The reactor block constitutes a highly complex key component in a pool-type breeder reactor, the difficulties involved being considerably heightened for Creys-Malville by its prototype character, involving a considerable scale-up from Phénix, with several important design changes. Problems arose in the course of design engineering, but also throughout the manufacturing stage: steel plate elaboration, welding, shaping, etc. The construction approach adopted involved consistent interaction between structure fabrication and the structural analyses underlying design and sizing documents. These carefully defined procedures are described using certain typical examples. For the very large structures, factory fabrication preceded site workshop assembly. Related technical difficulties are evoked, as, for instance, welding problems, compliance with tight dimensional tolerances, technological test mock-ups, structure stress relieving, assembly of heat insulating material, the general reactor block assembly operations, with the associated constant cleanliness requirements, and the overall leaktight testing. The knowledge acquired is already being applied in connection with the Superphénix 2 preliminary design.