Reprocessing of spent LWR fuel is an intrinsic part of the closed fuel cycle. While current technologies treat recovered minor actinides as high level wastes, the primary objective of one of the U.S. DOE Nuclear Energy Research Initiative (NERI) projects is to assess the possibility, advantages and limitations of achieving ultra-long life VHTR (Very High Temperature Reactor) configurations by utilizing minor actinides as a fuel component. The postulated principal mechanism is an enhanced involvement of self-generated fissile compositions based on spent LWR fuel. Since pebble bed and prismatic core designs permit flexibility in component configuration, fuel utilization and management, it is possible to improve fissile properties of minor actinides by neutron spectrum shifting through configuration adjustments. Depending on neutron spectra, neptunium, americium and curium may contribute to small reactivity swings (self-stabilization) over prolonged irradiation periods. The presented analysis is focused on achievability of spectral variations and their potential impact. In principle, promising core features and performance characteristics have been demonstrated.

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