Project Pluto was a United States government program to develop nuclear-powered ramjet engines for use in cruise missiles.

On 1 January 1957, the U.S. Air Force and the U.S. Atomic Energy Commission selected the Lawrence Radiation Laboratory to study the feasibility of applying heat from a nuclear reactor to power a ramjet engine for a Supersonic Low Altitude Missile.

This research became known as Project Pluto, and was directed by Theodore Charles (Ted) Merkle, leader of the laboratory's R Division.

The need to maintain supersonic speed at low altitude and in all kinds of weather meant that the reactor had to survive high temperatures and intense radiation.

After a series of preliminary tests to verify the integrity of the components under conditions of strain and vibration, Tory II-A, the world's first nuclear ramjet engine, was run at full power (46 MW) on 14 May 1961.

[2] The concept of using a nuclear reactor to provide a heat source for a ramjet was explored by Frank E. Rom and Eldon W. Sams at the National Advisory Committee for Aeronautics Lewis Research Center in 1954 and 1955.

[5] At the time, the United States Atomic Energy Commission (AEC) was conducting studies of the use of a nuclear rocket as an upper stage of an intercontinental ballistic missile (ICBM) on behalf of the USAF.

[6] On 1 January 1957, the USAF and the AEC selected the Livermore Laboratory to study the design of a nuclear reactor to power ramjet engines.

Operating at Mach 3, or around 3,700 kilometers per hour (2,300 mph) and flying as low as 150 meters (500 ft), it would be invulnerable to interception by contemporary air defenses.

Operating costs would also be low, as keeping them in readiness would be cheaper than a submarine or bomber, and comparable with a missile silo-based ICBM.

The need to maintain supersonic speed at low altitude and in all kinds of weather meant that the missile would have to fly though much denser air.

The reactor, code-named "Tory", would therefore have to survive high temperatures that would melt the metals used in most jet and rocket engines.

The core of the reactor would be made of beryllium oxide (BeO),[10] the only available neutron moderator material that could withstand the high temperatures required.

[12] The tubes consisted of a BeO matrix with a grain size between 5 and 20 micrometers (0.00020 and 0.00079 in) in diameter containing a solid solution of urania (UO2), zirconia (ZrO2) and yttria (Y2O3).

[15] The tubes had a hexagonal cross-section measuring 7.5 millimeters (0.297 in) from one flat side to the opposite, with a 7.5-millimeter diameter hole in the center.

It was then blended with a binding mixture containing polyvinyl alcohol, methyl cellulose and water and extruded through a die at 55,000 to 69,000 kilopascals (8,000 to 10,000 psi) to form the tubes.

[25] Scientists monitored the tests remotely via a television hook up from a tin shed located at a safe distance that had a fallout shelter stocked with two weeks' supply of food and water in the event of a major catastrophe.

[7] Some 40 kilometers (25 mi) of 25-centimeter (10 in) oil well casing was necessary to store the approximately 540,000 kilograms (1,200,000 lb) of compressed air at 25,000 kilopascals (3,600 psi) used to simulate ramjet flight conditions for Pluto.

Three giant compressors were borrowed from the Naval Submarine Base New London in Groton, Connecticut that could replenish the farm in five days.

[25] In 1957, the Livermore Laboratory began working on a prototype reactor called Tory II-A to test the proposed design.

During the summer and early fall of that year,[31] the core was assembled at Livermore inside a special fixture in a shielded containment building.

Livermore now produced a second reactor, Tory II-C, which would be a fully functional engine for a ramjet missile.

[7] The main advantage of the SLAM was its ability to carry a larger payload, but the value of this was diminished by improvements in nuclear weapon design that made them smaller and lighter, and the subsequent development of multiple warhead capability in ICBMs.

[41] The other major problem with the SLAM concept was the environmental damage caused by radioactive emissions during flight, and the disposal of the reactor at the end of the mission.

The Department of Defense's Director of Research and Engineering, Harold Brown, favored the continuation of Project Pluto at a low level of funding to progress the technology.

[8] This was rejected by the House Appropriations Committee; the technology had been demonstrated by the successful Tory II-C tests, and if there was no longer a military requirement for it, there was no reason to continue funding.

[5] Merkle hosted a celebratory dinner at a nearby country club for project participants where SLAM tie tacks and bottles of "Pluto" mineral water were given away as souvenirs.

At its peak, Project Pluto had employed around 350 people at Livermore and 100 at Site 401, and the total amount spent had been about $260 million (equivalent to $2 billion in 2023).

Fuel elements from the Tory II reactors were removed from the hot cells in Building 2201 and taken to Area 6, from whence they were shipped to the Idaho National Laboratory.

[47] This article incorporates public domain material from Nevada National Security Site History: Project Pluto Factsheet (PDF).