S4W reactor

S4W reactor. The S4W was a pressurized water reactor designed for naval propulsion by the United States Navy during the Cold War. It served as the power plant for the Skipjack-class submarine, a pivotal class that introduced the revolutionary teardrop hull form to American submarine design. The reactor's development and integration were critical to achieving the high submerged speeds and operational endurance that defined this generation of nuclear submarine.

Design and development

The S4W's design was spearheaded by the Bettis Atomic Power Laboratory under the management of the Naval Reactors branch, led by Admiral Hyman G. Rickover. It evolved from earlier designs like the S2W reactor used on the USS Nautilus (SSN-571) and the S3W reactor that powered the Skate-class submarine. A key design goal was to create a compact, high-power-density unit to fit within the new HY-80 steel hulls being developed. The reactor plant, designated S4W, utilized a single pressurized water reactor to drive two steam turbines connected to a single propeller shaft, a configuration that maximized power output for the constrained space. This work was conducted in close collaboration with prime contractor Westinghouse Electric Company and involved extensive testing at facilities like the Idaho National Laboratory.

Operational history

The S4W reactor entered service with the commissioning of the lead boat, USS Skipjack (SSN-585), in April 1959. It proved to be a highly reliable power source throughout the Cold War, enabling the six submarines of the Skipjack class to conduct prolonged surveillance and deterrence patrols against the Soviet Navy. The reactor plants typically operated for over a decade between refuelings, a significant improvement in operational availability. The design's success directly influenced subsequent naval reactors, including the S5W reactor which became the U.S. Navy's standard for decades. All S4W-equipped submarines were eventually decommissioned, with the last, USS Scamp (SSN-588), being stricken in 1994.

Technical specifications

The S4W was a light water cooled and moderated reactor using highly enriched uranium-235 as fuel. It had a designed thermal power output of approximately 150 MWth. The primary cooling system operated at high pressure to prevent boiling, transferring heat through steam generators to a secondary loop that drove the main propulsion turbines. The reactor core was controlled using neutron-absorbing boron control rods and a boric acid chemical shim system for precise reactivity management. The integrated propulsion plant, including the reactor, pressurizer, and associated pumps, was engineered for quiet operation to enhance acoustic stealth, a critical factor during missions involving the North Atlantic Treaty Organization.

Naval service and deployment

Each S4W reactor was installed aboard a single Skipjack-class submarine, powering all six vessels: USS Skipjack (SSN-585), USS Scamp (SSN-588), USS Scorpion (SSN-589), USS Sculpin (SSN-590), USS Shark (SSN-591), and USS Snook (SSN-592). These submarines were primarily assigned to the Atlantic Fleet and the Pacific Fleet, operating from bases like Naval Submarine Base New London and Pearl Harbor. Their missions included tracking Soviet submarines, intelligence gathering, and serving as a component of the national nuclear triad before the advent of the Polaris missile. The loss of USS Scorpion (SSN-589) in 1968, though not attributed to reactor failure, marked a somber chapter in the class's service history.

Safety features and design philosophy

The S4W incorporated the stringent safety philosophy institutionalized by Naval Reactors, emphasizing inherent stability, passive safety, and defense-in-depth. The reactor was designed with a negative void coefficient of reactivity, causing power to decrease if coolant boiling occurred. Multiple, independent shutdown systems, including the control rods and the boron injection system, were present. The containment structure around the reactor was robustly built to withstand extreme pressures. The entire design process, overseen by Admiral Hyman G. Rickover, mandated rigorous testing and quality control, with standards often exceeding those of contemporary civilian plants like those at Shippingport Atomic Power Station. This culture prioritized the safety of the crew and the environment throughout the reactor's operational life.