Eight plants were constructed. Due to the requirement for a small physical size, all these reactors other than the MH-1A used highly enriched uranium (
HEU). The MH-1A had more space to work with, and more weight-carrying capacity, so this was a low-enrichment reactor; i.e., larger and heavier. The MH-1A was briefly considered for use in Vietnam, but the idea of anything nuclear in Vietnam was quickly rejected by the State Department. and a DOE document. Key to the codes: • First letter: S – stationary, M – mobile, P – portable. • Second letter: H – high power, M – medium power, L – low power. • Digit: Sequence number. • Third letter: A indicates field installation. Of the eight built, six produced operationally useful power for an extended period. Many of the designs were based on
United States Naval reactors, which were proven compact reactor designs.
SM-1 •
SM-1: 2 MW electric.
Fort Belvoir, Virginia, Initial criticality April 8, 1957 (several months before the
Shippingport Reactor) and the first
U.S. nuclear power plant to be
connected to an electrical grid. Used primarily for training and testing, rather than power generation for Ft. Belvoir. The plant was designed by the
American Locomotive Company (renamed ALCO Products, in 1955), and was the first reactor developed under the Army Nuclear Power Program. See the SM-1 image gallery, below. This plant was a tri-service training facility, with both the US Navy and Air Force sending personnel to be trained on shore-based facilities (the Navy had a different stand-alone program for ship-based nuclear power, which is still in operation). The SM-1 and associated training facilities at Ft. Belvoir were the only training facility for shore-based military power plants. The plant cooled its condensers using the waters of the Potomac River. For about the first 10 years of its operation, the SM-1 unknowingly released
tritium into the waters of the Chesapeake Bay, until the development of the Packard Tri-Carb detector, which was the first detector system capable of detecting the low-energy beta decay of tritium. The instrumentation in the SM-1 pre-dated the development of solid-state devices and used vacuum tubes.
SL-1 •
SL-1:
Boiling water reactor, 200 kW electrical, 400 kW thermal for heating,
National Reactor Testing Station, Idaho. Initial criticality August 11, 1958. The SL-1 was designed by the
Argonne National Laboratory to gain experience in boiling water reactor operations, develop performance characteristics, train military crews, and test components. Combustion Engineering was awarded a contract by the AEC to operate the SL-1 and in turn employed the Army's military operating crew to continue running the plant. This BWR was specifically designed to power
DEW line stations. : On January 3, 1961, the reactor was being prepared for restart after a shutdown of eleven days over the holidays. Maintenance procedures were in progress which required the main central control rod to be manually withdrawn a few inches to reconnect it to its drive mechanism; at 9:01 p.m. this rod was suddenly withdrawn too far, causing SL-1 to go
prompt critical instantly. In four milliseconds, the heat generated by the resulting
enormous power surge caused fuel in the core to explosively vaporize. The nuclear fission reaction directly heated the water, flashing a large amount into steam. Melting aluminum reacted with water producing hydrogen gas. The exploding fuel plates, violent metal-water reaction, and expanding water vapor pressed upwards on the water above the core, sending a pressure wave that struck the top of the reactor vessel. The force impinged on the lid of the reactor vessel, causing water and steam to spray from the top of the vessel. This extreme form of
water hammer propelled top head shielding, remnants of fuel plates, five loose shield plugs, a nozzle flange, and the entire reactor vessel upwards. A later investigation concluded that the vessel had jumped in the air before striking the overhead bridge crane drive shaft. The vessel settled back into its original location, leaving little evidence of this except scattered debris. The spray of water and steam knocked two operators onto the floor, killing one and severely injuring another. One of the loose shield plugs on top of the reactor vessel impaled the third man through his groin and exited his shoulder, pinning him to the ceiling. : It was later established that Byrnes (the reactor operator) had lifted the rod and caused the excursion, Legg (the shift supervisor) was standing on top of the reactor vessel and was impaled and pinned to the ceiling, and McKinley, the trainee who stood nearby, was later found alive by rescuers. All three men succumbed to injuries from physical trauma; the radiation from the nuclear excursion would have given the men no chance of survival. : This was the only fatal incident at a US nuclear power reactor, which destroyed the reactor. This incident was important in the development of commercial power because future designs prevented the core from going critical with the removal of a single rod.
PM-2A , Greenland • PM-2A: 2 MW electric, plus heating.
Camp Century, Greenland. Initial criticality October 3, 1960. The first "portable" nuclear power reactor. Brought to Greenland in parts, assembled, operated, disassembled, shipped back to United States. The PM-2A in Camp Century was designed by the American Locomotive Company to demonstrate the ability to assemble a nuclear power plant from prefabricated components in a remote, arctic location. PM-2A operated at a
uranium-235 enrichment of 93 percent. In 1961, after the SL-1 plant explosion, General
Alvin Luedecke, the General Manager of the AEC, temporarily prevented the startup of the PM-2A until an interlock could be installed on the central control rod. While the interlock could be operated by personnel, General Luedecke would have to be notified first. The PM-2A was the only reactor besides SL-1 that had a central control rod that could startup the reactor on its own. We gave explicit instructions on the 8th of January that this reactor, which was shut down at the time, would not be started until we had reviewed the situation. It was necessary for us to issue instructions to modify mechanisms of the PM-2A so that no single rod could be raised to a point where criticality could automatically occur.
PM-3A • PM-3A: 1.75 MW electric, plus heating and desalinization.
McMurdo Station, Antarctica. Owned by the Navy. Initial criticality March 3, 1962, decommissioned 1972. The PM-3A, located at McMurdo Sound, Antarctica, was designed by the Martin Company to provide electric power and steam heating to the Naval Air Facility at McMurdo Sound. PM-3A operated at a uranium-235 enrichment of 93 percent. : The PM-3A (Portable, Medium-power, 3rd generation) was a plant installed to provide power for the McMurdo Base in Antarctica. During 1970–1971, it achieved a world-record power run. It was one of the first shore-based power plants to use solid-state equipment. The PM-3A was not operated by the Army, but was under the NAVFAC (Naval Facilities Engineering Command), the shore-based power division of the US Navy. Although the majority of the personnel were Navy, the PM-3A was a tri-service stationing. The plant was air-cooled with the condensers and fan units running glycol.
Waste heat was also used for desalination using vacuum flash distillation. The reactor was located in buried tanks in the ground. : The plant suffered from a multitude of problems, including a fire and coolant leakage. It was shut down in September 1972. After decommissioning, the plant was cut into pieces and transported to the US for burial. The soil surrounding the tanks had become radioactive, so it was also removed and transported to Port Hueneme Naval Base, California, where it was incorporated into asphalt pavement.
SM-1A • SM-1A: 2 MW electric, plus heating.
Fort Greely, Alaska. Initial criticality March 13, 1962. The SM-1A was designed by
ALCO Products and was the first field facility developed under the Army Nuclear Power Program. This site was selected to develop construction methods in a remote, Arctic location, and study its economics compared with
fuel oil systems in a remote area. This plant was shut down in 1972. SM-1A operated at a uranium-235 enrichment of 93 percent. In 2022, the Army Corps of Engineers awarded a first stage contract to decommission and dismantle SM-1A, but after a contract appeal allocated it to a different contractor in 2023. Total decommissioning cost was estimated at $243 million.
MH-1A •
MH-1A: 10 MW electric, plus fresh water supply to the adjacent base. Mounted on the
Sturgis, a barge (no propulsion systems) converted from a
Liberty ship, and moored in the
Panama Canal Zone. Initial criticality at Ft. Belvoir (in Gunston Cove, off the Potomac River), January 24, 1967. It was the last of the eight plants to permanently cease operation. The MH-1A was designed by Martin Marietta Corporation. It remained moored at Gatun Lake in the Panama Canal from 1968 until 1977, when it was towed back to Ft. Belvoir for decommissioning. This reactor used low-enrichment uranium (LEU) in the range of 4 to 7 percent. It was moved to the
James River Reserve Fleet in 1978 for
SAFSTOR. The MH-1A had an elaborate analog-computer-powered simulator installed at the Training Division, USAERG, Ft. Belvoir. The MH-1A simulator was obtained by Memphis State University Center for Nuclear Studies in the early 1980s, but was never restored or returned to operational service. Its dismantling was completed in March 2019.
MM-1 • MM-1: ~2.5 MW electric, Conceptualized but never built. Conceived as the "Military Compact Reactor". A truck mounted liquid metal cooled reactor, with shorter start up and shut down times. Requiring no shielding of Earth or exclusion zones to protect the operators from radiation. With its reactor core containing the energy equivalent of over 8 million pounds of
gasoline. Envisioned to have higher power density; its power output meant for the first time the powerplant would weigh less than a
diesel generator of comparable output. While initially meant to power bases and field operations, the program was shifted to the Army's "Energy Depot Concept" to investigate the production of synthetic fuels. The reactor and associated trailers would produce liquid fuels for tanks, trucks, armored personnel carriers, and aircraft and drastically reduce the vulnerable petroleum logistical supply chain. The associated trailers would use chemical conversion processes to convert the reactor's waste heat energy into useful fuels using elements universally found in air and water (
hydrogen,
oxygen,
nitrogen and
carbon), potentially producing
methanol,
liquid hydrogen and/or
ammonia. ==Timeline==