The Belgonucleaire-CEN research and development work, performed in the scope of a joint USAEC-EURATOM Program, involves the development of both fabrication techniques and methods of design calculations for plutonium-enriched cores. Based on experience gained since 1963, a specific study of plutonium recycle in the SENA reactor was undertaken in cooperation with EURATOM and the Belgian utilities. This study, confirmed by experiments carried out in the Belgian VENUS critical facility, has led to proposed optimized schemes for the composition and the location of plutonium-enriched fuel rods. For the conditions prevailing around 1973, it appears that, in the case of the SENA reactor, there should be a slight economic advantage in mixing the available plutonium with slightly enriched uranium arising from the reprocessing of the fuel assemblies rather than with natural or depleted uranium.