Assembly exit thermocouples are chosen for the Clinch River Breeder Reactor Plant as the instrumentation providing the most useful information at the minimum cost. One thermocouple is positioned at the exit of each fuel assembly and at approximately half of the radial blanket assemblies. The number of thermocouples, their positions, and characteristics are selected to satisfy the reactor control, surveillance, and design verification functions. The various uncertainties affecting the assemblies’ coolant exit temperature measurements are quantitatively defined to correlate the measured temperature with the fuel rod design cladding temperature, which is the major parameter in determining the allowable fuel rod burn-up and lifetime. Thus, appropriate factoring of thermocouple measurements allows the fuel assembly burnup to be increased quite significantly, with related cost savings of hundreds of millions of dollars. Due to the tremendous economic leverage on operating costs over the plant lifetime, close attention to proper instrumentation should be paid in the design of future commercial liquid-metal fast breeder reactors.