U.S. Naval Reactors
(updated 24 April 2011)


US Aircraft Carriers by Norman Friedman
US Submarines, Post 1945
by Norman Friedman
US Destroyers, Second Edition by Norman Friedman
Adequacy of Current Organization: Defense and Arms Control – Appendix K by U.S. Government Publishing Office.
Rickover: Controversy and Genius – A Biography by Norman Polmar & Thomas B. Allen
N.S. Savannah Operating Experience by J.H. MacMillian, D.C. MacMillian, J.E. Robb, H.I. Lill, Jr, and R.O. Mehann (SNAME Transactions)
Nuclear Navy, 1939-1962 by Richard G. Hewlett and Francis Duncan, University of Chicago Press, 1975
Ending the Production of Highly Enriched Uranium for Naval Reactors by Chunyan Ma and Frank von Hippel
Report on Use of Low Enriched Uranium in Naval Nuclear Propulsion by Director, Naval Nuclear Propulsion; June 1995 (2.1 MB PDF)
Un-Labeled Diagram (13 kb GIF)

Reactor Fuel Materials

Modern U.S. Naval Reactors use fuel enriched to a minimum of 93% U235. This allows long core lives and compact reactor cores, as shown in the table below.

Equivalent Volumes of Fissile Energy in U235

93% Enriched HEU

1 cm3

20% Enriched LEU

4.7 cm3

5% Enriched LEU

18.6 cm3

0.72% Natural Uranium

129 cm3

Reactor Designations

From the early beginnings of the Navy's nuclear program to 1955; the Navy designated reactors by their application. For example, the reactor for the coming mass produced nuclear submarines was designated the Submarine Fleet Reactor, or SFR. The reactor being developed for aircraft carriers received the name Large Ship Reactor, or LSR.

This system, while functional for the early years of the Naval Reactor program, quickly became very confusing as more and more reactors were proposed. Thus in October 1955, Rickover's office issued a new system for designation naval reactors, reprinted below:

First Digit: Reactor Application

A: Aircraft Carrier
F: Frigate
C: Cruiser
D: Destroyer
S: Submarine

Second Digit: Number of reactors designed for that application by the manufacturer.

Third Digit: Manufacturer

C: Combustion Engineering
G: General Electric
W: Westinghouse
X: Unassigned (for conceptual future developments)

Combustion Engineering Reactors

S1C (Submarine Reactor, Small / SRS)

Notes: Land-Based Prototype for S1C.


Power: 2,500 SHP
SSN 579 Tullibee Class

General Electric Reactors

S1G (Submarine Intermediate Reactor, Mark A / SIR MK A)

Notes: Land Based Prototype of S2G.

S2G (Submarine Intermediate Reactor, Mark B / SIR MK B)

Uses: SSN 575 Seawolf Class

S3G (Submarine Advanced Reactor, Prototype / SAR-1)

Notes: Land Based Prototype of S4G.

S4G (Submarine Advanced Reactor, Ship / SAR-2)

Power: 34,000 SHP
Uses: SSRN 586 Triton (2 reactors)


Power: 90 MWth or 17,000 SHP
Core Lifetime: 10,000 Hours at full power.
SSN 671 Narwhal Class (1 reactor)


D1G-2 Core (SSN-688 to -718): 148 MWth (being replaced with D2W cores as the boats are refueled)
D2W Core (SSN-719 onwards):
165 MWth
Power: 30,000 to 35,000 SHP
Reactor Compartment Dimensions: 33 feet in diameter, 42 feet long; 1,680 tons
SSN 688 Los Angeles Class (1 reactor)


Notes: Prototype Land-based reactor which did not use control rods; reactivity being controlled by stationary gadolinium tubes partially filled with water. Water could be pumped from the portion of the tube inside the core up to a reservoir above the core, or allowed to flow back down into the tube. A higher water level in the tube slowed more neutrons in the core, causing more neutron capture by the gadolinium tube cladding rather than by the uranium fuel, thus lowering the power level. The system was configured with the pump running continually to keep the water level low; on loss of electrical power, all of the water would flow back into the tube, shutting down the reactor.


Power: 220 MWth or 60,000 SHP
Reactor Compartment Dimensions: 42 feet in diameter, 55 feet long; 2,750 tons
SSBN 726 Ohio Class (1 reactor)
Notes: Natural-circulation/Forced Circulation reactor. At low power levels, coolant is allowed to circulate via heat differential. At higher power levels, pumps kick in.


Power: 40,000 SHP
Core Lifetime: 33 Years
: SSN 774 Virginia Class


Notes: Land-Based Prototype for D2G. Built in West Milton, NY. Operated from 1962 to March 1996.


Power: 30,000 SHP
Reactor Compartment Dimensions: 31 ft in diameter, 37 feet deep; 1,400 tons
CGN 25 Bainbridge (2 reactors, 60,000 SHP)
CGN 35 Truxtun (2 reactors, 60,000 SHP)
CGN 36 California (2 reactors, 60,000 SHP)
CGN 38 Virginia (2 reactors, 60,000 SHP)

Westinghouse Reactors

A1W (Large Ship Reactor, Prototype / LSR)

Steam: 535 °F at ~600 PSI.
Notes: Land-Based prototype for A2W. The installation simulated two reactors driving a single propeller shaft. Operated from October 1958 to 1994.
The designations of the reactors were:

A2W (Large Ship Reactor, Ship / LSR)

Power: 120 MWth or 35,000 SHP
Steam: 535 °F at ~600 PSI.
Uses: CVN-65 Enterprise (8 reactors; 280,000 SHP)
Note: The eight A2Ws installed on Enterprise were designated as follows:


Power: 45,000 to 50,000 SHP (estimated, not backed up by source material)
Uses: CVN 67 John F. Kennedy (4 reactors) (cancelled)
Notes: Was built for CVN-67; but she was re-ordered as CV-67. The layout for CVN-67 would have been four reactors; a reduction by half of the eight needed for Enterprise.


Power: 104 MWTh or 130,000-140,000 SHP
Uses: CVN-68 Nimitz class (2 reactors)
Notes: This reactor was based off a 1960s design intended to replace the typical two reactor 60,000 SHP D2G plants found in DLGNs/CGNs with a single reactor.


Power: 40,000 SHP
Reactor Compartment Dimensions: 38 ft long, 37 ft wide, 42 ft deep, 2,250 tons
Uses: CGN-9 Long Beach (2 reactors, 80,000 SHP)
Notes: Originally was designed as a four reactor powerplant to produce the same output as the steam plant of a Des Moines class heavy cruiser – some 120,000 SHP.


Power: 60,000 SHP


Notes: Frigate reactor which would use the A1W core in a somewhat larger reactor.

S1W (Submarine Thermal Reactor Mark I / STR MK I)

Notes: Land Based Prototype for S2W. Operated from around 1951 to 1989.

S2W (Submarine Thermal Reactor Mark II / STR MK II)

Power: 13,400 SHP
Core Lifetime:
900 hours at full power
SSN 571 Nautilus Class (1 reactor)

S3W (Submarine Fleet Reactor / SFR)

Power: 7,300 SHP
Core Lifetime (Early): 2,000 hours at full power
Core Lifetime (Late): 2,500 hours at full power
SSN 578 Skate Class (1 reactor)
SSGN 587
Halibut Class (1 reactor)

S4W (Submarine Fleet Reactor /SFR)

Power: 7,300 SHP
Uses: SSN 579 Swordfish and SSN 584 Seadragon
Notes: Was a variant of the S3W with horizontal steam generators used in the S1W and S2W. During overhauls both boats which had been built with the S4W had it replaced with the S5W.

S5W (High Speed Submarine Reactor)

Power: 78 MWth or 15,000 SHP
Core Lifetime (Early Cores): 5,500 hours at full power
Core Lifetime (Later Cores): 10,000 hours at full power
Reactor Compartment Weight: 650 tons
Used in:
1955 BuShips LSST
SSN 585 Skipjack Class
SSN 593 Thresher/Permit Class
SSBN 598 George Washington Class
SSBN 608 Ethan Allan Class
SSBN 616 Lafayette Class


Power: 40,000 SHP (Supposedly)
Uses: SSN 21 Seawolf class

Bechtel Reactors


Uses: CVN 78 Gerald R. Ford Class

Reactor Developmental Designations

C1X (Cruiser Reactor)

D1X (Task Force Escort Reactor / FER)

Civilian Reactors

630A Nuclear Steam Generator

Company: General Electric
Light Water Moderated, Air Cooled.
Power: 66 MWTh or 10,000 SHP
Core Lifetime: 15,000 hours
Reactor Dimensions: 15 feet in diameter, 31 feet high; 312 tons
Notes: Was run at the Idaho National Laboratory (INL) to explore the feasibility of air cooled reactors for nuclear merchant ships. Program terminated in December 1964 due to the entire nuclear powered merchant program being cut in priority.
It was designed to replace the boiler of a conventional ship with no other propulsion equipment needing to be replaced or modified. A main blower pumps the coolant air through a flow passage to the reactor inlet. Passing through the reactor core at 400 psi, the air is heated to 1200 deg F. The air then passes over the boiler where its heat is extracted to produce steam at 880 psia and 950 deg F in the boiler. Leaving the boiler, the air returns to the blower, thus completing the cycle.

N/S Savannah Reactor

Service Life:
52,200 MW/d (original design value)
42,000 MW/d (later, revised design value), equal to 568 days at full power
N/S Savannah