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The Evolution of Modern Submarine Power Plants – Pt. II

The term “nuclear” or “atomic” submarine generally denotes a submarine that is powered by nuclear propulsion generated by a reactor. The performance advantages of nuclear submarines over “conventional” diesel-electric submarines are considerable in that nuclear propulsion, being entirely independent of the necessity for combustion air frees the submarine from the requirement to surface or snorkel recurrently to replenish air supply as is essential for conventional submarines. The limitless and abundant supply of electric power that can be generated utilizing a nuclear reactor allows nuclear submarines to operate at high speeds for protracted time spans and for extended distances without refueling allowing them virtually unlimited range with the only deployment time constraints being dictated by such factors as limited storage space for food supplies and other consumables, and crew endurance considerations. Current generations of nuclear submarines can complete their entire 25-30-year service lifecycles without having to refuel their reactors. Contrastingly, the limited power capacity of electric batteries dictate that even the most advanced conventional submarines can only remain submerged for a few days at slow speed, and a few hours at top speed, though recent advances in air-independent propulsion have improved this hindrance.

The prohibitively high costs associated with nuclear technology dictates that comparatively few of the world’s military powers can field nuclear submarine fleets. And that may be contributory to the reality that the most devastating nuclear radiation accidents have involved Soviet nuclear submarine mishaps.

The concept for a nuclear-powered submarine was first proposed in the United States Navy in 1939 by the Naval Research Laboratory’s physicist Ross Gunn who was enthralled with the prospect of utilizing nuclear power to propel ships, particularly submarines. And it ultimately led to the construction of the world’s first nuclear powered submarine that was made possible by the successful development of a nuclear propulsion plant by a group of scientists and engineers at the Naval Reactors Branch of the Bureau of Ships and the Commission. Subsequently, in July 1951 the U.S. Congress authorized construction of the world’ first nuclear submarine, Nautilus, under the leadership of Captain Hyman G. Rickover USN, the sub’s name borrowed from Captain Nemo’s fictional submarine Nautilus in Jules Verne’s Twenty Thousand Leagues Under the Sea.

The Westinghouse Corporation was selected to build the Nautilus’ reactor and after the submarine was completed at the Electric Boat Company in Groton CT, First Lady Mamie Eisenhower broke the traditional bottle of champagne over her bow to formally commission her USS Nautilus SSN-571 on 30 September 1954. Then later, on 17 January 1955, Nautilus put to sea to begin her sea trials. The submarine was 320 feet long, and cost around $55 million.

The success of Nautilus soon proved nuclear power viable for the propulsion of future strategic ballistic missile submarines too, greatly improving their capability to remain submerged and undetected for extended time spans. The world’s first operational nuclear-powered ballistic missile “SSBN” submarine was USS George Washington SSBN-598 armed with 16 Polaris A-1 missiles and which conducted the first SSBN deterrent patrol November 1960 through January 1961. The Soviets already had several diesel-electric missile boats of the Project 629 Golf class and were only a year behind the U.S. with their first nuclear SSBN, the ill-fated K-19 of Project 658 Hotel class, commissioned in November 1960. However, they carried the same three-missile armament as the Golfs. The first Soviet SSBN with 16 missiles was the Project 667A Yankee class that entered service in 1967, by which time the US had already commissioned 41 SSBNs, designated “41 for Freedom”.

The primary difference between conventional submarines and nuclear submarines is their power generation system. Nuclear submarines employ nuclear reactors to produce steam to either generate electricity that powers electric motors connected to the propeller shaft or that drives steam turbines. Reactors used in submarines typically use highly enriched fuel, often greater than 20%, to enable them to deliver a heightened quantity of power from a smaller reactor and operate longer between refueling. There are three main types of marine nuclear reactors: pressurized-water, natural-circulation, and liquid-metal.

A nuclear-powered ship is constructed with the nuclear power plant inside a section of the ship called the reactor compartment. The main components of the nuclear power plant include a high-strength steel reactor vessel, a heat exchanger(s), a steam generator(s), and associated piping, pumps, and valves. Each reactor plant contains over 100 tons of lead shielding, part of which is rendered radioactive through contact with radioactive material or by neutron activation of impurities in the lead. The reactor is really nothing more than a giant tea kettle utilized to generate heat that comes from the fissioning of nuclear fuel contained within the reactor. Since the fissioning process produces ionizing radiation, heavy lead shielding is installed around it to protect the crew and the external environment against radiation exposure.

Most USN nuclear propulsion plants use a pressurized water reactor design which has two basic systems or loops, a primary loop and a secondary loop. The primary system circulates ordinary seawater and consists of the reactor, piping loops, pumps and steam generators. Then the heat produced by the reactor is transferred to the water under high pressure so it does not boil. The water is pumped through the steam generators and then back into the reactor for re-heating. In the steam generators, the heat from the water in the primary system is transferred to the secondary system to create steam. The secondary system is isolated from the primary system so that the water in the two systems does not intermix. In the secondary system, the steam flows from the steam generators to drive the turbine generators, which supply the ship with electricity, and to the main propulsion turbines, which drive the propeller. After passing through the turbines, the steam is condensed back into water which is fed back to the steam generators by the feed pumps for reuse. Thus, both the primary and secondary systems are closed systems where water is recirculated and renewed.

Generally, uranium in a reactor produces heat by nuclear fission. In the reactor, the uranium is surrounded by a moderator which is vital to slow the reaction neutrons so that they will interact more efficiently with the uranium. In most reactors, the moderator is water which is also used to carry away the heat of the reaction. This heated water is called the primary loop water and is pressurized to prevent it from boiling and it flows through a heat exchanger in which the heat is passed to another secondary, water circuit. The heat exchanger is essentially a boiler and the secondary circuit or loop provides the steam that actually turns the turbine. So long as a sufficient seal is maintained the water of the primary loop cannot contaminate the rest of the power plant.

In most cases, the water in the primary loop is circulated by a pump. Reactors can also be arranged so that differences in temperature for example between that portion of the reactor containing the reacting fuel and the rest of the reactor force the water to circulate naturally. Typically, in these natural-circulation reactors cooled water from the heat exchanger is fed into the bottom of the reactor and it rises through the fuel elements as they heat it.

Contrastingly, a liquid-metal-cooled reactor operates on the principle that molten metal can carry considerably more heat than water so that a more compact turbine can be utilized. Against that advantage, molten metal can be made highly radioactive, so that leaks, which are dangerous enough in a pressurized-water plant, become much more so. Second, pumps in these reactors must be more powerful, and the simplicity of using the same substance as moderator and heat sink is lost. Finally, there is always the possibility that enough heat will be lost for the plant to seize up, the metal solidifying in the pipes, with catastrophic results.

In either case, the reactor also provides power to the submarine’s other subsystems, such as for oxygen generation, maintenance of environmental air quality, freshwater production by distilling saltwater from the sea, internal atmospheric temperature regulation, high pressure air compressors and hydraulic accumulators, and much more. Therefore, virtually all systems rely on the reactor for their root source of power. And all naval nuclear reactors currently in use are operated with diesel generators as a backup power system able to provide emergency electrical power to supply an emergency propulsion apparatus and to charge the main batteries.

Under the direction of Captain (later Admiral) Hyman G. Rickover, the U.S. Navy developed both pressurized-water and liquid-metal prototypes. It completed its first two nuclear submarines, the Nautilus and Seawolf to test the two types, but problems including leakage in the Seawolf reactor led to the abandonment of the liquid-metal scheme. Later the Navy also developed natural-circulation reactors. In USN attack submarines, except for USS Narwhal, the natural-circulation prototype is built with pressurized-water reactors, but the Ohio-class strategic submarines are powered by natural-circulation reactors. The latter are inherently quieter than pressurized-water units because they require no pumps, at least at low and moderate power.

The U.S. Navy, accustomed to traveling long distances to wage its battles was an early proponent of nuclear propulsion because besides eliminating the necessity for combustion air it promised to eliminate the need for enormous quantities of ship fuel and in so doing reducing the logistical demands of a fleet at sea. And, nuclear-powered submarines would have a virtually unlimited range too, be faster above and below the surface of the ocean, generate more power per volume than diesel engines, and operate more effortlessly than diesel engines.

Naval reactors undergo repeated power changes for ship maneuvering, unlike civilian counterparts which operate at a steady state. Therefore, nuclear safety, radiation, shock, quieting, and operating performance requirements in addition to operation in close proximity to the crew dictate exceptionally high standards for component manufacturing and quality assurance. The internals of a naval reactor remain inaccessible for inspection or replacement throughout a long core life, unlike a typical commercial nuclear reactor which is opened for refueling roughly every eighteen months. Due to the need for sailors to live on the ships during operation, reactor compartments are designed to attenuate radiation levels outside of the reactor compartment to extremely low levels. The external surface radiation levels for the normal conditions of transportation of the U.S. Navy submarines are a fraction of the prescribed 200 mrem per hour maximum dosage.

Nautilus was originally conceived as a test-bed submarine to demonstrate that nuclear power was safe and effective, and as such was planned to be unarmed. Fortunately, the decision was quickly reversed, and it was given what was considered a standard armament suite for its time. Six 533-millimeter torpedo tubes were built into the bow, along with room to store up to twenty-six torpedoes. During its career, it had made 2,507 dives and traveled 513,550 miles without incident. A trailblazer, Nautilus’s success ensured that the U.S. submarine fleet would maintain technological superiority over its Soviet Navy for the remainder of the Cold War.

There are 98 or so commercial nuclear power reactors that produce about 20% of America’s electricity. However, there are approximately another hundred nuclear reactors that power USN submarines and aircraft carriers, producing electricity, heat and propulsion. Therefore, America’s Nuclear Navy is one of the oldest and largest nuclear organizations in the world and has the world’s best safety record of any industry of any kind. In terms of work hazards apart from combat, it is safer to work on a U.S. nuclear submarine or aircraft carrier than it is to sit at a desk trading stocks. Thousands upon thousands of people, 22,000 people at any one time, have lived, worked, eaten and slept within a stone’s throw of these nuclear reactors for 60 years without any adverse effects from radiation at all. Annual radiation doses to Navy personnel have averaged only 0.005 rem/year, a thousand times less than the federal 5 rem/year allowed for road workers. Normal background radiation in the United States varies from 100 mrem/year to over 1,000 mrem/year.

The Navy is able to maintain its safety record and high standards due to its Nuclear Power School located in Goose Creek, South Carolina that trains enlisted sailors, and officers for shipboard nuclear power plant operation and maintenance of surface ships and submarines in the U.S. nuclear Navy. The United States Navy currently operates 95 total nuclear power plants including 71 submarines, each with one reactor, 11 aircraft carriers each having two reactors, and 4 training/research prototype plants.

The nuclear program is widely acknowledged as having the most demanding academic program in all the U.S. military. And the school functions at a fast pace with stringent academic standards in all subjects, and students typically spend 45 hours per week in the classroom and are required to study an additional 10 to 35 hours per week outside of lecture hours, five days per week. Because the classified materials are restricted from leaving the training building, students are banned from studying outside of the classroom. And the ones who fail tests and otherwise struggle academically are required to review their performance with instructors and the student may be given remedial homework or other study requirements. Failing scores due to personal negligence, rather than a lack of ability, can result in charges of dereliction of duty under the Uniform Code of Military Justice. Failing students may be held back to repeat the coursework with a new group of classmates, but such students are typically released from the Nuclear Power Program and are re-designated or discharged.