Moreover, concentrating a ship’s power plant in one place makes her vulnerable to a single underwater hit. Since before World War II, U.S. design practice had been to split power plants so that no single hit amidships could immobilize a ship. Enterprise violated that requirement because of the need to concentrate those eight reactors (they shared important auxiliary machinery). With their single funnels, conventional carriers did suffer from some concentration, but they still had dispersed power plants. Since they needed no funnels, reactor plants could, at least in theory, be spread out more widely than their conventional predecessors, giving their ships better survivability.
For a time, in the 1970s, there was a legal requirement that all U.S. combatants of over 8,000 tons be nuclear-powered, unless the president specifically waived that condition. The U.S. Navy built several large nuclear destroyers (later designated cruisers), but found them unsatisfactory. In contrast to a carrier, the nuclear power plant was too great a fraction of their building and operating cost. They proved cramped, and they lacked anti-submarine capability (they were too noisy, because it would have been too expensive to silence their power plants).
Nimitz embodied that potential. Each of her two quite separate reactors drives a pair of steam turbines. Physically separating the reactors made it possible to disperse other vital parts of the ship, such as magazines. The split power plant is less vulnerable to attack or to other damage. It is also much easier to open up the ship to refuel two widely separated reactors. George H.W. Bush, the U.S. Navy’s newest carrier, has much the same reactor arrangement as Nimitz, but Naval Reactors has been working hard over the intervening forty years.
Since Nimitz, Naval Reactors has sought to lengthen the interval between fuelings, because that cuts the cost of running a nuclear ship. This is a matter of the design of the reactor’s nuclear core (new cores are designed to fit existing reactors, so in effect all nuclear carriers are upgraded over time). A reactor does not simply run out of fuel; when it is shut down there is still a good deal of burnable uranium in the fuel rods. Instead, as the fuel is used, byproducts such as Xenon form in the rods. Xenon in particular can poison the reactor, because it absorbs the neutrons that drive the chain reaction powering it. Changes in core design make it possible to run longer before the rods must be removed and the material inside purged of Xenon. Once enough Xenon has been formed, the reactor has to shut down. The Xenon poisoning problem recalls the very old problem of ships burning coal: periodically they had to turn down their boilers so that the ashes choking them could be removed. The difference is that Xenon cannot simply be sloughed off and the reactor restarted. It has to be chemically extracted from fuel rods along with other byproducts of nuclear fission (new rods are inserted into the reactor at refueling time). The time scale is of course far longer now. The goal is a core that lasts the life of the ship, so that she is never refueled. That is being done for submarines. Current cores last 20 to 25 years, limiting a carrier to one refueling during her career. The next carrier, USS Gerald R. Ford, is to have a full-life (50 year) core. Her reactors are also to be about a quarter more powerful than those of George H.W. Bush.
Rickover envisaged an all-nuclear task force with unlimited endurance. For a time, in the 1970s, there was a legal requirement that all U.S. combatants of over 8,000 tons be nuclear-powered, unless the president specifically waived that condition. The U.S. Navy built several large nuclear destroyers (later designated cruisers), but found them unsatisfactory. In contrast to a carrier, the nuclear power plant was too great a fraction of their building and operating cost. They proved cramped, and they lacked anti-submarine capability (they were too noisy, because it would have been too expensive to silence their power plants).
Moreover, a carrier battle group cannot be completely independent of tankers. Naval aviation is a very demanding profession. Even when a carrier is not fighting, her pilots must keep flying to maintain their proficiency. The carrier must take on aviation fuel periodically. Her gas turbine-powered escorts burn the same fuel, so it is not so very difficult for the carrier to fuel them periodically. The carrier herself benefits hugely from her nuclear power plant. It turns out that carriers need layers of liquids in their sides as torpedo protection; in non-nuclear days they carried the ship’s fuel oil. Eliminating the need for the carrier’s own fuel left the layers of fuel for her aircraft (which gained more flying days between refueling) and for the escorts. This compromise has proven quite successful.
This article was first published under the title “Under way on Nuclear Power” in Freedom at Work: USS George H.W. Bush CVN 77.