PhD Thesis, Carlota Soto
Title: Development of SiC-based material for Flow Channel Insets in high-temperature DCLL blankets
Defense Date: 28/09/2018
Director: Carmen García-Rosales
The development of the breeding blanket is one of the crucial aspects to overcome in the framework of the EU fusion program towards DEMO. In the nuclear fusion power plants of the future, the blanket will be responsible of providing the necessary thermal energy to produce electricity in an efficient way, together with assuring the tritium self-sufficiency of the reactor. To these effects, one of the most promising blanket concepts is the so-called Dual-Coolant Lead-Lithium. In the DCLL, liquid PbLi acts at the same time as the main coolant and as breeder, allowing to achieve high efficiencies due to the high temperature reached by the liquid metal. Nevertheless, a high level of R&D is required to successfully overcome its design.
Among the technological challenges that must be addressed in the development of a high temperature DCLL, the design of the so-called Flow Channel Inserts (FCIs) deserves special attention. FCIs are hollow channels containing the hot flowing PbLi during blanket operation. They provide thermal insulation to protect the blanket steel structure from the high temperatures of the liquid metal, and at the same time, they provide electrical insulation, since the motion of the electrically conducting PbLi in presence of the toroidal magnetic field confining the fusion plasma results in Lorentz forces which may disturb the flow. The FCIs must also offer effective protection against PbLi corrosion and infiltration during the whole blanket operation time. Due to its excellent properties in terms of thermal and chemical stability at high temperatures, the main candidates for high-temperature FCIs are materials based on silicon carbide (SiC), being one of the possible approaches to develop a dense-porous SiC sandwich consisting of a porous SiC core, which provides the insulation properties, and a dense SiC coating, protecting the porous core against PbLi corrosion and infiltration. In the present work, the design of such a SiC-based material for FCIs is addressed.
On the one hand, the design is approached from a theoretical point of view. In this analysis, the origin and value of the thermally-derived stresses associated to the high thermal gradient across the FCIs is addressed. As a second part of the theoretical study, the possible MHD effects are analyzed, discussing its relationship with FCIs’ properties like the electrical conductivity or the configuration of the SiC-based sandwich. The study is carried out under relevant conditions for the DCLL, including two values for the toroidal magnetic field, 4 and 10 T. Besides, the heat transfer problem in the blanket is studied, analyzing the dependency of the resulting temperature field on the FCIs’ configuration. A guideline for the material’s production is extracted from these studies, suggesting the use of a dense coating of 200 μm and a porous core of porosities near 40%. A mitigation of the MHD effects is predicted if materials with an electrical conductivity <1 S/m are used in the porous core.
As a second part of the results presented in this dissertation, the experimental production of a SiC-based sandwich material with the required properties for FCIs is carried out. A route to fabricate porous SiC with tailored porosity is developed using powder metallurgy techniques (uniaxial pressing and liquid phase sintering). A controlled amount of porosity is introduced by removing a carbonaceous sacrificial phase previously added to the initial powders; by this route, high-quality SiC materials with porosities in the range 35-50% are successfully fabricated. The processes governing the sintering are analyzed, and the resulting porous SiC materials are characterized in terms of thermal and electrical conductivities, mechanical strength and elastic properties, achieving promising values for FCIs. To produce and study a SiC-based sandwich material, a dense CVD-SiC coating was deposited on the fabricated porous SiC, being the resulting samples tested against PbLi in corrosion experiments. By last, the adaptation of the fabrication method for porous SiC to the gel-casting route is explored, which corresponds to an industrially-scalable technique that offers possibilities for the future fabrication of large pieces with complex geometries. The first lab-size SiC-based FCI prototypes were fabricated by the gelcasting route.