A locking mechanism (95, 125, 130) for a nuclear fuel assembly (2) having seed and blanket (6) subassemblies is configured to selectively lock the seed subassembly within the blanket subassembly, preventing separation thereof. The locking mechanism contains one or more detents (95) on one of the seed subassembly or the blanket subassembly, that are configured to engage with a surface feature on the other of the seed subassembly or the blanket subassembly. The locking mechanism contains a locking member movable between two positions. In a first position, the locking member (100) is configured to prevent the one or more detents from disengaging from the surface feature (125, 130), thereby holding the blanket subassembly and the seed subassembly together. In a second position, however, the detents are able to move into a non- locking position, such that the one or more detents may disengage from the surface feature as the seed subassembly is separated from the blanket subassembly.