[en] Most CDFR design work in the UK in the past has related to simple cylindrical cores of fuel surrounded by breeder material. It became apparent, however, that with this design there was room for improvement in the response to certain hypothetical accidents in which sodium coolant was expelled from the core, and a number of alternative designs have been examined in recent years. Most of these variants involved simple alterations in core layout using existing subassembly designs, principally by introducing local breeder regions into the fuelled core, in the form of radial spokes, annuli or clusters. A few others modified the subassembly design, using axial shortening, introduction of breeder layers, and reducing the sodium fraction; the effect of reducing the core size was also examined. (author)