The United States

    All U.S. submarines are nuclear-powered. Nine out of its 12 aircraft carriers are nuclear-powered and two additional nuclear-powered aircraft carriers are under construction. The United States has abandoned nuclear power for other surface ships. There has been a steady increase in core lifetime, from the original core of the first U.S. nuclear-powered submarine, the Nautilus, which lasted for about two years, to the cores for the new Virginia-class attack submarines, which are designed to lastforthe submarine'sentire 33-year design life. Current cores in the Nimitz-class aircraft carrier, Los Angeles-class attack submarine, and Ohio-class ballistic missile submarine last an average of about 20 years. Efforts are continuing to develop lifetime cores for new aircraft carriers (50 years) and the next-generation ballistic missile submarine (40 years).

During the 1980s, the U.S. Navy ordered four to five metric tons of U-235 in HEU per year. However, the size of the US nuclear submarine fleet has declined from 139 in 1990 to 73 (18 ballistic missile submarines and 55 attack submarines) in 2000 and the number of naval propulsion reactors has decreased to 97. Given that improved uranium efficiency is likely to have contributed to the greater longevity of the new reactors, the annual requirement for American nuclear submarines today is probably very roughly two tons U-235. For a nominal core life of 20 years, this would imply an average of five cores per year containing about 400 kilograms (kg) of U-235 each.

Russia

    Most of Russia's nuclear submarines, four of its six nuclear-powered icebreakers, and its three nuclear-powered cruisers each have two reactors (see Table 2). The fuel used in Russia's first two generations of naval reactors was 21-percent enriched but some of the fuel in third-generation reactors is as much as 45-percent enriched. The enrichment of icebreaker fuel is up to 90 percent. The amount of U-235 in each reactor core has increased from 50 kg in the first generation to 70 kg in the second generation, to 115 kg in the third generation as the power of the reactors increased. The reactors require refueling every seven to 10 years.

The Soviet Union had about 197 nuclear submarines in 1990. Russia inherited this large fleet but did not need and could not support so many nuclear vessels. As a result, by 2000, Russia had reduced its nuclear fleet to about 44 operational nuclear submarines (19 attack submarines, 16 ballistic missile submarines, and nine cruise missile submarines), three missile cruisers, six icebreakers, and one Arctic transport with a total of about 91 reactors (see Table 1). The missile cruisers are likely to be reduced soon, and the number of nuclear submarines is expected to decline still further. Assuming an average core life of eight years and an initial charge of about 115 kg U-235, the average annual requirement of U-235 of the Russian nuclear fleet would be about 1.3 tons. This is less than the estimate for the U.S. Navy, but, except for the icebreakers, Russia's nuclear ships spend relatively little time at sea.

United Kingdom

    British submarines are fueled with weapon-grade uranium. The refueling interval for the new Vanguard-class ballistic missile submarine is anticipated to be eight to nine years. The reactor core for the new generation Astute-class attack submarine is designed to last the 25 to 30 year design-life of the submarines.

According to current plans, Britain is expected to deploy fewer than its current 16 nuclear submarines in 2010. We assume that British nuclear submarines use about half as much U-235 per year as U.S. submarines because of their smaller size, lower power, and the shorter distances traveled. The fuel requirements of the British nuclear fleet would then be about 0.16 tons of U-235 per year. Britain has declared its total stockpile of HEU as 21.9 tons. As with the United States and Russia, post-Cold War downsizing of the U.K. nuclear stockpile has made a significant fraction of this HEU available for naval reactor fuel. In case of need, Britain could also continue to buy HEU from the much larger U.S. stockpile.

France

    Different classes and generations of French submarines use different fuel enrichments. The first three of France's first-generation (1970s) Redoutable-class ballistic missile submarines were reportedly fueled by LEU. However, the fourth and fifth ballistic missile submarines in this series were shifted to HEU. France's first generation of attack submarines, the Rubis class, and a second generation of ballistic missile submarines, the Triomphante class, returned to LEU fuel (see below). In the wake of France's decision to end the production of HEU, its intention appears to be to stay with LEU enriched to less than 10 percent.

Based on its current plans, in 2015 France is expected to have the same number of nuclear-powered submarines as it has in January 2001: four ballistic-missile submarines and six attack submarines - plus one nuclear-powered aircraft carrier.

HEU vs. LEU in Naval Reactor Fuel

    The details of naval reactor design are closely held military secrets. However, France has published diagrams of the internal layout of the LEU-fueled Rubis-class attack submarine and of the prototype of the reactor that powers it. The Norwegian government has also made public some basic data it received about the HEU-fueled reactor of a compact Russian icebreaker. Finally, a considerable amount of experience has been accumulated in converting compact research reactors from HEU to LEU.

Submarine reactors must be compact, both because of the limited space available on submarines and because of weight constraints. The reactors and their associated coolant loop are surrounded by massive quantities of material to shield crew members from the penetrating neutron and gamma radiation emitted by the core and from the primary coolant.

There is also an incentive to pack as much U-235 as possible into naval-reactor cores in order to maximize the time between refuelings. Refueling a nuclear submarine is costly and time consuming. For U.S. and Russian submarines, it involves cutting through the submarine hull and removing the core. France's Rubis-class submarines, however, have large hatches that reduce the time required for refueling from years to months.

The cumulative amount of energy that can be extracted from a naval reactor core depends upon two factors:

• Quantity of U-235 in the core. The fission of one gram of U-235 releases about one Megawatt-day (MWd) of thermal energy. The thermal energy is converted into mechanical energy using a steam turbine. The overall efficiency of conversion of thermal to mechanical energy is about 20 percent.
  
  
• Fractional Burn-up. The "bum-up" of fuel is usually measured in terms of the cumulative number of MWd generated per kg of uranium originally in the fuel. Below, in order to compare HEU and LEU fuel, we will quote burn-up in terms of MWd per kg of U-235 originally in the fuel. If all the U-235 were completely fissioned and no other fissile material were generated and fissioned, the burnup would be 940 Megawatt-days per kilogram of U-235 fissioned [MWd/(kg-U-235)].
  
  

Claims from the US Office of Naval Nuclear Propulsion (ONNP)

    In his 1995 report to the U.S. Congress, the Director of the Office of Naval Nuclear Propulsion (ONNP) asserted that:

[U.S.] 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 cladding, 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.

Assuming this constraint, he reported results for two alternative approaches forusing 20-percent enriched uranium in nuclear-propulsion-reactor cores:

1. Keep the size of the cores fixed and replace the weapon-grade uranium with an equal amount of LEU. This would reduce the amount of U-235 in the cores by a factor of 4.7. According to the ONNP report, such a reduction would reduce the core life for the Virginia-class submarine from 33 to 7.5 years, and, for Trident-class submarines and Nimitz-class aircraft carriers equipped with 45-year cores, to 14 and 10.4 years respectively.
   
   
2. Increase the volume and hence the amount of uranium until the same core life can be achieved with LEU. The ONNP report states that the volume of the core would have to be increased by approximately a factor of three. This is less than the ratio of 4.7 between the amount of 20-percent LEU and weapon-grade uranium containing 93 percent U-235 because some of the uranium-238 (U-238) added to the fuel would be converted by neutron absorption to fissile plutonium fuel. Also, the larger reactor would have the same power as the smaller reactor and therefore would not require proportionally more cooling.
   
   

The ONNP report explained the compounding effect of a three-times larger core on the size of the vessel as follows:

... the sizes and weights of the reactor vessel, pressurizer, and other primary plant components must be increased to accommodate the larger core. This in turn increases the size and weight of the reactor compartment and the amount of shielding needed to protect the crew. Consequently, the ship's volume must be increased to add buoyancy to compensate for the increase in reactor compartment and shielding size and weight.

In a design study for the new Virginia-class attack submarine, which has a submerged displacement of 7,700 tons, it was found that the ultimate result of a threefold increase in core size would be an increase in the displacement of the submarine by 12 percent. Thus, assuming that the original core contained about 0.4 tons of weapon-grade uranium, the addition of about two tons of U-238 to dilute the U-235 down to LEU would, according to the ONNP report, have a compounding effect that would increase the weight of the submarine by 1,000 tons. The effects on the larger ballistic missile submarine and aircraft carrier were less dramatic.

The principal reason for the large effect on the size of the attack submarine was apparently an increase in the diameter of the hull by about one meter (about three feet) to about 11.4 meters to accommodate the larger reactor. However, the French have shown that it is possible, with an integrated design which places the steam generator inside the reactor pressure vessel, to build a 48-MWt nuclear power plant with a 10-year core life into the 7.6-meter-diameter hull of the 2,700-ton-displacement Rubis attack submarine. This not only makes the system more compact but it eliminates the need for the heavy shielding around the external steam generators shown for U.S. naval reactors. It seems likely that, with a more creative approach, the U.S. Navy could manage to accommodate a larger core in the Virginia without significantly increasing its size.

Advanced Non-nuclear Propulsion for Submarines

    Nuclear propulsion is being abandoned for surface ships other than for U.S. and French aircraft carriers and Russian icebreakers. For aircraft carriers, a detailed Greenpeace study, subsequently confirmed by a U.S. General Accounting Office study, showed that nuclear-powered aircraft carriers are more costly and have no measurable performance superiority over their oil-powered counterparts in actual operations.

In contrast, the advantages of nuclear propulsion for submarines seem obvious. They can travel for months at high speed without surfacing. However, few countries need such a capability. As a result, some countries have decided against nuclear submarines because of the high costs and safety and environmental concerns. For their needs - primarily anti-submarine and anti-ship missions in coastal seas - diesel-electric submarines are adequate.

Diesel-electric submarines operate on battery power while submerged and on diesel power for propulsion and battery recharging while snorkeling or on the surface. The United States and Britain are the only countries that do not have such submarines. Forty-two other countries do. Britain, China, Germany, France, the Netherlands, Russia, and Sweden export diesel submarines.

During the past decade, new types of "air-independent propulsion" (AIP) submarines have been developed to provide greater underwater endurance at low speed. This involves storing liquid oxygen in the submarine and using it to burn fuel in a closed-cycle diesel, turbine, or Stirling engine, or to react with hydrogen in a fuel cell. Reportedly, AIP can extend underwater operations at low speed to more than two weeks. At a speed of five knots, such a submarine could travel 2,400 miles in 20 days without snorkeling.

As for the strategic deterrent mission, before the development of nuclear-powered ballistic missile submarines, the Soviet Union used diesel submarines as missile-launch platforms. Beginning in 1956, variants of the Golf-class submarine were equipped with two and then three nuclear-armed ballistic missiles each.

China still uses a Golf-class submarine as a test platform for underwater launches of ballistic missiles. This submarine launched the 1,700-km-range JL-1 ballistic missile in 1982 and is expected to be the test platform for the successor missile JL-2, a variant of the new 8,000-km-range DF-31 intercontinental ballistic missile.

It is possible that, in a future era of small arsenals and long-range ballistic missiles, countries could shift their submarine-launched ballistic missiles to AIP submarines. In the early 1980s, the United States considered such a basing option for the 10-warhead MX missile. Because of its long range, the MX could reach Soviet targets from U.S. coastal waters. According to one proposal, a small (3,300-ton submerged) diesel submarine could carry four 100-ton MX missiles in steel capsules strapped into hollows on its upper deck. The launch of a MX missile would have been accomplished by releasing one of these capsules, which would rise to the surface to float vertically with one end out of the water. That end of die capsule would then be blown off and the MX launched out of the canister. The submarines were estimated to cost perhaps one-fifth as much and have crews one-fourth as large as the 19,000-ton-displacement Trident submarines, which carry 24 submarine-launched ballistic missiles each.

If this solution were viable for the MX, it would certainly be viable for the smaller U.S. and Russian submarine-launched ballistic missiles, which also have long enough ranges to reach the other country from their home coastal waters. The missiles on British and French submarines also have enough range to reach Russia from European coastal waters. With the deployment of the JL-2 missile, China could reach the United States from I near-coastal waters.

Chunyan Ma is a Researcher in Weapon System Development and Arms Control Studies in China's Defense Science and Technology Information Center. Work on this paper was done while she was a Fellow at the Monterey Institute of International Studies (January-June 2000) and a Visiting Researcher at Princeton University's Center for Energy and Environmental Studies (July 2000-January 2001).

Frank von Hippel is a Professor of Public and International Affairs at Princeton University. His articles focus broadly on the technical basis for new nuclear disarmament and nonproliferation initiatives, including: deep cuts in the nuclear arsenals, taking U.S. and Russian missiles off hair-trigger alert, banning the production of fissile materials for weapons, and assisting Russia in down-sizing its nuclear weapon production complex.