The commercial success of current pressurized water reactor (PWR) nuclear power stations is the seed for research and development (R&D) work to carry this technology even further. Among the goals for future plants, significant improvement of fuel utilization is clearly prominent. There are various means for accomplishing this task. All basically concern the modification of the spectral conditions in the reactor core, with hardening being the most promising on theoretical grounds. Several studies based on investigations in physics, thermohydraulics, emergency core cooling, and mechanical design indicate that it should be possible to introduce systems with moderator-to-fuel volumetric ratios in the range of 0.5 to 1.0, drawing to the largest extent possible on the proven technology available. The Kraftwerk Union AG high conversion reactor represents a quasi-standard PWR with fuel assemblies of more or less uniformly enriched fuel rods, arranged in a tight hexagonal array with a pitch-to-diameter ratio p/d ≅ 1.12. High fuel enrichment as well as a high conversion ratio of ∼0.9 will provide the potential for high burnup values up to 70 000 MWd/tonne and a low fissile material consumption. The overall objective of the actual R&D program is to have the technical feasibility, including that for licensibility, established by the early 1990s as a prerequisite for deciding whether to enter a demonstration plant program.