Thermal Performance of Deep-Burn Fusion-Fission Hybrid Waste in a Repository
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Thermal Performance of Deep-Burn Fusion-Fission Hybrid Waste in a Repository James A. Blink, Veraun Chipman, Joseph Farmer, Henry Shaw, and Pihong Zhao Lawrence Livermore National Laboratory ABSTRACT The thermal characteristics of the spent fission fuel from a hybrid fusion-fission system (LIFE) capable of extremely high burnups are described. The waste has higher thermal output per unit mass of heavy metal, but lower thermal output per unit energy generated. Plausible designs for interim storage containers and cooling configuration can remove the heat without exceeding fuel temperature limits. Calculations show that a spent LIFE fuel repository would perform within the limits established for the proposed repository at Yucca Mountain. INTRODUCTION The Laser Inertial Confinement Fusion Fission Energy (LIFE) Engine [1] combines a neutron-rich but energy-poor inertial fusion system with an energy-rich but neutron-poor subcritical fission blanket. Because approximately 80% of the LIFE Engine energy is produced from fission, the requirements for laser efficiency and fusion target performance are relaxed compared to a pure-fusion system, hence a LIFE engine prototype could be based on target performance in the first few years of operation of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). Similarly, because of the copious fusion neutrons, the fission blanket can be run in a subcritical, driven, mode, without the need for control rods or other sophisticated reactivity control systems. Further, because the fission blanket is inherently subcritical, fission fuels that can be used in LIFE Engine designs include thorium, depleted uranium, natural uranium, spent light water reactor fuel, highly enriched uranium, and plutonium. Neither enrichment nor reprocessing is required for the LIFE Engine fuel cycle, and burnups to 99% fissions of initial metal atoms (FIMA) are envisioned. This paper discusses initial calculations of the thermal behavior of spent LIFE fuel following completion of operation in the LIFE engine [2]. The three time periods of interest for thermal calculations are during interim storage (probably at the LIFE engine site), during the preclosure operational period of a geologic repository, and after closure of the repository. CALCULATIONS AND DISCUSSION Interim Storage Period During the interim storage period, which is at least the first five years after removal from the operating LIFE Engine, the thermal power from the decay of fission products will require immersing the spent fuel in a heat-transfer medium. The vessel under the LIFE engine, designed to cool the pebbles during a loss of coolant situation, could be used. If the LIFE power plant is being decommissioned, interim storage in that vessel would be appropriate. If the LIFE power plant is being refurbished with new hardware for a second generation of LIFE power production, that vessel or a similar vessel could be used at an on-site location for the interim storage. For calculation purposes, the interim
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