even under irradiation and large temperature gradients during operation. FCIs must not fail in such a way that Pb–17Li enters the FCI and changes its electrical or thermal conductivity significantly. Another important issue for the DCLL is the development of a suitable tritium extraction from the Pb–17Li to achieve low tritium partial pressure, thus facilitating decisive tritium control. In this paper, the state of DCLL development in the US is presented including recent design modifications and results from recent RD Pb–17Li material capability and infiltration studies; simulations of MHD Pb–17Li flow characteristics and of resultant temperature distributions; and the analysis of FCI stress states based on these thermal loads. In addition, tritium extraction from Pb–17Li based on a vacuum permeator concept is shown to have the potential to achieve the desired tritium control. A discussion of DCLL optimization and unresolved DCLL issues and future R&D needs is also presented.

Laser flash measurements of thermal diffusivity of ZrO2 single crystals partially and fully stabilized with Y2O3 were compared with measurements for polycrystalline cubic ZrO2, and single crystals and polycrystals of Al2O3 and MgAl2O4. In general, the thermal diffusivities of the ZrO2 materials examined initially decrease with increasing temperature, although significantly less than for the Al2O3 and MgAl2O4 materials. The diffusivity subsequently rises with increasing temperature for the single crystals of ZrO2 but not for polycrystalline cubic ZrO2, with this increase for ZrO2 crystals occurring at much lower temperatures than for Al2O3 and MgAl2O4 crystals. As the ZrO2 materials went from fully stabilized with 20 wt% Y2O3 to partially stabilized with 5 wt% Y2O3, the room-temperature diffusivity increased from 0.70x10-6 to 0.97x10-6 m2/s. This indicated that the lattice defects, which increase with Y2O3 content, are more important sources of phonon scattering than are the precipitates which appear at lower Y2O3 compositions. On the other hand, at 1000°C the diffusivity of the fully cubic ZrO2 crystals with 20 wt% Y2O3 was 1.10x10-6 m2/s while that of the partially stabilized ZrO2 crystals with 5 wt% Y2O3 was 1.05x10-6 m2/s. The increased diffusivity at higher temperatures can be attributed to radiative heat transfer, with the lower relative diffusivity of the partially stabilized crystals resulting from photon scattering by the precipitate structures present.

The effect of interfacial decohesion, due to the thermal expansion mismatch, on the thermal diffusivity of a hot-pressed glass matrix with a dispersed phase of nickel was investigated by the laser-flash technique at 25° to 600°C. The interfacial gap formed on cooling acts as a barrier to heat flow and lowers the thermal diffusivity to values below those predicted from composite theory and also creates a strongly positive temperature dependence of the thermdl diffusivity. Preoxidation of the Ni spheres promotes interfacial bonding and yields values of thermal diffusivity higher than those for nonoxidized spheres and a thermal diffusivity which is relatively temperature-independent. The results of the present study also confirm the criteria for the effective thermal diffusivity of composites established by Lee and Taylor.

The hypothesis is presented that the maximum tensile thermal stresses in brittle ceramics can be reduced significantly by a redistribution of the temperature profile using a spatially varying thermal conductivity. On the basis of a hollow circular cylinder subjected to radially inward or outward steady-state heat flow, it is shown that such a spatially varying thermal conductivity can lead to significant decreases in magnitude of the maximum tensile thermal stress. Possible techniques of creating such thermal conductivity variations are discussed briefly.

The nature of the dependence of the thermal shock resistance of ceramics on the various thermo-mechanical properties of these materials can be used to design, in a systematic way, ceramics with improved thermal shock resistance. Great versatility in designing for thermal shock resistance and other desired properties can be achieved via ceramic composite approaches where properties of the two (or more) phases can be used to tailor the properties of the composite.

The hypothesis is presented that the maximum tensile thermal stresses in brittle ceramics can be reduced significantly by a redistribution of the temperature profile using a spatially varying thermal conductivity. On the basis of a hollow circular cylinder subjected to radially inward or outward steady-state heat flow, it is shown that such a spatially varying thermal conductivity can lead to significant decreases in magnitude of the maximum tensile thermal stress. Possible techniques of creating such thermal conductivity variations are discussed briefly.