Project Pluto
| Tory II-C | |
|---|---|
 Tory-IIC nuclear ramjet | |
| Reactor concept | High temperature gas-cooled reactor |
| Status | Decommissioned |
| Location | Jackass Flats |
| Coordinates | 36°48′59″N 116°9′52″W / 36.81639°N 116.16444°W |
| Main parameters of the reactor core | |
| Fuel (fissile material) | highly enriched uranium oxide |
| Fuel state | solid |
| Neutron energy spectrum | slow |
| Primary control method | Boron control drums |
| Primary moderator | Beryllium oxide |
| Primary coolant | air |
| Reactor usage | |
| Primary use | propulsion |
| Power (thermal) | 600 MWth (design capacity) [citation needed] |
| Criticality (date) | 20 May 1964 |
| Operator/owner | Lawrence Radiation Laboratory |
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 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, 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 $10 million in 2024) on 21 square kilometers (8 sq mi) 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
[edit]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.[1] Research into missiles was coordinated by its Missile Projects Branch.[2] 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.[3][4] 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.[5]
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.[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.[7] This research became known as Project Pluto.[5] It was directed by Theodore C. (Ted) Merkle, leader of the Laboratory's R Division.[7]
Development
[edit]_Test_198_1963.jpg)
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 23,000 and 91,000 kilograms (50,000 and 200,000 lb), permitting a payload of over 23,000 kilograms (50,000 lb). 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. 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 10 megatonnes of TNT (42 PJ), and delivered with greater accuracy. Moreover, unlike an ICBM, it could be recalled.[8]
It was estimated that the unit cost of each missile would be less than $5 million (equivalent to $40 million in 2024), 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.[8] 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.[9] 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.[5]
The solution arrived at was to use ceramic fuel elements, extruded into hollow hexagonal rods.[10] The core of the reactor would be made of enriched uranium dispersed in beryllium oxide (BeO),[11] the only available neutron moderator material that could withstand the high temperatures required.[12] 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.[13] The tubes consisted of a BeO matrix containing, in a solid solution, a dispersion of urania (UO
2), zirconia (ZrO
2) and yttria (Y
2O
3) with a grain size between 5 and 20 micrometers (200 and 790 μin) in diameter.[11] The zirconia and yttria stabilized the urania against phase transition to triuranium octoxide (U
3O
8) at temperatures around 1,200 °C (2,190 °F). The dispersed fuel particles of the urania-zirconia-yttria mixture (known as "horseradish") were mostly from 0.5 to 1 micrometer (20 to 39 μin) in size, although some were smaller or larger.[14] The uranium was in the form of uranium enriched to 93.2 percent uranium-235 (known as "oralloy").[15]
The tubes had a hexagonal cross-section measuring 7.5 millimeters (0.297 in) from one flat side to the opposite, with a 5.8 millimeters (0.227 in) diameter hole in the center.[16] They were closely packed to form a honeycomb pattern.[17] Over 80 percent of the fueled tubes were 9.97 centimeters (3.925 in) long; the rest varied in length so as to achieve the correct column length and arrangement.[18] The metal tie rods were made of René 41 and Hastelloy R235 and were cooled so they did not exceed 760 °C (1,400 °F).[19] The ceramic tubes surrounding the tie rods (known as guard tubes) were unfueled and had smaller 3.3-millimeter (0.130 in) diameter holes.[18] The core was surrounded by neutron reflectors on all sides. The forward reflector was 250 millimeters (9.7 in) thick and the aft reflector 61 millimeters (2.4 in) thick. Both were composed of BeO tubes. The side reflector consisted of 51 millimeters (2 in) of BeO tubes around which was 25 millimeters (1 in) of nickel shims.[20] 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 230 millimeters (9 in) 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 7.6 centimeters per second (3 in/s) 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.[18] The shim rods were 1,607 millimeters (63.25 in) long and 25 millimeters (1.0 in) in diameter, with a 100-centimeter (40 in) travel.[21]
The contract to manufacture the fuel elements was awarded to the Coors Porcelain Company.[5] 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.[22] Because the process involved oralloy, criticality safety required a long, narrow geometry for the mix tanks. The mixture was filtered, dried and calcined at 538 °C (1,000 °F). 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. The tubes were dried, the binder was burned out by heating to 820 °C (1,500 °F), and they were fired in hydrogen at 1,700 °C (3,090 °F) to densify them.[22]