Nuclear propulsion

Reactors

A nuclear reactor provides the heat that powers a steam turbine, which in turn drives a propeller. There are three main types of marine nuclear reactor: pressurized-water, natural-circulation, and liquid-metal.

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Ships and Underwater Exploration

Generally, uranium in a reactor produces heat by nuclear fission. In the reactor, the uranium is surrounded by a moderator, which is required 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 reaction. This heated water is called the primary loop water. Pressurized to prevent it from boiling, it runs 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 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.

The liquid-metal-cooled reactor operates on the principle that molten metal can carry much more heat than water, so that a more compact turbine can be used. 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 much 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.

Adoption by navies

Under the direction of Captain (later Admiral) Hyman 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. U.S. attack submarines (except for USS Narwhal, the natural-circulation prototype) are 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 other nuclear navies have employed pressurized-water or natural-circulation reactors with one exception, the Soviet Union in its very fast Alfa-class attack submarines, which were built in the 1970s and ’80s with liquid-metal reactors.

Strategic submarines

Strategic submarines are valuable because they are so difficult to find and kill, and they have become even more important as long-range SLBMs have become more accurate. Accurate missiles can destroy missiles in fixed land sites; were all strategic missiles so based, the side firing first could hope to disarm its enemy. However, if a nuclear power had its missiles based at sea, such a first strike would become virtually impossible—barring some breakthrough in submarine detection. To the extent that preemptive attack is impractical, therefore, a force of strategic submarines has become an effective deterrent against enemy attack. For this reason, the United States, the Soviet Union (and its successor state, Russia), Great Britain, France, China, and India have all built submarines designed to be armed with SLBMs.

Strategic submarines actually predated the nuclear-propulsion era, in that during the 1950s both the U.S. and Soviet navies developed missile-carrying diesel-electric submarines. The U.S. submarines were armed with Regulus cruise missiles, and the Soviet ships carried SS-N-3 Shaddock cruise missiles and SS-N-4 Sark short-range SLBMs. (The “SS-N” designations were given by NATO to each series of surface-to-surface naval missiles produced by the Soviet Union and Russia.) However, these missiles had to be launched from the surface, and the submarines themselves could not remain submerged indefinitely. Strategic submarines did not become truly effective until nuclear power plants and dive-launched missiles enabled them to operate continuously without exposing themselves on the surface in any way.

The first modern strategic submarines were of the U.S. George Washington class, which became operational in 1959. These 5,900-ton, 382-foot (116-metre) vessels carried 16 Polaris missiles, which had a range of 1,200 nautical miles (2,200 km). In 1967 the first of the Soviet Union’s 8,000-ton Yankee-class submarines were delivered, which carried 16 SS-N-6 missiles of 1,300-nautical-mile (2,400-km) range. These were followed a decade later by Delta-class vessels fitted with 16 SS-N-18 missiles. Each SS-N-18 had a range of 3,500 nautical miles (6,500 km). In 1982 the Soviet Union began to deploy its Typhoon class; at an estimated surface of 25,000 tons and a length of 170 metres (560 feet), these were the largest submarines ever built. They have continued in the service of the Russian navy since the dissolution of the Soviet Union in 1991, carrying 20 R-39 SLBMs (NATO SS-N-20 Sturgeon), each of which can carry its warheads a distance of 4,500 nautical miles (8,300 km). As the Typhoon and Delta vessels have aged, Russia has proceeded with plans to introduce its new Borey class of submarines, the first of which was launched in 2007. The Borey submarines have been designed to carry the new Bulava (NATO SS-N-30) missiles, which have a range similar to that of the R-39.

Beginning in 1970, the United States fitted its Lafayette-class submarines with 16 Poseidon SLBMs, which could launch its warheads a distance of 2,500 nautical miles (4,600 km). To carry as many as 24 Trident missiles, improved versions of which could travel about 6,500 nautical miles (12,000 km), the U.S. Navy commissioned 18 Ohio-class submarines between 1981 and 1997 (see photograph)—though some of them have since been converted to non-SLBM use under the terms of arms control treaties. These vessels displace 16,600 tons at the surface and are about as long as the Soviet/Russian Typhoons.

Britain’s first strategic submarines, of the Resolution class, entered service in 1967 with 16 Polaris missiles. Between 1994 and 1999 four Vanguard-class vessels, comparable to the U.S. Ohio vessels, entered service to carry as many as 16 Trident missiles each.

To supplement the Redoutable class of the 1970s, France built L’Inflexible. This 8,000-ton submarine, which entered service in 1985, carried 16 M-4 SLBMs, each with a range of 2,800 nautical miles (5,200 km). Between 1997 and 2010 four Triomphant-class submarines entered service; as replacements of L’Inflexible and the older Redoutable class, these are designed to carry 16 M45 or M51 SLBMs, which have ranges of 6,000 and 8,000 nautical miles (11,000 and 15,000 km), respectively.

In 1981 China launched its first Type 092 strategic submarine, which was based on an attack submarine derived from older Soviet designs. The Xia class, as it was called by NATO, was armed with 12 JL-1 missiles (NATO designation CSS-N-3), which had a range of 1,500 nautical miles (2,800 km). The Type 092 program was followed in 2004 by the launching of the first vessel of the Type 094 program (called the Jin class by NATO). These submarines are designed to carry 12 JL-2 SLBMs (NATO designation CSS-N-5), with a range of 4,300 nautical miles (8,000 km).

In 2009 India launched the Arihant, its first strategic submarine, built in India with Russian technical assistance. The nuclear-powered vessel, developed over more than a decade in India’s secret Advanced Technology Vessel program, is expected to go into service armed with India’s K-15 SLBM, which has a range of 375 nautical miles (700 km). Future vessels are expected to be armed with the longer-range K-4 missile, capable of reaching 1,900 nautical miles (3,500 km).

Attack submarines

After the rise of nuclear-powered strategic submarines, it seemed that only other nuclear submarines would be able to maneuver in three dimensions and remain in contact long enough to destroy them. Surface ships were clearly handicapped because their sonars could not operate as freely as those of a submarine. That situation changed somewhat when surface warships began to tow passive sonar arrays at submarine-like depths and when ship- or helicopter-launched homing torpedoes acquired a fair chance of holding and killing their targets. Both submarines and surface ships, therefore, became effective antisubmarine weapons, but only submarines could operate near an enemy’s bases, where hostile submarines would be easier to find, and only they could lie in ambush with little chance of being detected. For these reasons it was inevitable that navies with nuclear-powered strategic submarines would also build nuclear-powered attack submarines.

Almost all modern nuclear attack submarines are capable of two basic functions: to attack enemy surface ships and to destroy enemy submarines. To these basic functions some have added other roles, the most important one being the ability to strike enemy installations on land. Other roles, also important in post-Cold War submarine navies, are minelaying, electronic intelligence gathering, and special operations support. A good example of this trend is four generations of U.S. nuclear attack submarines that spanned the Cold War and post-Cold War eras: the Sturgeon class, 37 vessels commissioned between 1967 and 1975; the Los Angeles class, 51 vessels commissioned between 1976 and 1996; the Seawolf class, 3 boats commissioned between 1997 and 2005; and the Virginia class, 18 projected vessels, of which the first was commissioned in 2004. The Sturgeon and Los Angeles submarines, designed at the height of the Cold War, originally carried not only conventional torpedoes for antisubmarine warfare but also rocket-launched nuclear depth bombs, known as SUBROCs. The Seawolf submarines, also Cold War designs (though commissioned after the collapse of the Soviet Union), were dedicated "sub hunters," capable of maintaining high speeds while making little sound and diving to exceptional depths. Too expensive to be justified since the end of the Cold War, they have been succeeded by the Virginia vessels, which are intended to serve a number of roles near shore as well as in midocean. All U.S. attack submarines are armed with conventional torpedoes as well as underwater-launched Harpoon missiles for attacking surface ships from as far away as 70 nautical miles (130 km). Since the 1980s they have been fitted with Tomahawk cruise missiles, which can be programmed to strike ships 250 nautical miles (450 km) away or, in a strategic variant, to hit land targets with a nuclear warhead at ranges up to 1,300 nautical miles (2,500 km) with either a conventional or a nuclear warhead. In addition, many submarines have been either designed or retrofitted with special compartments or pods for launching and retrieving special operations personnel.

The Soviets tended to divide their attack submarines between antisubmarine and cruise-missile duties. The most prominent submarine-hunting vessels were of the three Victor classes. The Victor I vessels, which entered service beginning in 1968, introduced the "tear-drop" hull configuration to the underwater Soviet navy. These and the 6,000-ton Victor II and III classes of the following decades were fitted with rocket-launched torpedoes or nuclear depth bombs, giving them a battle range extending to 50 nautical miles (90 km). Beginning in 1971, the SS-N-7 Starbright cruise missile, which could be launched underwater and could strike ships 35 nautical miles (65 km) away, was deployed in Soviet Charlie-class submarines. The SS-N-7 began a series of dive-launched antiship cruise missiles of increasing range, culminating in the SS-N-19 Shipwreck, a supersonic missile that could carry a nuclear warhead 340 nautical miles (630 km). Twenty-four of these weapons were carried aboard the gigantic 13,000-ton, 150-metre (500-foot) Oscar submarines, which entered service in 1980.

Adding a land-attack role to Soviet attack submarines after 1987 was the SS-N-21 Sampson cruise missile, a weapon with a nuclear capability and range similar to those of the U.S. Tomahawk. These were carried by the Akula-class submarines, 7,500-ton, 111.7-metre (366-foot) vessels that continued to enter service with the Russian navy through the 1990s. In 2010 Russia launched its first Yasen-class submarine (called Graney by NATO), which carried the mixed armament of the Akula vessels—antisubmarine and antiship torpedoes and missiles as well as long-range cruise missiles.

The British Swiftsure class (six vessels, commissioned 1974–81) and Trafalgar class (six vessels, commissioned 1983–91) displaced between 4,000 and 4,500 tons at the surface and were about 87 metres (285 feet) long. They were originally armed only with torpedoes and dive-launched Harpoon missiles, consistent with their Cold War role of hunting and killing enemy submarines and surface ships. However, beginning in the 1990s some of them were fitted with Tomahawk cruise missiles, giving them a capability to attack land targets as well. The post-Cold War Astute class (a minimum of four vessels, the first being commissioned in 2010) has been designed from the beginning to carry cruise missiles.

In France the first nuclear attack submarine, the Rubis, was laid down in 1976 with antisubmarine torpedo and sonar systems inherited from the diesel-electric Agosta class. Beginning in 1984, the four vessels of this class were given improved sonar and silencing and were fitted with dive-launched Exocet antiship missiles. The Rubis vessels, the smallest nuclear attack submarines ever put into service, displaced about 2,400 tons at the surface and were about 71 metres (235 feet) long. They were followed in the early 1990s by two similar but slightly larger Amethyste-class submarines. In the late 1990s France brought its submarine posture into the post-Cold War era with plans for the Barracuda class, six submarines displacing some 4,000 tons at the surface and carrying land-attack cruise missiles and advanced electronic surveillance equipment as well as the usual torpedoes and antiship Exocets. Construction of the first Barracuda submarine began in 2007.

China began to plan for a nuclear attack submarine fleet in the 1950s. The first keel of the Type 091 vessel (known as the Han class to NATO), based partly on Soviet designs, was laid down in 1967, and the completed boat was commissioned in 1974. Four more Type 091 boats were commissioned over the next two decades. They were followed by the Type 093 class (NATO designation Shang), the first of which was commissioned in 2006. The Type 093 boats displace some 6,000 tons submerged and are about 110 metres (360 feet) long. Reflecting China’s strategic goal of asserting its presence against other navies in waters adjacent to its coasts, Chinese nuclear attack submarines are mainly torpedo-equipped sub hunters, though they can be fitted with antiship missiles as well.

Speed

Increased speed required increased power. Since the resistance a submarine encounters is a function of its surface area, the ideal was to achieve greater power without increasing the volume or weight of the power plant and, therefore, the size of the submarine. A more powerful (and therefore noisier) engine could be silenced, but only by increasing the size of the submarine, which in turn would lower its speed. These complex trade-offs were illustrated by the Sturgeon and Los Angeles submarines. Reactor power approximately doubled between these two generations, but overall size increased enormously, from about 3,600 to 6,000 tons surfaced. The Soviets, meanwhile, achieved very high speed (about 40 knots, compared to slightly over 30 knots for fast Western submarines) in their Alfa class, but probably at the cost of a great deal of noise at high speed.

Speed was prized for several quite different reasons. At first, the U.S. and Soviet navies developed fast submarines primarily as antiship weapons. In the 1950s the Guppy-style hull design of USS Nautilus gave it a submerged speed of over 20 knots, which was fast enough to evade surface ships but not to counterattack them. To make up this deficit, U.S. submarines then under design were altered by adapting nuclear power to the tapered “tear-drop” hull of the experimental submarine Albacore. The resulting Skipjack class, which entered service in 1959, came up with a top speed in excess of 30 knots.

In a spectacular demonstration of the Soviets’ fast attack capabilities, a Soviet nuclear submarine intercepted the nuclear aircraft carrier USS Enterprise in February 1968. The submarine was not quite as fast as the Enterprise, but it was fast enough to keep the carrier within weapon range while the carrier accelerated to top speed.

With the commencement of the Soviet fast nuclear program, the U.S. Navy shifted its emphasis to dual-purpose vessels capable of attacking submarines as well as surface ships. High speed, as achieved in the 1970s and ’80s by the Los Angeles class, was then required to keep up with the fast surface targets that the Soviet submarines were expected to attack.

High sustained speed also made it possible for submarines to deploy more efficiently to distant patrol stations. Although nuclear submarines’ fuel supplies were effectively unlimited, they were limited in their capacity for stores and could not expect to remain at sea for more than about 60 to 90 days. The more rapidly they could reach their patrol area, therefore, the more productive time they could spend there.

As in the case of nonnuclear submarines, higher speed was also valued for evasion after an attack. However, when that higher speed was bought at the cost of louder operation, submarines became easier to detect. Also, from the mid-1950s the main antisubmarine weapons were homing torpedoes, which became significantly faster than the submarines they sought, and nuclear depth bombs, which might be dropped effectively anywhere in the vicinity of a submarine. In all of these cases, sheer speed was no longer a guarantee of evasion, although it did make attack more difficult.

Depth

Deeper diving was valued for several reasons. As in the past, it could be combined with higher speed for better evasion. In addition, a deep-diving submarine could make better use of its own sonar, partly because it could operate in several quite different layers of the sea. This advantage was reflected in a change in U.S. submarine sonars that began about 1960. Previous submarine units had been cylindrical, producing broad, fan-shaped beams that could determine target range and bearing but not target depth. The new sonars were spherical, producing narrow, pencil-shaped beams that could distinguish between targets at different depths. They could also make better use of sonar reflection off the sea bottom and surface to achieve greater range.

Finally, greater maximum operating depth became particularly important at high speed, when there was always a possibility that a submarine would accidentally tip down and descend below a safe operating depth before the downward motion could be corrected. It is no surprise, then, that the greatest reported diving depth (about 2,800 feet) was associated with the highest reported maximum speed (about 43 knots), in the Soviet Alfa class. (Diving depth of most other modern attack submarines was reportedly between 1,000 and 1,500 feet.)

Greater depth required a stronger (and heavier) hull, and increased power required a stronger power plant. Attempts to combine the two required a larger hull (to provide enough buoyancy); that in turn added underwater resistance, which cut the speed advantage gained from the more powerful engine. This tension between different requirements explains the characteristics of many modern submarines. For example, the Los Angeles class was said to have sacrificed some diving depth in order to achieve higher speed. In the Alfa class, weight was saved by adopting an expensive titanium-alloy hull and a very compact power plant.

Silencing

Until the late 1950s, submarines were usually detected by active sonar; that is, by sound waves bounced off their hulls. Because these sound waves could also be detected by the hunted submarine, they gave it warning that it was in danger of attack. Also, because water can support only so much sonar energy, active sonar was limited in range. Beginning in the early 1950s, then, the U.S. and British navies began to investigate passive sonar, in which sensors detected noises emanating from the submarine itself. Early nuclear submarines were quite susceptible to such detection because their machinery was very noisy. In particular, the pumps required to circulate the coolant, which could not be turned off without melting the reactor core, could be heard at a considerable distance.

Beginning at that time, silencing became a major thrust in submarine design. The pumps of pressurized-water reactors were redesigned to be quieter, and in many submarines the machinery was carried clear of the hull on sound-absorbing mounts. All of this added to the size and weight of the machinery and to the expense of construction; it also added to the attraction of natural-circulation plants.

As a further step in silencing, hulls were coated with sound-absorbing material. Even relatively simple coatings could drastically reduce the effectiveness of homing torpedoes.