Although the recycling of plutonium as thermal mixed-oxide (MOX) fuel in pressurised water reactors (PWRs) is now well-established on a commercial scale, many physics questions remain. The main question addressed in this report is the number of times plutonium can effectively be recycled in a PWR. This report describes in particular an exercise based on a realistic, multiple-recycle scenario, which followed plutonium through five generations of recycling in a PWR. It considered both a standard PWR design currently in use and a highly moderated design. The latter is a possible option for a dedicated, MOX-fuelled PWR in which it would be possible to optimise the moderation for plutonium. The study of these two designs in parallel has provided a better understanding of their relative merits, as well as insight into the limitations of multiple recycling and the long-term toxicity of fission products and actinides.