The Instrumented Subassembly System supplies the means of measuring operating parameters of specimen fuels, instruments, and related components in the EBR-II core. The system consists of an instrumented subassembly, an extension tube with seals and connections for the instrument lead wires, a drive to make it compatible with fuel handling operations, and recording and data logging equipment. The instrumented subassembly is located in the fifth row of the core (replacing a control rod), is cooled by flowing sodium of 700 to 900°F, and is exposed to a total flux of ∼ 1.9 × 1015 n/(cm2 sec) at 50-MW reactor power. A prototype subassembly, containing 23 instruments was successfully tested in the reactor for 140 days (3856 MWd).