Nuclear Propulsion

High fuel enrichment gives the naval reactors a compact size, and a high reactivity reserve to override the xenon poison dead time. Naval reactors use boron as a burnable neutron poison. The fuel is an alloy of 15 percent zirconium and 85 percent uranium enriched to a level of about 93-97 percent in U235. Burnable poisons and high enrichment allow a long core lifetime and provide enough reactivity to overcome the xenon poisoning reactor dead time. Axial doping provides long core life, and radial doping provides even power and fuel burnup distributions.

With a high enrichment level of 93-97 percent, modern naval reactors were designed for a refueling after 10 or more years over their 20-30 years lifetime, whereas land based reactors use fuel low-enriched to 3-5 percent in U235, and need to be refueled every 1-1 1/2 years period. New cores, with an enrichment level of up to 97.3 percent in U235, are designed to last 50 years in carriers and 30-40 years in submarines [the design goal of the Virginia class submarines].

Burnable poisons such as gadolinium or boron are incorporated in the cores. This allows for a high initial reactivity that compensates for the build-up of fission products poisons over the core lifetime, as well as the need to overcome the reactor dead time caused by the xenon poison changes as a result of operation at different power levels. Axial direction doping provides a long core life, and a radial doping provides for an even power and fuel burnup distributions.

Over the years operators have to pull the rods out higher and higher to expose more of the fuel, as the fuel on the bottom of the core is used up, or as the fission products keep it from reacting efficiently. Eventually a point comes where there are so many fission product poisons and enough fuel used up that the reactor no longer has efficient fissioning, and the heat generation is not efficient, and core has to be replaced.

The Sturgeon-class attack submarines had to refuel every eight years, or three to five times over its lifespan. The Los Angeles-class, the Navy’s next fast attack sub, refueled only once or twice over its 33-year lifecycle. The Virginia-class does away with the process altogether.

Flow-induced noise increases strongly with flow rate and pump input power. The Naval fuel system has allowed achieving high reactor power for relatively low flow rate and main coolant pumping power, eliminating coolant pumping and flow noise as detectability concerns in modem submarines.

Naval reactor cores have evolved in compactness to the point where the maximum amount of uranium is packed into the smallest volume, and the only way to make more volume available for uranium would be to remove cladcting, structure or coolant. In other words, no more uranium could be packed into a modern long-lived core without degrading the structural integrity or cooling of the fuel elements.

The natural-circulation reactor concept was simple. Two different temperatures in portions of a system set up a current, for the cooler, more dense water forced the warmer water to rise. By using natural circulation to the maximum extent feasible, it might be possible to eliminate the large reactor coolant pumps. As the heat-transfer medium would still be water kept under pressure, the reactor would be a pressurized-water type. Beneath the attractiveness of the theory was one major uncertainty: no one could be sure that the system would operate at sea where a ship was subject to rolling and pitching. To Rickover the natural-circulation reactor was attractive primarily because it promised simplicity. The silencing features were less important.

Reactor Performance Objectives

Reactor plant safety was the single overriding design criteria. Against this standard Rickover tolerated no compromise. Closely associated with safety was reliability: the ship had to be assured of the constant availability of propulsive and auxiliary power. The criteria called for spare capacity to be designed into the propulsion plant systems and components, and the plants were designed to allow the crew to carry out preventive maintenance and repairs. Finally, all the nuclear submarines had an independent means of propulsion for emergencies. He opposed automation, particularly in systems that were vital to safety. Such controls were not absolutely reliable, and a failure, especially if undetected, could lead to a severe accident.

Speed was important because it gave a commanding officer a greater chance to engage, break off action, or evade an enemy. An increase in speed was not easy to attain, for the greater the velocity of a body through the water, the greater its resistance. Consequently, gaining a fraction of a knot at high speed demanded far more power than achieving the same fraction at a lower speed. The rate a ship moved depended not only upon the propulsion plant but upon hull design and other features associated with naval architecture.

Silence was also important in underwater operations. Sound betrayed a submarine. By other means of detection, the submerged ship was almost impossible to find. Sound traveled vast distances under water, as far as 2,000 miles under certain conditions, and its behavior was hard to predict. Temperature, pressure, and salinity influenced its transmission, and these factors differed in various parts of the world, from one body of water to another, and even from one time of day to another Despite all the variables, one thing was certain: postwar development had made it possible to hear a submerged submarine at long distances.

Roughly speaking, ship noises came from the movement of the hull through water and from the vibration of machinery, particularly those components having reciprocating or rotary elements. Steam plants, with their turbines, pumps, valves, and auxiliary systems, presented many problems. Of these, reduction gears were among the most important.

Beautiful pieces of machinery, their function was to reduce the high rotary speed at which the turbine was most efficient to the much slower rotary speed that was best for the propeller. No matter how well made and how well engineered, they remained a major source of noise. Silencing a propulsion plant was a never-ending battle; no sooner was one component dampened than another took its place as the major culprit. Quiet operation was an essential element of a submarine force that had to depend on excellence instead of numbers.

Rector Safety

As of 2012 Naval Nuclear Propulsion Program [NNPP] reactors had accumulated over 6,300 reactor-years of operation and had steamed over 233 million kilometers (145 million miles). There had never been a reactor accident, nor any release of radioactivity that has had an adverse effect on human health or the quality of the environment. The US Navy’s nuclear-powered ships had an unparalleled record of safety, reliability, and environmental compliance.

The Naval Reactors' program has shown the world that nuclear power can be handled safely, with no adverse effects on the public or the environment. While others have stumbled with this challenging technology, the Naval Reactors' program stands out-in the private sector as well as in the public sector-for vision, discipline, and technical excellence.

No members of the general public have received measurable radiation exposure as a result of operations of the Naval Nuclear Propulsion Program. Procedures used by the Navy to control releases of radioactivity from navy nuclear powered ships and support facilities have been effective in protecting the environment and public health.

To ensure the high level of reliability needed for shipboard application of nuclear power, the program required its own special discipline, which must be adhered to — a discipline that is in effect to this day. Initially, the program had to develop new materials, design new components, ensure proper fabrication, and instill the new rigorous approach to training sailors for safe reactor operations. Then the new engineering concept had to be fitted inside a submarine pressure hull and designed to operate in the ocean depths. History showed that the program was successful in meeting all of the challenges.

The Soviet November-class was roughly contemporary with the U.S. Skipjack-class. The November was 352 feet long, had 26.1-foot beam, drew 21.1 feet, and displaced 4,069 tons submerged. Skipjack-class submarines were 251 feet long, 31.8 feet wide, drew 27 feet, and displaced 3,515 feet submerged. In addition, US reactor designs were safer: four of the fourteen November-class boats were lost due to reactor accidents. For comparison, only one of the six Skipjack-class submarines was lost due to an accident, and only two US nuclear subs (the Skipjack-class Scorpion with 99 aboard and the Permit-class Thresher with 125 aboard) have been lost - neither due to nuclear problems.

Reactor Design

One of the design challenges was to build a small reactor (to fit inside a small submarine hull) yet make it last a long time (refueling a submarine is costly and reduces its availability for fleet support). This reactor must withstand battle shock and rapid changes in power demands. These requirements led to the use of highly enriched uranium (HEU) as the nuclear fuel. As time has passed, the demands for long life and more powerful reactors have increased. These requirements reinforced the early decisions to use HEU to meet the current military requirements for nuclear powered warships.

A nuclear-powered ship is constructed with the nuclear power plant inside a section of the ship called the reactor compartment. The components of the nuclear power plant include a high-strength steel reactor vessel, heat exchanger(s) (steam generator), and associated piping, pumps, and valves. Each reactor plant contains over 100 tons of lead shielding, part of which is made radioactive by contact with radioactive material or by neutron activation of impurities in the lead.

The propulsion plant of a nuclear-powered ship or submarine uses a nuclear reactor to generate heat. The heat comes from the fissioning of nuclear fuel contained within the reactor. Since the fisioning process also produces radiation, shields are placed around the reactor so that the crew is protected.

The nuclear propulsion plant uses a pressurized water reactor design which has two basic systems - a primary system and a secondary system. The primary system circulates ordinary water and consists of the reactor, piping loops, pumps and steam generators. The heat produced in the reactor is transferred to the water under high pressure so it does not boil. This water is pumped through the steam generators and 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 into water which is fed back to the steam generators by the feed pumps. Thus, both the primary and secondary systems are closed systems where water is recirculated and renewed.

Since there is no step in the generation of this power which requires the presence of air or oxygen, this allows the ship to operate completely independent from the earth’s atmosphere for extended periods of time.

Naval reactors undergo repeated power changes for ship maneuvering, unlike civilian counterparts which operate at steady state. 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.

Unlike commercial nuclear power plants, Naval reactors must be rugged and resilient enough to withstand decades of rigorous operations at sea, subject to a ship's pitching and rolling and rapidly-changing demands for power, possibly under battle conditions. These conditions -- combined with the harsh environment within a reactor plant, which subjects components and materials to the long-term effects of irradiation, corrosion, high temperature and pressure -- necessitate an active, thorough and far-sighted technology effort to verify reactor operation and enhance the reliability of operating plants, as well as to ensure Naval nuclear propulsion technology provides the best options for future needs.

The nuclear propulsion plants in United States Navy ships, while differing in size and component arrangements, are all rugged, compact, pressurized water reactors designed, constructed, and operated to exacting criteria. The nuclear components of these plants are all housed in a section of the ship called the reactor compartment. The reactor compartments all serve the same purpose but may have different shapes depending on the type of ship. For submarines, the reactor compartment is a horizontal cylinder formed by a section of the ship's pressure hull, with shielded bulkheads on each end. Cruiser reactor compartments are shielded vertical cylinders or shielded rectangular boxes deep within the ship's structure.

The propulsion plants of nuclear-powered ships remain a source of radiation even after the vessels are shut down and the nuclear fuel is removed. Defueling removes all fission products since the fuel is designed, built and tested to ensure that fuel will contain the fission products. Over 99.9% of the radioactive material that remains is an integral part of the structural alloys forming the plant components. The radioactivity was created by neutron irradiation of the iron and alloying elements in the metal components during operation of the plant. The remaining 0.1% is radioactive corrosion and wear products that have been circtiated by reactor coolant, having become radioactive from exposure to neutrons in the reactor core, and then deposited on piping system internals.

The fuel in a reactor contains uranium atoms sealed within metal cladding. Uranium is one of the few materials capable of producing heat in a self-sustaining chain reaction. When a neutron causes a uranium atom to fission, the uranium nucleus is split into parts producing atoms of lower atomic number cded fission products. When formed, the fission products initially move apart at very high speeds, but they do not travel very far, ody a few thousandths of an inch, before they are stopped within the fuel cladding. Most of the heat produced in the fission process comes from stopping these fission products within the fuel and converting their kinetic energy into heat.

Radioactivity is created during fission because some of these fission products are highly radioactive when they are formed. Most of the radioactivity produced by nuclear fuel is in the fission products. The uranium fuel in naval nuclear proptision reactor cores uses highly corrosion-resistant and highly radiation-resistant fuel and cladtig. As a resdt, the fuel is very strong and has very high integrity. The fuel is designed, built, and tested to ensure that the fuel construction will contain and hold the radioactive fission products. Naval fuel totally contains fission products with the fuel - there is no fission product release from the fuel in normal operation.

Fissioning of uranium also produces neutrons while the nuclear power plant is operating. Most of the neutrons produced are absorbed by the atoms within the fuel and continue the chain reaction. However, some of the neutrons travel away from the fuel, go outside the fuel, and are absorbed in the metal structure which supports the fuel or in the walls of the reactor pressure vessel. Trace amounts of corrosion and wear products are carried by reactor coolant from reactor plant metal surfaces. Some of these become radioactive born exposure to neutrons.

Reactor coolant carries some of these radioactive products through the piping systems where a portion of the radioactivity is removed by a purification system. Most of the remaining radionuclides transported from the reactor core deposit in the piping systems. These neutrons, when absorbed in the nucleus of a nonradioactive atom like iron, can produce a radioactive atom. For example, iron-54 contains a total of 54 particles. Adding an additiond neutron produces an atom containing 55 particles, called iron-55. This atom is radioactive. At some later time, it changes into a nonradioactive manganese-55 atom by releasing energy in the form of radiation. This is called radioactive decay.

Due to the need for sallors 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 cruisers and LOS ANGELES Class and 0HI0 Class submarines are expected to be a fraction of the 200 mrem per hour on contact under 49CFR173.