Development of Porous Yttrium-Chromium Hydride as an Increased-Performance and Reduced-Cost Microreactor Moderator

Statement of the problem or situation that is being addressed: Yttrium hydride (YHx) is an attractive moderator material for fission microreactors due to its ability to retain large quantities of hydrogen to high temperatures and its relatively low impact on neutron economy. However, use of YHx in the form of a high-density monolith is difficult due to microcracking that occurs as a result of volumetric expansion upon hydriding as well as poor resistance to thermal shock. Microcracking and poor thermomechanical properties can result in degradation of the YHx moderator under neutron irradiation. Most significantly, hydrogen gradually depletes after high temperature, high-fluence neutron irradiation, greatly limiting the useful moderator lifetime of YHx. Statement of how this problem or situation is being addressed: In previous work for DOE involving development of a tritium breeder for use in fusion energy systems, Ultramet developed processing for fabrication of a porous “reverse foam” structure that is produced by melt infiltrating the desired material into highly porous open-cell carbon foam, after which the foam core is removed by oxidation. The process leaves a robust, ~90 vol% dense structure with an interconnected network of internal microchannels. The size and spacing of the microchannels can be varied to optimize the structure for the particular application. Unlike solid brittle materials, which are susceptible to catastrophic cracking due to large thermal gradients and mechanical impact, the reverse foam structure at 90 vol% dense has been shown to exhibit high resistance to thermal and mechanical shock. In this project, Ultramet, in collaboration with Idaho National Laboratory (INL), will determine the feasibility of modifying existing reverse foam processing for fabrication and optimization of porous YHx and yttrium-chromium hydride moderator materials for microreactors. Previous research has shown that the addition of a small percentage of chromium significantly increases the thermal shock resistance of YHx. In addition to the excellent thermal/mechanical shock resistance of reverse foam, the high surface area internal microchannels will allow for increased hydriding and reduced stress relative to existing solid moderators. The thickness of solid moderator between passages inside reverse foam will be fairly low and tailorable, which will minimize the distance required for hydrogen diffusion during the hydriding process. The most prominent benefit of this design is that it is anticipated to allow for in-core hydrogen regeneration, which will significantly increase moderator lifetime and reduce microreactor cost. What is to be done in Phase I? Using existing reverse foam tomography data, INL will perform hydrogen flow modeling for initial optimization of the foam density, size of the internal microchannels, and moderator geometry. Modeling of stresses that occur in yttrium and yttrium-chromium reverse foams due to volumetric expansion upon hydriding will be performed and compared with stresses in a solid hydride. Processing will be established for yttrium and yttrium-chromium reverse foam fabrication at Ultramet via melt infiltration of a carbon foam skeleton followed by foam removal by oxidation to open internal microchannels. Reverse foam characterization will include hydrogen flow and pressure drop testing. Commercial applications and other benefits: Low-power, transportable microreactors are under development by DOE and industry to generate clean, reliable electricity for commercial use, remote communities, military bases, and emergency response/disaster relief, or for non-electric applications such as district heating, water desalination, and hydrogen fuel production. The reactors utilize passive safety systems to prevent overheating or reactor meltdown.