Gas core reactor rocket
Encyclopedia
Gas core reactor rockets are a conceptual type of rocket that is propelled by the exhausted coolant of a gaseous fission reactor
. The nuclear fission reactor core may be either a gas
or plasma
. They may be capable of creating specific impulse
s of 3,000–5,000 s (30 to 50 kN·s/kg, effective exhaust velocities 30 to 50 km/s) and thrust
which is enough for relatively fast interplanetary travel. Heat transfer
to the working fluid
(propellant
) is by thermal radiation
, mostly in the ultraviolet
, given off by the fission
gas at a working temperature of around 25,000 °C.
and core wall structural temperatures, which are distanced from the hottest regions of the gas core. Consequently, nuclear gas core reactors can provide much higher temperatures to the propellant
. Solid core nuclear thermal rockets can develop higher specific impulse than conventional chemical rockets due to the extreme power density
of the reactor core, but their operating temperature
s are limited by the maximum temperature of the solid core because the reactor's temperatures cannot rise above its components' lowest melting
temperature.
Due to the much higher temperatures achievable by the gaseous core design, it can deliver higher specific impulse and thrust than most other conventional nuclear designs. This translates into shorter mission transit times for future astronauts or larger payload fractions. It may also be possible to use partially ionized plasma from the gas core to generate electricity magnetohydrodynamically, subsequently negating the need for an additional power supply.
.
fuel is usually highly enriched uranium
pellets or a uranium containing gas (U-235
or U-233
). Sometimes uranium tetrafluoride is required due to its chemical stability; the propellant is usually hydrogen
.
usually made up of beryllium oxide
. The propellant also provides moderation.
absorbing drums or the radial moderator.
The advantage of the open cycle design is that it can attain much higher operating temperatures than the closed cycle design, and does not require the exotic materials needed for a suitable closed cycle design.
al or counter flow toroidal. Since there are issues regarding the loss of fissile fuel with the cylindrical and toroidal designs, the counter-flow toroidal gas core geometry is the primary source of research. The counter flow toroid is the most promising because it has the best stability and theoretically prevents mixing of the fissile fuel and propellant more effectively than the aforementioned concepts. In this design, the fissile fuel is kept mostly in a base injection stabilized recirculation bubble by hydrodynamic confinement. Most designs utilize a cylindrical gas core wall for ease of modeling. However, previous cold flow tests have shown that hydrodynamic containment is more easily achieved with a spherical internal wall geometry design.
The formation of the fuel vortex
is complex. It basically comes down to flow over a projectile shape with a blunt base. The vortex is formed by placing a semi-porous wall in front of the desired location of the fuel vortex but leaves room along its sides for hydrogen propellant. Propellant is then pumped inside the reactor cavity along an annular inlet region. A dead space then develops behind the semi-porous wall; due to viscous and shear
forces a counter toroidal rotation develops. Once the vortex develops, fissile fuel can be injected through the semi-porous plate to bring the reactor critical. The formation and location of the fuel vortex now depends on the amount of fissile fuel that bleeds into the system through the semi-porous wall. When more fuel bleeds into the system through the wall, the vortex moves farther downstream. When less bleeds through, the vortex moves farther upstream. Of course, the upstream location is constrained by the placement of the semi-porous wall.
environment present in the reactor, specifically particle bombardment from the nearby fission reactions. This barrage of particles can lead to sputtering
and eventual wall erosion.
One closed cycle gas core rocket design (often called the nuclear lightbulb
) contains the fissioning gas in a quartz
enclosure that is separate from the propellant. First, the hydrogen coolant runs through the nozzle and inside the walls of the quartz enclosure for cooling. Next, the coolant is run along the outside of the quartz fuel enclosure. Since the fissile gas would be directly in contact with the walls, the operating temperature is not as great as other designs because the walls would eventually ablate away.
, β.
When β<1 magnetic confinement is possible (most fusion
schemes have a β close to 0.05). However, the pressures in a gas core rocket are much higher than pressures in fusion devices, approximately 1000 atm
(100 MPa). For these pressures, the necessary magnetic field strength required is close to 16 tesla
s just to produce β=1. For a magnetic field of this magnitude
, superconducting technology is necessary and the added mass of such a system would be detrimental. Also, even with a β<1, resistive diffusion will cause the fuel core to collapse almost immediately unless β<<1, which would require an even larger magnetic field.
effects to decrease core containment by 35% if all other flow-rates are held constant from a zero g startup. Ultimately, the fuel-propellant flows will have to be throttled until the rocket approaches some sort of steady state.
Due to the inability to perform live testing on earth, research is focused primarily on computational modeling of such a system. It was previously mentioned that the specific impulse could be as high as or higher than 3000 s. However, results of computational modeling point towards this number being somewhat optimistic. When thermal hydraulics were modeled more completely for a typical base injection stabilized recirculation bubble gas core rocket by D. Poston, the specific impulse dropped from >3000 s to <1500 s. In the base injection stabilized recirculation bubble gas core rocket concept, it is thought that some additional method of fuel confinement will be beneficial. As mentioned earlier, relying completely on magnetic containment of the fuel bubble is not yet practical. However, a magnetic field may be able to assist in containment or help suppress turbulence that would lead to fuel-propellant mixing.
The primary areas of future research for such an OCGCR would therefore be centered on keeping the fuel and propellant from mixing as much as possible. Although this article has focused on enriched uranium for the fuel and hydrogen for the propellant, this may not be the optimal choice for either. Other fuels, such as plutonium, and other propellants, including helium or even helium-3, have also been considered and in certain situations provide advantages.
Gaseous fission reactor
A gas nuclear reactor is a nuclear reactor in which the nuclear fuel is in a gaseous state rather than liquid or solid. In this type of reactor, the only temperature-limiting materials are the reactor walls. Conventional reactors have stricter limitations because the core would melt if the fuel...
. The nuclear fission reactor core may be either a gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
or plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. They may be capable of creating specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
s of 3,000–5,000 s (30 to 50 kN·s/kg, effective exhaust velocities 30 to 50 km/s) and thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
which is enough for relatively fast interplanetary travel. Heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
to the working fluid
Working fluid
A working fluid is a pressurized gas or liquid that actuates a machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder...
(propellant
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...
) is by thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
, mostly in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, given off by the fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
gas at a working temperature of around 25,000 °C.
Theory of operation
Nuclear gas-core-reactor rockets can provide much higher specific impulse than solid core nuclear rockets because their temperature limitations are in the nozzleNozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
and core wall structural temperatures, which are distanced from the hottest regions of the gas core. Consequently, nuclear gas core reactors can provide much higher temperatures to the propellant
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...
. Solid core nuclear thermal rockets can develop higher specific impulse than conventional chemical rockets due to the extreme power density
Power density
Power density is the amount of power per unit volume....
of the reactor core, but their operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
s are limited by the maximum temperature of the solid core because the reactor's temperatures cannot rise above its components' lowest melting
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
temperature.
Due to the much higher temperatures achievable by the gaseous core design, it can deliver higher specific impulse and thrust than most other conventional nuclear designs. This translates into shorter mission transit times for future astronauts or larger payload fractions. It may also be possible to use partially ionized plasma from the gas core to generate electricity magnetohydrodynamically, subsequently negating the need for an additional power supply.
General features of the nuclear reactor
All gas-core reactor rocket designs share several properties in their nuclear reactor cores, and most designs share the same materials. The closest terrestrial design is the gaseous fission reactorGaseous fission reactor
A gas nuclear reactor is a nuclear reactor in which the nuclear fuel is in a gaseous state rather than liquid or solid. In this type of reactor, the only temperature-limiting materials are the reactor walls. Conventional reactors have stricter limitations because the core would melt if the fuel...
.
Nuclear fuel
The fissileFissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...
fuel is usually highly enriched uranium
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
pellets or a uranium containing gas (U-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
or U-233
Uranium-233
Uranium-233 is a fissile isotope of uranium, bred from Thorium as part of the thorium fuel cycle. It has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years....
). Sometimes uranium tetrafluoride is required due to its chemical stability; the propellant is usually hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
.
Neutron Moderator
Most gas core reactors are surrounded by a radial first wall capable of taking the brunt of the extreme environment present inside the core, a pressure shell to hold everything together, and a radial neutron moderatorNeutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
usually made up of beryllium oxide
Beryllium oxide
Beryllium oxide , also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and actually exceeds that of some metals. As an amorphous solid, beryllium...
. The propellant also provides moderation.
Reactor coolant / Rocket propellant
The hydrogen propellant cools the reactor and its various structural parts. Hydrogen is first pumped through the nozzle, then through the walls and back down through the core region. Once it passes through the core region, the hydrogen is exhausted. If cooling from the propellant is not enough, external radiators are required. The internal gas core temperatures in most designs vary, but the designs with the highest specific impulses generally have fissioning gas plasmas heating a low mass propellant. This heating occurs primarily through radiation.Control
Control can be accomplished by either changing the relative or overall densities of the fissile fuel and the propellant or by having outside control drives moving neutronNeutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
absorbing drums or the radial moderator.
Open cycle versus closed cycle
There are two main variations of the gas core reactor rocket: open cycle designs, which do not contain the fuel within a vessel, and closed cycle designs, which contain the gas reaction core within a solid structure.Open cycle designs
The disadvantage of the open cycle is that the fuel can escape with the working fluid through the nozzle before it reaches significant burn-up levels. Thus, finding a way to limit the loss of fuel is required for open-cycle designs. Unless an outside force is relied upon (i.e. magnetic forces, rocket acceleration), the only way to limit fuel-propellant mixing, is through flow hydrodynamics. Another problem is that the radioactive efflux from the nozzle makes the design totally unsuitable for operation within Earth's atmosphere.The advantage of the open cycle design is that it can attain much higher operating temperatures than the closed cycle design, and does not require the exotic materials needed for a suitable closed cycle design.
Flow hydrodynamics in open cycle designs
The shape of the fissile gas core can be either cylindrical, toroidToroid
Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....
al or counter flow toroidal. Since there are issues regarding the loss of fissile fuel with the cylindrical and toroidal designs, the counter-flow toroidal gas core geometry is the primary source of research. The counter flow toroid is the most promising because it has the best stability and theoretically prevents mixing of the fissile fuel and propellant more effectively than the aforementioned concepts. In this design, the fissile fuel is kept mostly in a base injection stabilized recirculation bubble by hydrodynamic confinement. Most designs utilize a cylindrical gas core wall for ease of modeling. However, previous cold flow tests have shown that hydrodynamic containment is more easily achieved with a spherical internal wall geometry design.
The formation of the fuel vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
is complex. It basically comes down to flow over a projectile shape with a blunt base. The vortex is formed by placing a semi-porous wall in front of the desired location of the fuel vortex but leaves room along its sides for hydrogen propellant. Propellant is then pumped inside the reactor cavity along an annular inlet region. A dead space then develops behind the semi-porous wall; due to viscous and shear
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
forces a counter toroidal rotation develops. Once the vortex develops, fissile fuel can be injected through the semi-porous plate to bring the reactor critical. The formation and location of the fuel vortex now depends on the amount of fissile fuel that bleeds into the system through the semi-porous wall. When more fuel bleeds into the system through the wall, the vortex moves farther downstream. When less bleeds through, the vortex moves farther upstream. Of course, the upstream location is constrained by the placement of the semi-porous wall.
Closed cycle designs
The closed cycle is advantageous because its design virtually eliminates loss of fuel, but the necessity of a physical wall between the fuel and the propellant leads to the obstacle of finding a material with extremely optimized characteristics. One must find a medium that is transparent to a wide range of gamma energies, but can withstand the radiationRadiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
environment present in the reactor, specifically particle bombardment from the nearby fission reactions. This barrage of particles can lead to sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
and eventual wall erosion.
One closed cycle gas core rocket design (often called the nuclear lightbulb
Nuclear lightbulb
A nuclear lightbulb is a hypothetical type of spacecraft engine using a Fission reactor to achieve Nuclear propulsion. Specifically it would be a type of Gas core reactor rocket that separates the nuclear fuel from the coolant/propellant with a quartz wall. It would be operated at such high...
) contains the fissioning gas in a quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
enclosure that is separate from the propellant. First, the hydrogen coolant runs through the nozzle and inside the walls of the quartz enclosure for cooling. Next, the coolant is run along the outside of the quartz fuel enclosure. Since the fissile gas would be directly in contact with the walls, the operating temperature is not as great as other designs because the walls would eventually ablate away.
Magnetic confinement
Barring an external force, hydrodynamic containment is the only way to increase the residence time of the fuel in the reactor. However, one may ask why bar an outside force, could not magnetic confinement be used since the fuel would be highly ionized (three or four times ionized) while the propellant is only partially ionized? To answer this question one must understand a little about magnetic plasma confinement. The key parameter of interest for magnetic confinement is the ratio of kinetic pressure to magnetic pressureMagnetic pressure
Magnetic pressure is an energy density associated with the magnetic field. It is identical to any other physical pressure except that it is carried by the magnetic field rather than kinetic energy of the gas molecules. Interplay between magnetic pressure and ordinary gas pressure is important to...
, β.
When β<1 magnetic confinement is possible (most fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
schemes have a β close to 0.05). However, the pressures in a gas core rocket are much higher than pressures in fusion devices, approximately 1000 atm
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
(100 MPa). For these pressures, the necessary magnetic field strength required is close to 16 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
s just to produce β=1. For a magnetic field of this magnitude
Magnitude (mathematics)
The magnitude of an object in mathematics is its size: a property by which it can be compared as larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which it belongs....
, superconducting technology is necessary and the added mass of such a system would be detrimental. Also, even with a β<1, resistive diffusion will cause the fuel core to collapse almost immediately unless β<<1, which would require an even larger magnetic field.
Impact of rocket acceleration
Another important aspect to GCRs is the impact of the rocket acceleration on the containment of the fuel in the fuel bubble. A rocket acceleration of only 0.001 g (10 mm/s²) will cause buoyancyBuoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
effects to decrease core containment by 35% if all other flow-rates are held constant from a zero g startup. Ultimately, the fuel-propellant flows will have to be throttled until the rocket approaches some sort of steady state.
Neutronic considerations
Since steep temperature gradients will be present in any such gas core reactor, several implications for neutronics must be considered. The open-cycle gas-core reactor (OCGCR) is typically a thermal/epithermal reactor. Most types of OCGCR require external moderation due to the steep temperature gradients inside the gaseous core. Neutrons born in the fuel region travel relatively unimpeded to the external moderator where some are thermalized and sent back into the gas core. Due to the high core temperatures, however, on the return trip the neutrons are up scattered in the fuel region, which leads to a significant negative reactor worth. To achieve criticality, this reactor is operated at very high pressure and the exterior radial wall is made up of a moderator of some sort, generally barium oxide. Moderation can also come from introducing moderating particles into either the fuel or propellant streams, but by doing so, the benefits in neutronics is canceled by loss of rocket performance.Technology summary and outlook
The open-cycle gas-core rocket has many unique design attributes that make it a serious challenger to other proposed propulsion for interplanetary missions. Due to the necessity of having a transparent wall inside the reactor for a closed cycle concept, the benefit of moving to a gas core from a solid core are nearly negated. The high specific impulse and large thrust possible for the OCGCR correspond to shorter mission times and higher payload fractions. However, the technical challenges and unknowns inherent in its design are many. Additionally, any test of the system performed on earth would be under a gravity field of 1 g, which would bring buoyancy effects into play inside the gaseous core.Due to the inability to perform live testing on earth, research is focused primarily on computational modeling of such a system. It was previously mentioned that the specific impulse could be as high as or higher than 3000 s. However, results of computational modeling point towards this number being somewhat optimistic. When thermal hydraulics were modeled more completely for a typical base injection stabilized recirculation bubble gas core rocket by D. Poston, the specific impulse dropped from >3000 s to <1500 s. In the base injection stabilized recirculation bubble gas core rocket concept, it is thought that some additional method of fuel confinement will be beneficial. As mentioned earlier, relying completely on magnetic containment of the fuel bubble is not yet practical. However, a magnetic field may be able to assist in containment or help suppress turbulence that would lead to fuel-propellant mixing.
The primary areas of future research for such an OCGCR would therefore be centered on keeping the fuel and propellant from mixing as much as possible. Although this article has focused on enriched uranium for the fuel and hydrogen for the propellant, this may not be the optimal choice for either. Other fuels, such as plutonium, and other propellants, including helium or even helium-3, have also been considered and in certain situations provide advantages.
See also
- Gaseous fission reactorGaseous fission reactorA gas nuclear reactor is a nuclear reactor in which the nuclear fuel is in a gaseous state rather than liquid or solid. In this type of reactor, the only temperature-limiting materials are the reactor walls. Conventional reactors have stricter limitations because the core would melt if the fuel...
- Spacecraft propulsionSpacecraft propulsionSpacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
- Nuclear lightbulbNuclear lightbulbA nuclear lightbulb is a hypothetical type of spacecraft engine using a Fission reactor to achieve Nuclear propulsion. Specifically it would be a type of Gas core reactor rocket that separates the nuclear fuel from the coolant/propellant with a quartz wall. It would be operated at such high...
- Nuclear materialNuclear materialNuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium , uranium-233, and...
- Atomic physicsAtomic physicsAtomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and...
- Project OrionProject Orion (nuclear propulsion)Project Orion was a study of a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft...
- Nuclear pulse propulsionNuclear pulse propulsionNuclear pulse propulsion is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by DARPA, after a suggestion by Stanislaw Ulam in 1947...
- Cavradyne engine