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Nuclear thermal rocket

From Academic Kids

In a nuclear thermal rocket a working fluid, usually hydrogen, is heated in a high temperature nuclear reactor, and then expands through a rocket nozzle to create thrust. The nuclear reactor's energy replaces the chemical energy of the reactive chemicals in a traditional rocket engine. Due to the high energy of the nuclear reactions compared to chemical ones, about 107 times, the resulting efficiency of the engine is at least twice as good as chemical engines even considering the weight of the reactor, and even higher for advanced designs.

Contents

Theoretical designs

A nuclear thermal rocket can be categorized by the construction of its reactor, which can range from a relatively simple solid reactor up to a much more complicated but more efficient reactor with a gas core.

The most typical type uses a conventional (albeit light-weight) nuclear reactor running at high temperatures to heat the working fluid that is moving through the reactor core. This is known as the solid-core design, and is the simplest design to construct.

A NERVA solid-core design
Enlarge
A NERVA solid-core design

The solid-core has the downside that it can only be run at temperatures below the melting point of the materials used in the reactor core. Since the efficiency of a rocket engine is strongly related to the temperature of the working fluid, the solid-core design needs to be constructed of materials that remain strong at as high a temperature as possible. Even the most advanced materials melt at temperatures below that which the fuel can actually create, meaning that much of the potential energy of the reactions is lost. Generally the solid-core design is expected to deliver specific impulses on the order of 800 to 900 lbf新/lb (8 to 9 kN新/kg) , about twice of LH2-LOX designs such as the SSME.

The weight of a complete nuclear reactor is so great that solid-core engines would be hard-pressed to achieve a thrust-to-weight ratio of 1:1, which would be needed to overcome the gravity of the Earth on launch. Nevertheless the overall weight of the engine and fuel for a given amount of total impulse is lower. This means that solid-core engines are only really useful for upper-stage uses where the vehicle is already in orbit, or close to it, and the required thrust is lower. To be a useful launch engine, the system would have to be either much lighter, or provide even higher specific impulse. Both would, of course, be even better.

One way to increase the temperature, and thus the specific impulse, is to isolate the fuel elements so they no longer have to be rigid. This is the basis of the particle-bed reactor, also known as the fluidized-bed, dust-bed, or rotating-bed design. In this design the fuel is placed in a number of (typically spherical) elements which "float" inside the hydrogen working fluid. Spinning the entire engine forces the fuel elements out to walls that are being cooled by the hydrogen. This design increases the specific impulse to about 1000 lbf新/lb (10 kN新/kg), allowing for thrust-to-weight ratios just over 1:1, although at the cost of increased complexity. Such a design could share design elements with a pebble-bed reactor, several of which are currently generating electricity.

Dramatically greater improvements can be had by mixing the nuclear fuel into the working fluid, and allowing the reaction to take place in the liquid mixture itself. This is the basis of the so-called liquid-core engine, which can operate at higher temperatures beyond the melting point of the fuel. In this case the maximum temperature is whatever the container wall (typically a neutron reflector of some sort) can handle, while actively cooled by the hydrogen. It is expected that the liquid-core design can deliver performance on the order of 1300 to 1500 lbf新/lb (13–15 kN新/kg).

These engines are difficult to build however; the reaction time of the nuclear fuel is much higher than the heating time of the working fluid, meaning that some system must be used to trap the fuel inside the engine while still allowing the working fluid to easily exit through the nozzle. Most liquid-phase engines have focussed on rotating the fuel/fluid mixture at very high speeds, forcing the fuel to the outside due to centrifugal force (uranium is heavier than hydrogen). In many ways the design mirrors the particle-bed design, although operating at even higher temperatures.

An alternative liquid-core design, the nuclear salt-water rocket has been proposed by Robert Zubrin. In this design, the working fluid is water, which serves as neutron moderator as well. The nuclear fuel is not retained, drastically simplifying the design. However, by its very design, the rocket would discharge massive quantities of extremely radioactive waste and could only be safely operated well outside the earth's atmosphere and perhaps even entirely outside earth's magnetosphere.

The final classification is the gas-core engine. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a toroidal pocket of gaseous uranium fuel in the middle of the reactor, surrounded by hydrogen. In this case the fuel does not touch the reactor wall at all, so temperatures could reach several tens of thousands of degrees, which would allow specific impulses of 3000 to 5000 lbf新/lb (30 to 50 kN新/kg). In this basic design, the "open cycle", the losses of nuclear fuel would be difficult to control, which has led to studies of the "closed cycle" or nuclear light bulb engine, where the gasseous nuclear fuel is contained in a super-high-temperature quartz container, over which the hydrogen flows. The closed cycle engine actually has much more in common with the solid-core design, but this time limited by the critical temperature of quartz instead of the fuel stack. Although less efficient than the open-cycle design, the closed-cycle design is expected to deliver a rather respectable specific impulse of about 1500-2000 lbf新/lb (15–20 kN新/kg).

Practical testing

Although engineering studies of all of these designs were made, only the solid-core engine was ever built. Development of such engines started under the aegis of the Atomic Energy Commission in 1956 as Project Rover, with work on a suitable reactor starting at LANL. Two basic designs came from this project, Kiwi and NRX.

Kiwi was the first to be fired, starting in July 1959 with Kiwi 1. The reactor was not intended for flight, hence the naming of the rocket after a flightless bird. This was unlike later tests because the engine design could not really be used, the core was simply a stack of uncoated uranium oxide plates onto which the hydrogen was dumped. Nevertheless it generated 70 MW and produced an exhaust of 2683 K. Two additional tests of the basic concept, A' and A3, added coatings to the plates to test fuel rod concepts.

The Kiwi B series fully developed the fuel system, which consisted of the uranium fuel in the form of tiny uranium oxide (UO2) spheres embedded in a low-boron graphite matrix, and then coated with niobium carbide. Nineteen holes ran the length of the bundles, and through these holes the liquid hydrogen flowed for cooling. A final change introduced during the Kiwi program changed the fuel to uranium carbide, which was run for the last time in 1964.

Using information developed from the Kiwi series, the Phoebus series developed much larger reactors. The first 1A test in June 1965 ran for over 10 minutes at 1090 MW, with an exhaust temperature of 2370 K. The B run in February 1967 improved this to 1500 MW for 30 minutes. The final 2A test in June 1968 ran for over 12 minutes at 4,000 MW, the most powerful nuclear reactor ever built. For contrast, the largest hydroelectric plant in the world, Itaipu, produces 12,600 MW, 25% of all the power used in Brazil.

A smaller version of Kiwi, the Peewee was also built. It was fired several times at 500 MW in order to test coatings made of zirconium carbide (instead of niobium carbide) but also increased the power density of the system. An unrelated water-cooled system known as NF-1 (for Nuclear Furnace) was used for future materials testing.

While Kiwi was being run, NASA joined the effort with their NERVA program (Nuclear Engine for Rocket Vehicle Applications). Unlike the AEC work, which was intended to study the reactor design itself, NERVA was aiming to produce a real engine that could be deployed on space missions. A 75,000 lbf (334 kN) thrust baseline design was considered for some time as the upper stages for the Saturn V, in place of the J-2s that were actually flown.

The design that eventually developed, known as NRX for short, started testing in September 1964. The final engine in this series was the EX, which was the first designed to be fired in a downward position (like a "real" rocket engine) and was fired twenty-eight times in March 1968. The series all generated 1100 MW, and many of the tests concluded only when the test-stand ran out of hydrogen fuel. EX produced the baseline 75,000 lbf (334 kN) thrust that NERVA required.

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NASA-nuclear-destruction.jpg
A KIWI engine being destructively tested

All of these designs also shared a number of problems that were never completely cured. The engines were also quite easy to break, and on many firings the vibrations inside the reactors cracked the fuel bundles and caused the reactors to break apart. This problem was largely solved by the end of the program, and related work at Argonne National Laboratory looked promising. However, while the graphite construction was indeed able to be heated to high temperatures, it likewise eroded quite heavily due to the hydrogen. The coatings never wholly solved this problem, and significant "losses" of fuel occurred on most firings. This problem did not look like it would be solved any time soon.

The NERVA/Rover project was eventually cancelled in 1972 with the general wind-down of NASA in the post-Apollo era. Without a manned mission to Mars, the need for a nuclear thermal rocket was unclear. It was also becoming clear that there would be intense public outcry against any attempt to use a nuclear engine.

Although the Kiwi/Phoebus/NERVA designs were the only to be tested in any substantial program, a number of other solid-core engines were also studied to some degree. The Small Nuclear Rocket Engine, or SNRE, was designed at the Los Alamos National Laboratory (LANL) for upper stage use, both on unmanned launchers as well as the Space Shuttle. It featured a split-nozzle that could be rotated to the side, allowing it to take up less room in the Shuttle cargo bay. The design provided 73 kN of thrust and operated at a specific impulse of 875 lbf新/lb (8.58 kN新/kg), and it was planned to increase this to 975 with fairly basic upgrades. This allowed it to achieve a mass fraction of about 0.74, comparing with 0.86 for the SSME, one of the best conventional engines.

A related design that saw some work, but never made it to the prototype stage, was Dumbo. Dumbo was similar to Kiwi/NERVA in concept, but used more advanced construction techniques to lower the weight of the reactor. The Dumbo reactor consisted of several large tubes (more like barrels) which were in turn constructed of stacked plates of corrugated material. The corrugations were lined up so that the resulting stack had channels running from the inside to the outside. Some of these channels were filled with uranium fuel, others with a moderator, and some were left open as a gas channel. Hydrogen was pumped into the middle of the tube, and would be heated by the fuel as it travelled through the channels as it worked its way to the outside. The resulting system was lighter than a conventional design for any particular amount of fuel. The project developed some initial reactor designs and appeared to be feasible.

Risks

There is an inherent possibility of atmospheric or orbital rocket failure which could result in a dispersal of radioactive material, and resulting fallout. Catastrophic failure, meaning the release of radioactive material into the environment, would be the result of a containment breach. A containment breach could be the result of an impact with orbital debris, material failure due to uncontrolled fission, material imperfections or fatigue and human design flaws. A release of radioactive material while in flight could disperse radioactive debris over the Earth in wide and unpredictable area. The zone of contamination and its concentration would be dependent on prevailing weather conditions and orbital parameters at the time of re-entry.

Atmospheric plutonium releases

Three to five megagrams of plutonium (about 8,816 pounds or 11.8 petabecquerels) were released into the atmosphere by weapons testing from the end of WWII (1945) until the Partial Test Ban Treaty was signed in 1963. Though no deaths have been definitively detected from these events, it is virtually impossible to determine if any cases of cancer have actually been induced by inhalation or ingestion of plutonium particles dispersed in the atmosphere or lithosphere as a result of these releases. Often, antinuclear organizations will state that one gram (1/28 of an ounce) of plutonium is enough to give everyone on Earth cancer. However this statement assumes that such a sample would be monodispersed into a fine powder and then each person on earth "dosed" with a portion of the sample; this is an incredibly unrealistic scenario. After the 1963 treaty between the U.S. and the USSR all nuclear weapons testing by these two nations was done underground. Between 1963 and 1980 the U.S. exploded 316 nuclear devices underground.

Source: Lillie, David W. Our Radiant World, Ames, IA: Iowa State University Press, 1986. ISBN 0813812968. An easy to read narrative of the various sources of radiation, the effect upon humans and the many ways in which radiation is used. Nuclear weapons, nuclear medicine, and nuclear power are discussed, as well as nonionizing radiation. The book is recommended as an excellent supplementary text for science courses.

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