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NS Savannah - Propulsion

In simplified terms, the difference between a nuclear-powered ship and a conventional ship is that the nuclear reactor, rather than an oil-fired boiler, produces the steam to drive the turbines. In addition to different methods of boiling water, these two propulsion systems required different designs for the storage and delivery of fuel. A conventional steamship requires the storage of vast and fluctuating quantities of fuel, as well as complex pumping and piping systems. A nuclearpowered ship substituted a central, internally constrained storage, delivery, and consumption system.

Savannah featured the same general type of atomic power plant as USS Nautilus (1955) and the Shippingport Atomic Power station (1957), a Pressurized Water Reactor (PWR). A PWR reactor operates on the principle that water under great pressure (1,750 psi) can be heated to a high temperature without boiling. In the reactor's primary system, a separate and contained loop of pressurized water is heated to a temperature of more than 500 degrees. It then comes into contact with a heat exchanger, which transfers this heat to water of a secondary system. Under much lower pressure, the enclosed secondary water boils rapidly, producing steam. This steam passes through and powers reduction turbines, turning the shaft of the ship's propeller. The enclosed pipes of the secondary system then pass through cold seawater, condensing the steam back to water. The cycle continues as this water is then pumped back to the heat exchanger, where the primary system heats it back to steam. The major features of the reactor were installed in duplication so either could be operated independently of the other. Therefore, in case of a leak or failure in one of the secondary loops, the other could continue to generate steam.

Within the larger containment vessel, the reactor itself was housed within a "primary shield." This shield was a waterfilled, 17' high, 2" to 4" thick lead tank. The reactor's active core was a circular right cylinder 62" in diameter and 66" high. The core was made up of 32 fuel elements. Each fuel element comprised 164 stainless steel fuel rods, .5" in diameter. The rods contained uranium oxide pellets, enriched to an average of 4.4 percent of U-235. The fuel rods in the centermost 16 fuel elements contained uranium oxide at an enrichment of 4.2 percent U-235, and in the outer 16 fuel elements the enrichment was 4.6 percent U-235. This compares to the longer lasting, 90 percent enriched uranium used in Navy reactors.7 Savannah's uranium oxide pellets, were .4255" in diameter, and the space between the pellets and the inner tube wall contained helium gas under pressure to assure good heat transfer across the fuel rod.

Reactivity control was provided by 21 cruciform control rods. Each control rod was a composite of boron-stainless steel jacketed with stainless steel plate. They measured 8" across, tip to tip, and were .375" thick. Each rod had an effective length of 66". The amount of heat generated by atomic fission in the reactor depended upon how far the control rods were raised. When the rods were in the full down position, they absorbed the neutrons emitted by the nuclear fuel. Raising the rods permitted the neutrons to bombard the surrounding fissionable uranium atoms and sustain the chain reaction necessary to produce heat continuously. The higher the rods were raised, the greater the heat that was generated. Inversely, lowering the rods restricted the fissioning action and reduced the heat. In the full down position, the chain reaction was cut off entirely. Dropping all rods quickly and simultaneously to the full down position is called "scramming." An emergency "scram" insertion would lower the control rods in 1.6 seconds. The control rods were originally driven by hydraulic pistons, which were later replaced with more reliable electric motors.

A radiation monitoring system determined the amount of radiation at selected points throughout the ship and gave an alarm if the level became dangerous. There were 32 monitoring points; 12 were constantly monitored, the remaining 20 were scanned automatically or manually as conditions dictate.

The steam propulsion machinery area is located aft of the nuclear space. This space is 55' long and 78' wide. It extends from the tank top to "C" deck, a distance of 32'. This space contained all the major machinery required to propel and service the ship.

The main control room is located in the upper level of the machinery compartment just aft of the propulsion units. From here the reactor engineers monitored the reactor and controlled the speed of the ship. The bulkhead common with the engineroom was fitted with large, double-thickness glass windows to permit observation of the main control console from the visitors gallery and the machinery space. Likewise, the console operators could visually monitor the engineroom situation at all times. The main control console was a nerve center whose primary function was to monitor, display and control all the essential functions pertaining to the ship's nuclear reactor, propulsion system, and electrical power plant. For emergency take-home power, a 750 hp electric motor could be coupled to the main turbine through the main gearing. This was originally a non-reversible, low-starting torque motor, designed to take the ship back to port if a loss of nuclear power occurred. It was later upgraded to a reversible, high-starting torque motor affording greater maneuverability, capable of moving the ship clear of its pier in the event of a nuclear hazard while in port. Two supplemental 750 kw diesel generators were designed to start automatically upon failure of either turbine generator. This ensured reliable power for reactor heat removal after shutdown, and also provided sufficient power for the take-home motor and other essential electrical demands.

While the non-reactor machinery components were typical of those found in conventional steam turbine plants, numerous departures from conventional arrangements were necessary. As reported in a maritime journal of the day: ...the saturated steam conditions produced by the reactor required special precautions by the turbine manufacturer to prevent blade erosion. Consequently, the last three stages of the 9-stage impulse-type, high pressure turbine incorporated moisture-collecting provisions in the diaphragm design. Similar provisions were provided in 6 of the 7 stages of the L-P turbine. In addition, the exhaust from the H-P entered a twostage moisture separator, which by baffles and centrifugal force, dried the steam before it entered the L-P. Another departure from conventional practice was the 750-hp take-home motor, which required a special coupling to be designed by the gear manufacturer to engage the high-pressure, high-speed pinion.



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Page last modified: 22-07-2011 17:41:18 ZULU