Project Pluto was a United States government program to develop nuclear-powered ramjet engines for use in cruise missiles. Two experimental engines were tested at the Nevada Test Site (NTS) in 1961 and 1964 respectively.
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 would have many advantages over other contemporary nuclear weapons delivery systems: operating at Mach 3, or around , and flying as low as , it would be invulnerable to interception by contemporary air defenses, carry more nuclear warheads with greater nuclear weapon yield, deliver them with greater accuracy than was possible with intercontinental ballistic missiles (ICBMs) at the time, and, unlike them, could be recalled.
This research became known as Project Pluto, and was directed by Theodore Charles (Ted) Merkle, leader of the laboratory's Radiation Division. Originally carried out at Livermore, California, testing was moved to new facilities constructed for $1.2 million (equivalent to $ million in ) on at NTS Site 401, also known as Jackass Flats. The test reactors were moved about on a railroad car that could be controlled remotely. 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. Ceramic nuclear fuel elements contained highly enriched uranium oxide fuel and beryllium oxide was used both as neutron moderator and as neutron reflector.
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. A larger, fully-functional ramjet engine was then developed called Tory II-C. This was run at full power (461 MW) on 20 May 1964, thereby demonstrating the feasibility of a nuclear-powered ramjet engine. Despite these and other successful tests, ICBM technology developed quicker than expected, and this reduced the need for cruise missiles. By the early 1960s, there was greater sensitivity about the dangers of radioactive emissions in the atmosphere, and devising an appropriate test plan for the necessary flight tests was difficult. On 1 July 1964, seven years and six months after it was started, Project Pluto was canceled.
Origins During the 1950s, the United States Air Force (USAF) considered the use of nuclear powered aircraft and missiles as part of its Aircraft Nuclear Propulsion project, which was coordinated by the Aircraft Nuclear Propulsion Office. Research into missiles was coordinated by its Missile Projects Branch. The concept of using a shieldless 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. The principle behind the nuclear ramjet was relatively simple: motion of the vehicle pushed air in through the front of the vehicle (the ram effect). If a nuclear reactor heated the air, the hot air expanded at higher speed out through a nozzle at the back, providing thrust.
At the time, the United States Atomic Energy Commission (AEC) was conducting studies of the use of a nuclear thermal rocket as an upper stage of an intercontinental ballistic missile (ICBM) on behalf of the USAF. The AEC farmed this work out to its two rival atomic weapons laboratories, the Los Alamos Scientific Laboratory (LASL) in Los Alamos, New Mexico, and the Lawrence Radiation Laboratory at Livermore, California. By late 1956 improvements in nuclear weapon design had reduced the need for a nuclear upper stage, and the development effort was concentrated at LASL, where it became known as Project Rover.
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. This research became known as Project Pluto. It was directed by Theodore C. (Ted) Merkle, leader of the Laboratory's R Division.
Development
The proposed use for nuclear-powered ramjets would be to power a cruise missile, called Supersonic Low Altitude Missile (SLAM). It would have many advantages over other nuclear weapons delivery systems. It was estimated that the reactor would weigh between , permitting a payload of over . Operating at Mach 3, or around and flying as low as , it would be invulnerable to interception by contemporary air defenses. It could carry more nuclear warheads than the sixteen aboard a Polaris ballistic missile submarine, they could be larger, with nuclear weapon yields of up to , and delivered with greater accuracy. Moreover, unlike an ICBM, it could be recalled.
It was estimated that the unit cost of each missile would be less than $5 million (equivalent to $ million in ), making them much cheaper than a Boeing B-52 Stratofortress bomber. 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. Range would not be unlimited, but would be determined by the fuel load. Merkle calculated that a MW-day of energy would burn about one gram of highly enriched uranium. A 490 MW reactor with 50 kilograms of uranium would therefore burn 1 percent of its fuel each day. Assuming that an accumulation of neutron poisons could be avoided, the missile could fly for several days. The success of the project depended upon a series of technological advances in metallurgy and materials science. Pneumatic motors necessary to control the reactor in flight had to operate while red-hot and in the presence of intense ionizing radiation. 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. In turn, this meant that it would encounter much greater air resistance and have to generate more power to overcome it. 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 solution arrived at was to use ceramic fuel elements, extruded into hollow hexagonal rods. The core of the reactor would be made of enriched uranium dispersed in beryllium oxide (), the only available neutron moderator material that could withstand the high temperatures required. The Tory II-A reactor used a dispersion of enriched uranium in BeO, but by the time Tory II-C was built zirconia and yttria was added in a 1.06:1:1 molar ratio of urania:zirconia:yttria. The tubes consisted of a BeO matrix containing, in a solid solution, a dispersion of urania (), zirconia () and yttria () with a grain size between in diameter. The zirconia and yttria stabilized the urania against phase transition to triuranium octoxide () at temperatures around . The dispersed fuel particles of the urania-zirconia-yttria mixture (known as "horseradish") were mostly from in size, although some were smaller or larger. The uranium was in the form of uranium enriched to 93.2 percent uranium-235 (known as "oralloy").
The tubes had a hexagonal cross-section measuring from one flat side to the opposite, with a diameter hole in the center. They were closely packed to form a honeycomb pattern. Over 80 percent of the fueled tubes were long; the rest varied in length so as to achieve the correct column length and arrangement. The metal tie rods were made of René 41 and Hastelloy R235 and were cooled so they did not exceed . The ceramic tubes surrounding the tie rods (known as guard tubes) were unfueled and had smaller diameter holes. The core was surrounded by neutron reflectors on all sides. The forward reflector was thick and the aft reflector thick. Both were composed of BeO tubes. The side reflector consisted of of BeO tubes around which was of nickel shims. The reactor was controlled through the movement of hafnium control rods that moved axially within the tie rods. Twelve of the rods, known as shim rods, were located about from the central axis of the core, while two were located closer to the reflector; one was a vernier rod and the other as a safety rod. Normally the movement of the rods was restricted to but in the event of a scram they could be moved in 1.5 seconds. The shim rods were moved by four actuators, each of which handled three shim rods. The shim rods were long and in diameter, with a travel.
The contract to manufacture the fuel elements was awarded to the Coors Porcelain Company. The process of making horseradish involved mixing sinterable BeO powder with oralloy uranyl nitrate, yttrium nitrate and zirconium nitrate to form a slurry which was coprecipitated by adding ammonium nitrate. Because the process involved oralloy, criticality safety required a long, narrow geometry for the mix tanks. The mixture was filtered, dried and calcined at . It was then blended with a binding mixture containing polyvinyl alcohol, methyl cellulose and water and extruded through a die at to form the tubes. The tubes were dried, the binder was burned out by heating to , and they were fired in hydrogen at to densify them. The maximum permissible effect on reactivity due to impurities in the tubes was 2 to 3 percent. In practice it was only 0.5 percent.
Test facilities Tests were conducted at new facilities constructed for $1.2 million (equivalent to $ million in ) on of Jackass Flats at the AEC's Nevada Test Site (NTS), known as Site 401. The facilities there were shared with Project Rover. The complex included of roads, critical-assembly building, control building, assembly and shop buildings, and utilities.
An aggregate mine was purchased to supply the concrete for the walls of the disassembly building, Building 2201, which were thick. Building 2201 was designed to allow radioactive components to be adjusted, disassembled or replaced remotely. Operations in the main disassembly bay could be viewed through lead glass viewing windows. Hot cells adjacent to the disassembly bay were used to monitor the control rod actuators. Vaults within each cell were equipped with remote manipulators.
All controls were located in the central control room, which was air conditioned with a positive pressure so that air always flowed toward the disassembly bay and the hot cells, and the used air from them was passed through filters. The main disassembly bay and the hot cells were accessible through openings that were normally covered with lead plates. There were showers and a radiation safety room for workers. Building 2201 also contained a maintenance shop, darkroom, offices, and equipment storage rooms. 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.
Some of oil well casing was necessary to store the approximately of compressed air at 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. A five-minute, full-power test involved of air being forced over 14 million diameter steel balls that were held in four steel tanks which were heated to .