Recent research and development for the US Dual-Coolant Lead-Lithium blanket

The Dual-Coolant Lead-Lithium, or DCLL, blanket concept with flow channel inserts (FCIs) serving as thermal and electrical insulator was initiated in the ARIES-ST study and became the reference blanket in ARIES-CS. It has been also investigated under the APEX program and Test Blanket Module effort in the US, and in the EU as well. The DCLL is constructed from Reduced Activation Ferritic/Martensitic (RAF/M) steel with the first wall and all internal structures cooled by high pressure helium to a temperature below the allowable operating temperatures for RAF/M steel. The flowing lead-lithium (PbLi) coolant/breeder, however, is thermally and electrically isolated from the He-cooled structures by loose-fitting, flow channel inserts (FCIs) that serve no structural function, but which are compatible with PbLi at much high temperatures that the RAF/M steel. With such an arrangement, the DCLL He coolant emerges from the blanket at typical temperatures ∝450 C, but with the PbLi coolant emerging with outlet temperatures ∝700 C for improved thermal efficiency. The circulation speed of the PbLi is still fast enough that tritium partial pressure can be controlled to a relatively low level with a high efficiency extraction technique. For the DCLL blanket itself, the flow channel insert is the critical piece. FCIs must have low electrical and thermal conductivity and be compatible with PbLi 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 PbLi enters the FCI and changes its electrical conductivity appreciably. Another important issue for the DCLL is the development of a suitable tritium extraction from PbLi to achieve a tritium partial pressure <1 Pa, 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 R and D efforts. Such R and D includes the progress on development of SiC/SiC composites and SiC foams as FCI candidates; electrical and thermal conductivities for FCI materials, PbLi capability and infiltration studies, simulations MHD flow characteristics and of resultant temperature distributions; and the analysis of FCI stress states and deformation based on these thermal loads. In addition, tritium extraction from PbLi based on a vacuum permeator is shown to have the potential to achieve desired tritium control. A discussion of unresolved DCLL issues and future R and D needs and plans in the US is also presented. (orig.)