The discipline of applied-mechanics analysis has had a significant influence in designing the Fast Test Reactor (FTR). The FTR is a sodium-coolod fast flux reactor being constructed to test candidate materials and fuels for the U.S. Liquid Metal Fast Breeder Reactor (LMFBR) program. The influence of applied mechanics is more evident in the design of a liquid metal cooled reactor such as the FTR than it is in the more conventional water-cooled designs, primarily because the combination of environmental conditions in a liquid metal reactor results in an interplay between the mechanics analyst and the reactor designer never before required. Specifically, these environments include a fast neutron spectra, high neutron fluence (flux-time) exposures, an elevated thermal environment, and a high conductivity coolant. The fast neutron spectrum (65% greater than 0.1 MeV) and high flux exposures (greater than 10 23 nv), when coupled with the thermal environment (1100°F in the active core), causes a metal growth phenomenon and a radiation-induced creep effect, particularly in the active core region. The high flux and fast spectra also cause metal embrittlement which must be factored in to the elastic design of the more permanent, nonremovable structure within the reactor. The combination of an elevated temperature, a high conductivity coolant (sodium) and (in the case of the FTR) the diverse structures required in a test reactor, creates unique thermal stress conditions not previously encountered in water reactors. Criteria presently being used for structural design include residual ductility requirements for permanent structures; strain-limited design requirements for removable, highly irradiated components (such as the cladding); and an intermediate combined criteria for such items as the in-core component ducts. Areas where the analysis design interplay are particularly required include: - Duct and core restraint design and duct distortion/dilation analysis - Inlet tube-sheet elastic design and neutron embrittlement considerations - Outlet instrumentation structures and thermal stress analysis of these structures under postulated transient conditions. These areas, along with thermal environments, are noted on fig. 1, an elevation view of the FTR. This paper discusses the significant effect of these phenomena on reactor design, describes the analytic methods being used to formulate design solutions, and covers the resultant unique FTR features evolving from the analysis/ design iterations.