System Integration and Testing of a Robust Cellular Solid Breeder Material with Enhanced Tritium Production and Release

Solid breeder materials that have been considered in the past are all lithium based ceramics, including lithium oxide, silicate, titanate, and zirconate, in pebble bed configurations to enhance tritium release and recovery. However, the current pebble bed configuration imposes severe design and operational limitations as a result of poor thermal conductivity and high Hertzian stress between pebbles at the contact points, which leads to pebble deformation, cracking, fragmentation, and sintering. These failures, along with low pebble packing fraction (<65 vol% dense), limit the operating temperature range and increase the need for neutron multipliers. Purge gas blockage due to these failure modes reduces tritium breeding ratio, temperature control, and safety. In previous work for DOE, Ultra-met and Digital Materials Solutions (DMS) developed an advanced lithium zirconate (Li2ZrO3) solid breeder material in the form of a cellular ceramic. The breeder is melt infiltrated into highly porous open cell carbon foam, after which the carbon foam is removed by oxidation. The process leaves a nominally 90 vol% dense breeder material with an internal network of interconnected micro channels for enhanced tritium release. Thermal conductivity is increased relative to pebble beds, high temperature sintering is eliminated, and tritium breeding ratio and breeder durability are increased. Ultra-met teamed with DMS and the University of California, Los Angeles (UCLA) to expand on the previous work by focusing on system integration as well as continued breeder material optimization. Design and modeling of a new, high performance blanket configuration using the cellular breeder structure was performed at DMS. Tritium purge gas (helium) flow testing and tensile testing of development specimens was performed at UCLA. The established cellular Li2ZrO3 processing conditions were modified at Ultra met toward fabrication of a cellular breeder containing lithium titanate (Li2TiO3). Although a cellular Li2ZrO3 breeder is anticipated to offer substantial benefits over a pebble bed made of any breeder material due to its higher density, higher thermal conductivity, and greater structural integrity, the increased lithium content and waste disposal benefits of a cellular breeder containing Li2TiO3 could yield a further improved cellular breeder. Cellular solid breeder material optimization, blanket integration, and performance testing will be expanded. Emphasis will be placed on integration of the cellular breeder within the blanket in terms of optimizing material geometry and thermal contact with the steel structure. DMS will perform multi physics modeling and blanket design. Material optimization will include fabrication and testing of both Li2ZrO3 and Li2ZrO3Li2TiO3 cellular breeders. UCLA will establish cellular solid breeder thermomechanical properties and assist in blanket design. Deuterium absorption and release testing will be performed at Idaho National Laboratory (INL). The project will conclude with fabrication and thermal testing of a blanket segment prototype. Nuclear fusion offers a replacement for carbon negative fossil fuel energy sources. Other alternatives to fossil fuels (e.g. wind, solar, geothermal) cannot generate sufficient energy to meet current and future needs. Development of an advanced solid breeder material is necessary to enable the U.S. industrial base to participate in near term commercial applications of fusion energy (e.g. ITER test blankets, DEMO, and beyond). Key words: magnetic fusion energy, tritium, breeder, blanket, cellular, open cell foam, lithium zirconate, lithium titanate. Nuclear fusion offers a technically viable means of generating energy consistent with current consumption levels and environmental preservation. Establishing the commercial viability of fusion requires the development of advanced materials and structures that allow reliable operation in the demanding reactor environment.