Abstract: In order to ensure the operation safety of nuclear power plant, the ²³⁵U enrichment of nuclear fuel assembly in reactor has strict design requirements. Therefore, all reactor fuel rods must undergo 100% enrichment and loading uniformity nondestructive testing. The enrichment detection methods include active method and passive method. Through the research on the age of UO₂ pellets in the active method and passive method, it is proposed that the active detection method is affected by the age of UO₂ pellets at a lower activity, which makes the fuel rod enrichment cannot be detected correctly. The passive-placement-waiting method is needed to solve the problem of pellet age. However, the passive method can directly correct the pallets age in the detection, which meets the requirements of the detection and has been applied in engineering.