[en] The Thermal Fuels Behavior Program (TFBP) has developed, applied, and assessed instrumentation devices which are used to measure the characteristics of nuclear fuel rod assemblies during tests under normal, abnormal, and accident conditions. The devices have evolved over a series of tests and are now being successfully used to characterize nuclear fuel rod features during loss-of-coolant accident (LOCA)-, reactivity-insertion accident (RIA)-, power-cooling-mismatch (PCM)-, and gap conductance-type experiments. This paper describes the fuel assembly instrumentation, application, and performance as applied to the TFBP test assemblies