The NS Savannah was a groundbreaking vessel, not just for being the first nuclear-powered merchant ship, but for the sophisticated engineering that underpinned its propulsion system. Launched in 1959, the ship was a direct outcome of President Dwight D. Eisenhower's "Atoms for Peace" program, intended to demonstrate the peaceful applications of nuclear energy. At the core of this demonstration was a meticulously designed nuclear reactor and associated
machinery, representing a significant leap in maritime technology. The entire project, including the reactor and its fuel core, cost $28.3 million out of the total $46.9 million construction budget, highlighting the complexity and cost of this innovative power source.
The Pressurized Water Reactor
The heart of the NS Savannah was its 74 MW Babcock & Wilcox pressurized water reactor. This reactor was specifically designed to civilian standards, prioritizing safety and reliability over the shock resistance and compactness often seen in military propulsion reactors. Its placement within the ship was strategic, allowing for access from above for refueling operations. The reactor itself was a tall, narrow cylinder, encased within a cylindrical containment vessel with rounded ends. A 14-foot-diameter vertical cylindrical projection from the containment vessel housed the control rods and refueling equipment.
The 50-foot-long containment vessel was a critical safety feature, housing the pressurized-water reactor, the primary coolant loop, and the steam generator. Constructed from steel, its wall thickness varied from 2.5 to 4 inches, engineered to withstand a gauge pressure of 186 psi, which could be generated by a ruptured primary coolant pipe. For additional safety, two 24-by-18-inch manholes in the bottom of the containment vessel were designed to admit water if the ship sank in more than 100 feet of water, preventing the pressure vessel's collapse. The containment vessel was not occupied during operational conditions but could be accessed within 30 minutes of reactor shutdown. Shielding was extensive: the lower half was protected by a 4-foot concrete barrier, while the upper half featured 6 inches of lead and 6 inches of polyethylene. A collision mat, composed of alternating layers of 1-inch steel and 3 inches of redwood in a 24-inch assembly, shielded the sides of the vessel.
Reactor Core and Propulsion Machinery
The reactor core itself was 17 feet high, with a diameter of 62 inches and a height of 66 inches. It contained 32 fuel elements, each comprising 164 fuel rods. These rods, clad in stainless steel, housed uranium oxide pellets enriched to an average of 4.4% U-235. The enrichment varied slightly, with the 16 center elements at 4.2% and the outer 16 elements at 4.6%. The pellets were 0.4244 inches in diameter, with pressurized helium gas filling the annular space between the pellets and the element walls. Control was managed by twenty-one control rods, each 66 inches long, 8 inches across, and 3/8 inches thick, capable of full insertion in a rapid 1.6 seconds by an electric drive.
The nuclear reactor generated steam to power a relatively standard steam plant, which included a nine-stage high-pressure turbine and a seven-stage low-pressure turbine, driving a single propeller shaft. The steam and water chemistry program for the Savannah was managed by the Bull & Roberts Company, mirroring systems used in U.S. Navy nuclear-powered aircraft carriers and submarines. The turbines were specially adapted to efficiently utilize the saturated steam characteristic of a nuclear power source. An unusual feature was a 750-horsepower electric motor geared to the high-pressure turbine for emergency use. This motor, later upgraded for greater torque and reversibility, could be powered by the ship's steam turbogenerators or 750 kW emergency diesel generators, providing basic propulsion and reactor coolant pump operation. The propulsion plant was designed for 20,000 horsepower, aiming for a 20-knot speed, but in practice, it delivered about 22,000 horsepower, achieving a maximum speed of 24 knots.
