Various embodiments include a rotor key member, along with a related assembly and steam turbine. Particular embodiments include a rotor key member for retaining a circumferential seal in a steam turbine, the rotor key member including: a main body; a first arm extending from the main body; a second arm extending from the main body, wherein the first, arm, the second arm and the main body define a space spanning axially between the first arm and the second arm; and a hook extending from the second arm in a direction axially away from the space, the hook sized to contact a radially inner surface of the circumferential seal.