What Would Converting to Fusion Mean for the “Nuclear Navy”?

The operator of the most nuclear reactors on the planet isn’t some utility operator, or a government research facility – it is the US Navy. From the launch of the USS Nautilus in 1954 to the USS Iowa (SSN797) launched on April 5, 2025, the US Navy has launched a total of two hundred nineteen (219) nuclear-powered warships. Across these warships (and a span of over seventy years), the US Navy deployed 562 reactor cores. Today, the US Navy operates a total of seventy-nine (79) nuclear-powered warships: 22 aircraft carriers, 50 attack submarines, and 18 strategic submarines.

While the Navy has built nuclear-powered ships other than aircraft carriers and submarines (the USS Long Beach and USS Bainbridge among them), today the Navy only deploys nuclear reactors in aircraft carriers and submarines. In the latest aircraft carriers (the USS Gerald Ford class, shown in the picture above), each ship has two (2) Bechtel A1B reactors, with a power output of 125 megawatts of electrical power and 350,000 shaft horsepower each, for a total equivalent power of 700 thermal megawatts (MWth) per reactor. These are the most powerful reactors in the US Navy inventory, with the two in a USS Gerald Ford class aircraft carrier providing 1.4 thermal gigawatts (GWth) of equivalent power. The USS Virginia-class nuclear attack submarine utilizes a single S9G nuclear reactor, with an equivalent power output of 210 MWth.

A Fusion Navy???

So what would it mean for the US Navy to switch from nuclear power to fusion power? In some ways, the US Navy would be a logical organization to be one of the first users of fusion power – they were (and still are) the world leader in the use of nuclear fission power. From a fuel standpoint, it also makes sense as seawater is the primary source for deuterium, the exact environment that the Navy operates in. So what class of ships would make the most sense to utilize a fusion reactor? Clearly, aircraft carriers need the most power – 1.4GW is roughly the same power as a commercial fusion power plant would use. On the other hand, the 210 MWth of the S9G is a power that early-on magnetic confinement fusion (MCF) machines such as the Commonwealth Fusion Systems (CFS) SPARC (50-100 MWth) and ARC (400 MWth) MCF machines, or Helion’s hybrid magnetic-inertial post-Polaris fusion machines (targeting 500 MWth) will likely be able to achieve in the next 5-10 years.

The Challenges of Putting a Fusion Machine In a Submarine

By far and away, the fundamental challenge of putting a fusion machine in a submarine is the diameter of the submarine’s pressure hull. The USS Virginia class nuclear attack submarine, while long at 377 feet, is 34 feet in diameter. The S9G reactor itself is roughly 7 meters (23 feet) high by 12 meters (39-1/3 feet) in diameter. While the SPARC and ARC tokamaks could  fit into the volume of the S9G reactor, their support systems (pulsed power, cryogenic plant, etc.) would not be able to be accommodated in this space currently allocated to the S9G reactor in US attack submarines, or to the reactor in US ballistic missile submarines.

On the positive side, there is a lot of flexibility in where these systems are placed, but they would take considerably more volume today than nuclear fission technologies such as the S9G reactor require. Inertial Confinement Fusion (ICF) systems would have similar challenges. Fusion power for submarines will not likely be possible until the components in either MCF (pulsed power systems, cryogenic systems) or ICF (laser drive train, cryogenic systems) can be considerably reduced in size. Finally, masking the significant electromagnetic signature of an MCF machine within a submarine would be a critical requirement, as the signature would likely impact the detectability of these vessels.

Why Aircraft Carriers Probably Makes the Most Sense for Fusion Power

The greatest advantage that current US supercarriers have over attack or ballistic missile submarines is simply their size. The table below highlights these differences for both of these classes of warships:

While displacement is not an exact measure of the internal volume of a ship, the displacement of the USS Gerald Ford is nearly 13 times that of the USS Virginia. The USS Gerald Ford is also nearly three times as long, as well as significantly higher and wider, making it easier to
find additional space within its hull for the systems required for a fusion machine. For  reference, the engine room on US Navy nuclear supercarriers such as the USS Gerald Ford are at the bottom of the cutaway image, slightly to the right of the center (inside red box).

 One of the additional interesting facts making an aircraft carrier potentially a better match for an early fusion machine is that the newest US aircraft carriers utilize a significant pulsed power system for launching aircraft, known as the Electromagnetic Aircraft Launch System (EMALS). While the EMALS system and a potential fusion machine would not likely utilize the same subsystems, the fact that the aircraft carrier crews and maintenance personnel would be familiar with pulsed power systems has benefits from a maintenance and up-time perspective.

Conclusion

The real question is “what benefit would the US Navy get from changing to fusion energy-powered ships?” The switch to the A1B reactors from the previous reactors in the USS Nimitz class ships was primarily driven by the need for more electrical power. This need is not likely to decrease over time; the potential deployment of directed energy weapons for point-defense and counter-drone systems, not to mention more powerful computers/supercomputers on ships, will only increase the need for more electricity on aircraft carriers. Additionally, the potential for aircraft carriers to increase in size is also not insignificant. The USS Enterprise of World War II (CV-6) had a fully-loaded displacement of only 32,060 tons and a length of 809 feet, roughly 30% of the displacement and ¾ of the length of the next USS Enterprise (CVN-80), the third Gerald R. Ford class aircraft carrier to be built. Requiring a 2GW (equivalent) power plant for the next generation of US Navy aircraft carriers is certainly not unforeseeable…