The irradiation behavior of the High Flux Isotope Reactor (HFIR) control cylinders has been assessed. After 48 000 MWd of irradiation, the nuclear worth of the control cylinder has not changed significantly. The maximum surface oxide buildup was 0.003 to 0.005 in. thick and occurred over the tantalumaluminum dispersion. The hardness of the 6061 aluminum cladding was found to have increased from 45 DPH at the low burnup end of the cylinder to 110 DPH over the tantalumaluminum region which is located ∼5 in. below the area of maximum fastneutron fluence. Extensive metallographic examination revealed no degradation or reaction in either the tantalum aluminum or Eu2O3-Al portions of the cylinder.