Undermoderated plutonium lattices in pressurized water reactors (PWRs) offer the prospect of very significant uranium savings as well as less degradation of the plutonium isotopic quality. There are, indeed, some uncertainties associated with such designs and, in 1984, an extensive program was launched to reduce these uncertainties. For the neutronic reactor physics research, the program included: (a) adaptation of codes—in particular, the cell code APOLLO and its neutronic data library; (b) an extensive experimental program (the ERASME, ICARE, and MORGANE experiments) to reduce the uncertainties in the different neutronic parameters; and (c) studies of important and specific future PWR problems. This program will allow us to qualify the neutronic codes for the undermoderated lattices in a very large range of moderator ratios and to validate future PWR designs.