An Additive Manufacturing Technology for the Fabrication and Characterization of Nuclear Reactor Fuel

Phase I of this Small Business Innovation Research (SBIR) project was conducted under Department of Energy's (DOE) Advanced Technologies for Nuclear Energy (NE) topic 19(b), entitled Advanced Technologies for the Fabrication, Characterization of Nuclear Reactor Fuel. Phase II preserves the two-pronged vision and goals outlined in Phase I. First, this project seeks to provide a technological answer to DOE-NE's needs, specifically in advancing the performance of accident tolerant fuel/cladding concepts in light water reactors (LWR) and TRISO fuel fabrication techniques. Second, this project will contribute to the advancement of the DOE-NE's long-term congressional mandate to enhance current reactor safety, reliability, and life, reduce nuclear proliferation risks, and improve the affordability of new nuclear reactors in part through more sustainable nuclear fuel cycles. The present proposal builds upon cross-cutting advances in the following fields: (i) additive manufacturing (AM), (ii) micro-electromechanical-systems (MEMS) design, (iii) micro- and nano-scale fabrication and (iv) ceramic matrix composites (CMCs). These advances are employed in the development of novel Silicon Carbide fiber (SiCf) rein- forced Silicon Carbide matrix (SiCm) CMC fuel structure. They allow the integration of the full functionality of TRISO fuel at a microscopic level encapsulated into SiC filaments, which are in turn embedded into a SiCm to form a fully integrated nuclear fuel and cladding structure. This innovation merges the functionality of TRISO fuels and cur- rent fuel rods into a single element. This novel fuel structure offers a passively safe, integral and energy efficient Silicon Carbide alternative to Zircalloy fuel rods assemblies in LWR with little change to reactor control architecture. The proposed fuel structure's elevated temperature capability and chemical inertness implies lower amounts of and easier to store generated nuclear waste. Finally, micro-encapsulation of fissile material into a CMC would make fissile material retrieval extremely difficult, thereby lowering the risk of nuclear proliferation. Phase I of this project successfully tested the feasibility of producing the basic elements of the proposed fiber-embed- ded structure. The following key steps were demonstrated: (1) full-density, beta-phase Silicon Carbide fibers with variable diameters and (2) direct-write of nanoporous carbon coatings to small sections of fibers. In the proposed Phase II, we intend to continue development of the concept; however given the nuclear regulatory requirements, completion and commercialization of a project of the proposed magnitude will not be feasible within the various constraints of a Phase II project. A sequential Phase II project will be necessary. The present project focus will address the issue of production scale-up of nuclear-technology bound SiC fibers. For a commercially viable fuel-in-fiber concept, the core SiC fibers must be manufacturable in large quantity at competitive prices. We will therefore devote 75% of the proposed effort on the goals of scaling up fabrication of nuclear- technology rated SiC fibers and characterizing CMC cladding tubes made using these fibers. The remaining 25% of the effort will focus on the implementation of the direct-write of coating microstructure with a surrogate material for nuclear fuel as well as neutronics and thermal modeling of the proposed fuel-in-fiber concept in a CMC fuel element.