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| Fig. 1: Integral-type marine PWR vessel. (Image source: B. Nicastro, after Ragheb. [2]) |
Approximately 40% of the US naval fleet is powered by nuclear reactors. [1] Although the principles of operation underlying these naval power plants are identical to commercial, land-based plants, considerations unique to the marine environment require that naval reactors differ substantially in fuel composition and design. Some basic elements of naval reactor fuels and designs are herein addressed.
The minimum dead time between shut-down and restart for Uranium nuclear reactors is bottlenecked by a phenomenon known as Xenon poisoning: when U-235 fissions, it may yield Te-135, which subsequently decays into I-135, and then into Xe-135. [2] Xe-135 has a neutron absorption cross-section of approximately 2.65 million barns, making it an exceedingly strong neutron poison. [2,3] During continuous operation, the equilibrium concentration of Xe-135 in reactors is not sufficiently high to preclude net power generation; however, if a reactor is shut down, decays of residual I-135 in the core cause the Xe-135 concentration to increase for a time (typically of order ~10 hours), during which interval the reactivity is driven negative, making restart impossible until a sufficient amount of Xenon poison has decayed. [2] The time after reactor shut-down required for Xenon concentration to fall to a level permitting restart is called the dead time. For land-based reactors, this dead time is approximately 24 hours. [2] However, for marine vessels, especially aircraft carriers and submarines, such a long period of powerplant inactivity would be intolerable.
To reduce reactor dead time, naval plants use much more highly enriched fuels than land-based plants, as the increased reactivity of the more fissile fuel offsets the poisoning effect of the Xe-135 build-up. According to Ragheb, naval reactor fuels typically consist of uranium enriched to a level of 93% in U-235, with U-238 and U-234 of order a few percent. [2] By contrast, land-based water-cooled reactors use fuels enriched to < 4% in U-235. [4]
As a result, naval reactor cores have far greater power densities than their commercial, land-based counterparts, the former generating up to 200 MW per cubic meter, in comparison to 100 MW per cubic meter in the latter. [5] However, while naval reactor cores are more space-efficient (and, therefore, mass-efficient), their total power demand tends to be significantly lower than land plants, around 10-30 MW of shaft power for the former for most propulsion applications, corresponding to hundreds of MW of thermal power output, in comparison to >3000 MW for the latter. [2]
Almost all modern plants are composed of pressurized water reactors (PWRs), with inexpensive light water being a natural choice of moderator given the high enrichment (and therefore, high baseline reactivity) of military-grade fuels. [6] The architectures of the primary and secondary fluid circuits in these PWRs fall into one of two types: loop-type and integral-type. In loop-type reactors, principal components (e.g. pumps, pressurizers, steam generators) are separate pressure vessels which are connected by piping. [7] In integral-type reactors, all components are contained within a single pressure vessel, as depicted in Fig. 1. [7] Modern designs prefer integral-type vessels, as they are more compact (i.e., less massive) and require less pumping power, both of which serve to increase the specific power of the reactor. [2] The lack of pipe connections in this integral design also eliminates modes of tertiary coolant (seawater) leakage into the secondary fluid circuit. [2]
The reactor pressure vessel is surrounded by two layers of shielding: the primary shield, typically consisting of water or lead, functions to prevent radiation (fast neutron and gamma ray) penetration into the plant containment vessel, allowing temporary human occupancy for the purposes of plant servicing, refueling, etc. [6] The secondary shield, typically consisting of concrete or polyethylene, functions to prevent radiation escape outside the containment vessel, allowing prolonged occupancy by ship crew. [6]
Unlike land-based reactors, marine reactors must be stable under mechanical vibration, collisions, and capsizing. For this reason, plants are located at or near the center of buoyancy of the ship, which is furthest from the most probable impact sites (the bow and stern), and is least impacted by hull oscillations about the pitch and roll axes. [6] The reactor is also located low in the hull, reducing the center of gravity of the vehicle, further suppressing ship roll. [6] This location furthermore minimizes moments produced on the ship by the weight of the powerplant. [6]
© Ben Nicastro. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The United States Naval Nuclear Propulsion Program," U.S. Department of the Navy, 2020.
[2] M. Ragheb, "Nuclear Naval Propulsion," in Nuclear Power - Deployment, Operation and Sustainability, ed. by P. M. Tsvetkov (IntechOpen, 2011).
[3] S. Bernstein et al., "Neutron Cross-Section of Xe-135 as a Function of Energy," Phys. Rev. 102, 823 (1956).
[4] M. Simnad, "Nuclear Reactor Materials and Fuels," in Encyclopedia of Physical Science and Technology, 3rd Ed., Vol. 10, (Academic Press, 2001), pp. 775-815.
[5] P. A. Duong et al., "Nuclear Propulsion For Merchant Ships: A Path to Sustainable Maritime Energy," Int. J. Sustain. Energy 44, 2579469 (2025).
[6] K. Holbert, "A Review of Maritime Nuclear Reactor Systems," J. Nucl. Eng. 6, 5 (2025).
[7] R. Strong and R. Nash, "The Nuclear Reactor: Impact on Submarine Design and Operation," J. Nav. Eng. 28, 193 (1984).