The TCLUST1 computer program was developed to study the transient thermal behavior of a subassembly of a liquid-metal fast breeder reactor (LMFBR) that is thermally coupled to its six neighboring subassemblies. Each of the seven subassemblies may be either a reflector subassembly, which contains a single solid hexagonal rod, or a pin-bundle subassembly. The TCLUST1 analytical model was developed based on conservation of energy, and the temperature distribution in the subassembly was obtained using the successive over-relaxation (SOR) technique. Measured temperatures obtained from a natural-convection flow test performed in EBR-II compare well with those predicted by TCLUST1. Appendixes describe the code input and go through a sample program.