This paper compares the sustainability of two light water reactor, LWR, fuel cycles: the once-through UOX (low-enriched uranium oxide) cycle and the twice-through MOX (Mixed Uranium-Plutonium Oxide) cycle (increasing the input efficiency of available uranium) by assessing their probable long-term competitiveness. With the retirement of diffusion enrichment facilities, enrichment prices have declined by one-third since 2009 and are likely to remain below $100-kgSWU for the foreseeable future. Here, initial uranium prices are set at $90/kgU and reprocessing costs at $2500 per kilogram of heavy-metal throughput, representative of "new-build" costs for reprocessing facilities. Substantial reprocessing cost reductions must be achieved if MOX is to be competitive, i.e., if it is to improve the sustainability of the LWR. However, results indicate that preserving the MOX alternative for spent fuel management later in this century has a large present value under several sets of assumptions regarding uranium price increases and reprocessing cost decreases. (C) 2014 International Atomic Energy Agency. Published by Elsevier Ltd. All rights reserved.