Which role is NOT directly responsible for maintenance and operation of nuclear systems?

The Sub Electrical Assistant (EA) is primarily focused on the electrical systems aboard a submarine, which includes operating and maintaining the ship's electrical components, systems, and equipment. However, while electrical systems are crucial for the overall operation of a nuclear submarine, the EA does not have direct responsibility for the maintenance and operation of the nuclear systems themselves.

In contrast, the roles of the Reactor Controls Assistant (RCA), Main Propulsion Assistant (MPA), and Engineer Officer (ENG) are all directly involved with the nuclear propulsion systems. The RCA manages controls related to the reactor operation, ensuring it runs safely and efficiently. The MPA oversees the main propulsion and works closely with the reactor systems to ensure the submarine can maneuver. The Engineer Officer is responsible for the overall operation and maintenance of the reactor systems, including ensuring safety protocols are followed.

Thus, the role of the Sub Electrical Assistant is distinct in that while it supports the operational capacity of the submarine, it does not directly engage with and maintain the nuclear systems, making it the correct answer to the question.