A simplified interassembly heat transfer model has been developed to satisfy liquid-metal fast breeder reactor core restraint system analysis needs that explicitly treats steady-state intra-assembly and interassembly heat transfer in core assemblies. The intra-assembly heat transfer inside reactor assemblies is modeled based on application of the subchannel concept together with the use of bulk parameters for coolant velocity and coolant temperature within a subchannel. The model utilizes a tri-grid system to treat interassembly heat transfer between assemblies. Because of this special nodal scheme, a set of finite difference equations, derived from the energy equation for all the subchannels, duct wall, and gap flow, is actually a rather special system of simultaneous linear algebraic equations which have a tri-diagonal matrix form. Due to this special form, an efficient method of solution for computers is used without matrix elimination and inversion. Although this model was developed for core restraint applications, it is also well suited for the determination of core-wide coolant temperature distributions.