S1G/S2G Liquid Sodium Reactor

Overview: The higher temperatures of a liquid Sodium reactor offered significant increase in the thermodynamic efficiency of the reactor plant over the pressurized water reactor (PWR) that was used in Nautilus. This would translate in to more miles of steaming for a submarine for the same amount of fuel in the reactor. Liquid sodium also offered several other advantages: (1) high-temperature operation could be achieved at low pressures which reduced the cost and weight of containing the primary coolant; and (2) its tendency to slow down neutrons was low so a significant fraction of power from the reactor could come from fissions caused by intermediate-energy neutrons where PWR reactors needed to rely on slow (or thermal) neutrons. However, to achieve these advantages many material and engineering problems would need to be solved and then proven before implementing in a submarine. Some of the challenging problems were that: (1) Sodium become highly-activated by the radiation as it passes through the reactor to provide cooling and than gives off high-energy gamma radiation. To address this, the primary coolant loops require significant shielding; (2) the radioactive-Sodium coolant stays too radioactive to permit reactor plant maintenance for much longer than water; (3) Sodium metal burns in the air requiring an oxygen-free system to maintained and also presenting a hazard if leaks develop as well as complicating maintenance; (4) Sodium freezes at 200F so a keep warm system must be needed to prevent the reactor from freezing - and this keep warm system is needed when the reactor is not available (shutdown conditions); and (5) while the higher temperatures provide the benefit of less fuel use due to better thermodynamic efficiency, it turned out that intermediate-energy neutrons tend to be less efficient at causing fission which would offset thermodynamic benefits. (1) (2)

Using information from Oak Ridge on liquid sodium reactor concepts and work by General Electric (which ran the Knolls Atomic Power Laboratory or KAPL) and initial reactor concept called the Submarine Intermediate Reactor (or SIR) was developed by KAPL. The name SIR distinguished it from the initial nautilus core (STR or Submarine Thermal Reactor).

By the time President Truman signed the authorization bill for the FY1952 shipbuilding (which included the 1st nuclear powered submarine) in August 1950, it was clear that the water-cooled STR Mark 1 would be ready before the sodium reactor. As a result, PWR was chosen for Nautilus.

While work on STR and Nautilus moved forward, the SIR was designed and a prototype plant was built at the West Milton site near Schenectady NY with S1R Mark A reactor. Preliminary deign work for SIR Mark A completed in February 1951 (about 5 months behind the Nautilus prototype), detailed design completed in December 1951, and construction of the prototype completed in June 1953 (about 7 months behind the Nautilus prototype.). A 225-foot diameter horton sphere was constructed as the containment building for the new reactor. (This sphere was later used for the D1G reactor and the sphere became known as the D1G Prototype or D1G Ball.) In early 1955, Naval Reactors was still struggling to get the SIR Mark A prototype (renamed as S1G by that time with SIR Mark B becoming S2G) fully operational and at now 2 years behind the PWR plant it was not yet a reliable system. One of the chief challenges was difficulty in ensuring the Sodium in the primary plant and the water in the secondary plant never intermingled particularly in the steam generator (this is where hot primary coolant from the reactor passes through tubes in the steam generator to heat boil water to drive the propulsion plants turbine generators). Challenges were also faced in valves and pumps to work reliably. (2)

By June 1955, the plant was operating well, and duplicated the Nautilus prototypes trans-Atlantic voyage by running at full-power for more than 2,000 simulated miles. However, in July 1955, more leaks were detected in the secondary steam side. With considerable effort, the leaks were repaired allowing resumption of high-power operation in January 1956. (2)

In parallel with the prototype work, the initial submarine core was planned for SEAWOLF (SSN 575) with the S1R Mark B reactor. SEAWOLF preliminary design started in September 1952, shipbuilding contract was placed in April 1953, and construction completed in November 1954. The highly parallel developments of the STR and SIR as well as NAUTILUS and SEAWOLF were the only way to meet the target of taking a nuclear powered submarine to sea in January 1955 given the challenges that each path faced. During testing of SEAWOLF in the summer of 1956, the plant performed well until during full power testing in August 1956, a leaks was detected. These were successfully repaired and SEAWOLF eventually went to sea in January 1957 followed by commissioning in March 1957. (2)

However, due to the success of the PWR and challenges of the Sodium plan, by that time Rickover had already decided to abandon the Sodium-cooled reactor approach in favor of PWR. In November 1956, Rickover informed the Atomic Energy Commission that he would replace the reactor in SEAWOLF with a PWR reactor similar to the one in NAUTILUS. The plan was to operate SEAWOLF with the Sodium plant until a new PWR plant was ready for installation. In October 1958, SEAWOLF completed a 60-day submerged voyage of 13,761 nautical miles before starting conversion to a PWR plant in December 1958. In total, SEAWOLF steamed 71,611 miles on her sodium plant. (2) (3)

According to the Boston Globe, the SEAWOLF S2G core was disposed of at a location off the Delaware coast in 1959 at a depth of 9100 feet. (3)

All technical information in this overview is taken from reference (2) which is available directly from the Government at energy.gov.