The Multimegawatt program included consideration of a wide range of power technologies:(1)
"A number of concepts need to be pursued in parallel because different SDI weapons will eventually have different power, energy and other functional requirements. As a result, one system may not be able to meet the demands of all weapons in an optimum fashion. Moreover, if a power system concept was chosen early in the program before the weapon power and energy requirements were reasonably well established, one may end up choosing a concept that did not meet all the weapon requirements or met the requirements only partially.
From the outset, it was recognized that meeting SDI MMW power requirements would entail significant advancements over existing systems:(2)
"A number of technologies need to be pursued in parallel to assure that, in making the order-of-magnitude advances required by the program, at least one path meets with success. Order-of-magnitude advancements are motivated by a need to significantly reduce power system weight, size and deployment costs.
"The recommended temperatures for operations for these [reactor] systems are well above the operating temperatures of existing systems. Higher temperatures of operation combined with requirements of higher burnups and higher neutron fluence levels impose stringent requirements on reactor fuels and materials...
In early 1986, the DOE San Francisco Operations Office funded seven industry team Concept Definition studies, in addition to four on-going DOE National Laboratory studies, all of which were completed by March 1987.
Concept Proponent
Ceramic Core Livermore National Lab
Cermet Argonne National Lab
Cermet Refractory Core General Electric
Foam Core Babcock & Wilcox
Gas Core INSPI
NERVA-Derivative Rockwell
NERVA-Derivative Westinghouse
Particle Bed Reactor Brookhaven National Lab
Pellet Bed Science Applications
RMBLR Rotating MMW Boiling Liquid-metal Reactor Hanford
RMBLR Rotating MMW Boiling Liquid-metal Reactor Battelle
Idaho National Engineering Laboratory (INEL) was the Energy Department executive agent for the Strategic Defense Initiative Multi-Megawatt Nuclear Space Power Program.(3) This effort evaluated four open cycle systems, proposed by Boeing, General Electric, Westinghouse and Grumman, as well as two closed cycle systems, proposed by General Atomics and Rockwell-Rocketdyne. In addition, INEL personnel have been actively involved in the technical panels of the Inter-Agency Nuclear Propulsion Steering Committee. Along with Livermore, Idaho National Engineering Lab has also investigated Fission Fragment Rockets of the type advocated by Sandia.(4)
In early 1988, DOE awarded a total of six $1 million, 10-month MMW Concept Assessment contracts,(5) which included awards to:
Concept Proponent
Open gas-cooled fast reactor Boeing
Open gas-cooled Brayton Graphite Moderator reactor Grumman
Open gas-cooled Brayton Ceramic fuel reactor General Electric
In-core Thermionic liquid metal cooled GA Technologies
Ceramic fuel liquid lithium Rankine fast reactor Rockwell
NERVA-Derivative Westinghouse
Of these, two open cycle burst mode concepts and one closed cycle continuous power concept were selected for Phase II evaluation.
1 - NERVA Derivatives
Westinghouse is the leading proponent of the NERVA Derivative Reactor (NDR), which has been evaluated for SDI multimegawatt power applications.(6) This reactor concept draws on the extensive experience Westinghouse accumulated in the NERVA program in the 1960s. Rejoining the NERVA industrial team of the 1960s, Westinghouse and Rockwell recently signed a memorandum of understanding concerning their joint work on NERVA-Derived Reactors for the Space Exploration Initiative -- as with the initial program, Westinghouse has concentrated on nuclear systems, with Rockwell focusing on system engineering.(7)
A Sandia review of space reactors concluded that:
"The retrieveabilty of the (NERVA) A-6 reactor design was verified by an onsite review of drawings, fabrication procedures, materials certification, etc... The final development of the NERVA derivative reactor will, nonetheless, require several years."
The basic NDR fuel element consists of six hexagonal graphite fuel elements (1.91 cm across the flats) surrounding a central tie tube, which provides structural support as well as a central coolant circulation channel. At least three different fuel element types have been considered for the NERVA-Derived Reactor. The standard NERVA design uses ZrC or NbC coated hexagonal graphite matrix in which UC2 fuel particles are embedded. These TRISO fuel particles are coated with layers of graphite. Alternate higher-performance configurations have a Binary Carbide UC-ZrC fuel dispersed in a coated graphite matrix, and a Ternary Carbide all-UC-ZrC hexagonal fuel elements without the coated graphite matrix. Additional improvements over the original NERVA configuration include addition
of a separate Helium coolant loop for the tie-tubes that would provide both a dual-mode capability and a redundant heat-rejection capability, and a modified fuel end cap, permitting faster temperature ramp-ups.
2 - NERVA/PLUTO
Along with Westinghouse, Livermore is also a proponent of the PLUTO/NERVA Derivative Reactor, which have been evaluated for SDI power applications.(8) This reactor concept combines the experience LLNL accumulated in the PLUTO nuclear ramjet program with the experience Westinghouse accumulated in the NERVA program in the 1960s. The PLUTO/NERVA Derivative reactor uses NERVA fuel (UC2 in graphite) in the PLUTO reactor configuration. This avoids the 2000 K operating temperature limit of the PLUTO fuel elements.
3 - PLUTO Derivative
The PLUTO-Derivative reactor is based on the Tory II-C reactor (which was tested at 500 MW thermal with a coolant outlet temperature of 1450 K), modified to eliminate internal structural elements.(9) The UB2 reactor uses the Pluto geometry with a UB2 fuel in a B4C matrix. Concepts in which the fuel is enriched in B11 to reduce B10 parasitic neutron absorption, as well as hard spectrum regimes which avoid this requirement have been considered.
The PLUTO fuel elements are composed of a homogeneous mixture of BeO moderator and 93% enriched UO2 fuel formed into hexagonal elements 0.683 cm across the flats perforated by a 0.4 cm diameter coolant channel. These 10 cm long fuel elements are stacked into multiple columns which are mounted on springs held against the upper and lower grid plates of the cylindrical reactor core. Reactivity is controlled by burnable poisons which help reduce long-term reactivity changes, as well as a variable leakage reflector to bring the reactor critical and to vary the power level.
4 - Particle Bed Reactors (PBR)
Initial research at Brookhaven National Laboratory was conducted on fluidized bed reactors for propulsion applications during the 1958-1973 period.(10) This included successful demonstrations of cold flow bed dynamics.(11) At least three configurations have been proposed for particle bed reactors:
Annular Reactors, in which the entire reactor's inventory of particles is confined between large hot and cold frits (Figure II-7);
Multi-Element Reactors, in which a number of identical fuel elements are composed of hot and cold frits containing fuel particles. These fuel elements may either be surrounded by a graphite moderator in a thermal reactor (Figure II-8), or be self-supporting in a fast reactor (Figure II-9).
Rotating Bed Reactors in which the reactor core is rapidly rotated, with fuel particles confined by centrifugal force against the cold frit, eliminating the need for a hot frit (Figure II-10).
The Rotating Particle Bed Reactor seeks to achieve improved performance, with exhaust temperatures in excess of 3000 K, and maximum power levels in excess of 5000 MWt. This configuration avoids the clogging and materials stress problems of the Particle Bed Reactor, by eliminating the hot side frit containing the particles. Instead the particle bed is contained in a rapidly rotating cylinder, supported by a cold-side frit, with the motion of the annular flow of the propellant offset by the centrifugal forces of the rotating bed. The rotating bed reactor can be operated fully settled (with all the particles compacted against the outer rotating frit), fully fluidized (with all the particles suspended by propellant gas flow), or in a mixed condition.
One early analysis proposed a non-nuclear Space Power Integrated Demonstration Reactor (SPIDER). The SPIDER demonstration would proceed in several phases, with an initial two year test of the system as a simulated reactor heat source, followed by two more years of power generation tests. Power levels could range from 3 to 10 Mwt with electrical heating, and up to 200 Mwt with a nuclear heat source. This project envisioned three different power reactor configurations:(12)
SPIDER Case 1 Case 2 Case 3
Thermal Power MWt 3 - 200 250 1000 5000
Radius cm 40 15 25 40
Length cm 40 15 50 80
Fuel particle diameter cm 0.05 0.05 0.05
Outlet temperature K 1200 3000 3000 3000
Cavity Pressure bar 100 100 100
Fuel mass Kg 42.2 206 800
A number of technical issues associated with the rotating-bed concept remain unresolved, including:(13)
" the effect of changes in bed geometry to neutronic stability, the confinement of fuel before startup and during loss of rotation, the loss of power system caused by the loss of a single reactor bearing, the startup torques and the steep power gradients caused by the external moderator".
Even proponents concede that the Rotating Bed Reactors(14)
"... are more complex mechanically than FBRs owing to the need for bearings and labyrinth seals... Development requirements are substantially greater than for the RBR than for the FBR. The fluid dynamics of volume heated fluidized beds has not been adequately explored, and fuel particle behavior in very high temperature H2 needs to be further investigated."
Although this reactor concept is not currently under consideration for nuclear thermal propulsion applications, the Rotating MMW Boiling Liquid Reactor (RMBLR), which uses a potassium working fluid, is under consideration as an electrical power source for the nuclear electric propulsion program.
Oak Ridge National Laboratory (ORNL) also has extensive experience with coated fuel particles under the High Temperature Gas Reactor program.(15) This experience has identified a variety of factors limiting the performance of these fuel particles. Massive fission product release from a particle may result from tensile stress on the coating caused by internal fission product gas pressure, and consequential rupture of the coating. The thermal gradient within the particle can lead to carbon transport and eventual migration of the Uranium fuel element into contact with the outer SiC coating, which will degrade the coating leading to fission product release. And fast neutron flux may lead to differential expansion and contraction of the pyrocarbon layer, leading to rupture of the coating. Testing demonstrates that TRISO particles generally demonstrate fission product fraction releases from 10-7 to less than 10-5 at temperatures below 1,900 K, but that release fractions rapidly approach 1% as temperatures exceed 2,100 K.
General Atomics (formerly GA Technologies) has fabricated in excess of 10,000,000,000,000 coated fuel particles, including those for the 40 MWe Peach Bottom electrical power reactor, and the 300 MWe Fort. St. Vrain electrical power reactor.(16) These particles consist of multiple layers of coatings surrounding the inner fuel particle, and are of two types:
BISO 1 - isotropic pyrocarbon outer layer
2 - low density porous carbon buffer inner layer
TRISO 1 - isotropic pyrocarbon outer layer
2 - SiC Silicon Carbide
3 - isotropic carbon
4 - porous carbon buffer inner layer
The BISO particles are suitable for applications with low internal pressure of gaseous fission products, while the TRISO particles are required for the higher internal gas pressures of higher fuel burnups (up to 50%). The multiple layers of these particles improve fission product retention, approaching 99.99%. The inner porous carbon layer protects the
outer layer from fission fragment recoil, as well as providing a void space for fission gases. The additional SiC layer in TRISO provides an additional barrier to metallic fission products that would migrate through the pyrocarbon layer, such as barium, strontium and cesium.
These fuel particles were designed for use in closed-cycle power reactors with inert-gas coolants (such as helium). Consequently, they are not suitable for use in open-cycle systems with hydrogen coolant or propellant, since their carbon coating would quickly react with the hot hydrogen, leading to rapid particle erosion. Thus the 500 micron particles used with hydrogen coolants would require an additional zirconium carbide coating to provide protection from hydrogen erosion.(17)
Brookhaven National Laboratory, in conjunction with Babcock and Wilcox, Grumman, and Garrett Corporation, undertook a study of PBR applications for SDI burst-mode multimegawatt power requirements from October 1985 through December 1986.(18)
The study noted:(19)
"Rhenium and ZrC can be used in hydrogen up to at least 2000 K... However, SiC coated particles should not directly contact rhenium at temperatures above 1500 K because of rhenium silicide formation."
According to this analysis:(20)
"The area available for heat transfer in the reactor fuel element is extremely large. (Approximately 7000 m2/m3 of bed) This large heat transfer area insures that the film temperature drop from the particle surface to the coolant gas is small (approximately 50 C - 100 C). Furthermore, since the dimension of the fuel particles are small (diameter 500 microns), the maximum fuel temperature is only marginally higher than the gas outlet temperature.... due to the particulate nature of the fuel, severe thermal gradients can be tolerated across the fuel element without fuel failure due to thermal stresses. This makes it potentially possible to start the reactor very rapidly....
"The issue of clogging of frits if fines are generated in the flow circuit has been raised... It is concluded that frits can be designed and filters incorporated that will prevent clogging, even if substantial amounts of fines are generated."
This analysis considered both open and closed cycle systems, using a fast reactor for 10 MWe Alert Mode power and a moderated reactor for 400 MWe Burst-Mode power.
5 - Pellet Bed
Science Applications International (SAIC) is a leading proponent of the Pellet-Bed Reactor, which has been evaluated for SDI power applications.(21) The Pellet Bed Reactor is similar to the Particle Bed Reactor, with the principle difference being the much larger Pellet Bed Reactor fuel elements.(22) The fuel-element pellets are contained between porous cylindrical screens referred to as frits. The reactor is controlled with rotating drums in the outer reflector. Hydrogen propellant flow moves in an inward radial direction, with a nominal exhaust temperature of 3,000 K. The pellets consist of fuel microspheres suspended in a graphite matrix coated in ZrC. Each microsphere consists of a UC-TaC or UC-NbC kernel coated in concentric layers of Pyrolytic Carbon and High-Density Carbon, with an outer coating of TaC or NbC.
This reactor concept uses 93% enriched UC2-coated spherical nuclear fuel pellets that may range from 0.5 to 2.0 cm in diameter, with typical diameters of about 1.0 cm, embedded in a graphite matrix. The fuel pellets are contained (without frits or other internal structure) in a cylindrical containment vessel, fabricated of refractory materials, with perforated end plates for coolant circulation. Rotatable control drums made of Beryllium or BeO with B4C neutron-absorber strips are mounted in the surrounding reflector.
Several possible circulation schemes have been considered for the reactor type. Earliest consideration was given to non-circulating fuel designs in which the propellant enters one end of the pressure vessel and exits the opposite end. More recent studies have focused on radial propellant flow, as well as circulating fuel particle flow.
A number of advantages of the Pellet Bed Reactor have been identified:
+ The concept is a modular system with high specific power levels, as well as high specific impulse and high thrust. The reactor has a high heat capacity and passive decay heat removal by radial conduction and radiation into space.
+ Low development cost and risk is made possible by full use of the available technology base.
+ Can be used in both pulsed and continuous modes, and in conjunction with dynamic conversion systems (Stirling, K-Rankine, or Direct Brayton) for NEP and dual-mode operations.
+ Safety features include subcritical water immersion and compaction, as well as two independent control systems, consisting of control drums and in-core safety rod drivers. The fuel pellet design provides a low thermal gradient and structural integrity to maintain containment of fission products. Long lifetime permits in-orbit refueling for mission reuse.
Major unresolved issues include:
- Development of reliable high temperature (above 3,000 K) fuel. Coating material choices are constrained by requirements for thermal expansion coefficients similar to that of the fuel, high thermal conductivity, and the ability to accommodate stress due to fission product build-up. Experimental data suggests that at temperatures above 2,000 K, Uranium will migrate from the UC fuel into ZrC layer, leading to the coating's failure. This migration is strongly dependent on operating temperature.
- Development of high temperature fuels, compatible with hydrogen, including a carbon/carbon turbopump, which pushes the state of the art.
6 - Cermet
General Electric (GE) is the leading proponent for the Cermet Reactor, which has been evaluated for SDI applications.(23) This concept was initially developed as part of the Aircraft Nuclear Propulsion Program (ANP) which was terminated in 1962, and subsequently developed under the 710 High Temperature Gas Reactor system developed in the 1960's. The 710 program, which was conducted in conjunction with Argonne National Lab from 1962 through 1968 in support of both propulsion and power generation, envisioned a Neon-cooled 210 kWe fast reactor fueled with UO2 in Tungsten operating at a temperature of between 1445 K and 1800 K. Some hardware was fabricated, although a full-scale reactor was never assembled.(24)
The principal features of the Cermet reactor are hexagonal refractory metal ceramic UO2/W fuel elements with multiple tubular propellant flow channels, supported by a refractory metal matrix.(25) The Mo-cermet has been considered as an alternate matrix, although it is incompatible with UN, which has been considered as an alternate fuel. From 20% to 40% of the fuel element is composed of the refractory metal. This fast neutron spectrum reactor includes radial BeO reflectors, and BeO and molybdenum end reflectors, with reactor control provided by rotatable Be drums in the radial reflector.
7 - RMBLR Rotating MMW Boiling Liquid-metal Reactor
The Rotating MMW Boiling Liquid-metal Reactor (RMBLR) concept has been evaluated by Hanford National Laboratory, as well as Batelle Laboratories. The fast cermet-fuel uses uranium nitride molybdnum fuel blocks with coolant channels. The reactor is cooled with a boiling liquid metal (potassium) which is vaporized in the core. An inward radial flow of liquid potassium is maintained by the rotation of the reactor in order to reduce thermal
stress on reactor components. The potassium vapor, with an outlet temperature of 1440 K, is passed through a Rankine conversion cycle, eliminating the need for a secondary cooling loop.
Principal advantages of this concept include:
+ Attractive thermal and mechanical properties of the cermet fuel;
+ Reduced reactor operating temperature due to radial coolant inflow;
+ Relatively low specific mass, due to the elimination of a separator system and a secondary coolant loop;
+ Turbine design compatible with low quality vapor.
Although this concept is based on proven cermet fuel element technology, additional development efforts required include thermal and hydraulic evaluation of the boiling core, modification of existing turbine designs to accomdate potassium coolant, and design of the thermal rejection system.(26)
8 - THOR
General Atomics has proposed an innovative thermionic reactor concept, designated THOR (Figure II-14) in which a very large number of thermionic fuel elements are coupled to individual heat sink canisters arranged on panels. The reactor is brought to criticality by folding the panels together to form a 9 meter cube. Following this power burst, the panels are unfolded to a flat array to dissipated the stored heat.
9 - Wire Core
Rockwell is a proponent for the Wire Core Reactor, which has been evaluated for SDI power applications,(27) although no major developmental work has been conducted. This reactor concept was originally developed in connection with the Aircraft Nuclear Propulsion program.
The reactor consists of continuous Tungsten-Rhenium clad UN fuel wires, 0.05 to 0.25 cm in diameter, shaped into a number of concentric annular fuel assemblies. The small wire diameter reduces maximum fuel temperature, and lower operating temperatures may permit use of UO2 fuel clad in Nichrome V. Alternating layers of fuel wires are separated by tension-stressed unfueled spacer wires to maintain fuel element spacing and facilitate propellant flow. Propellant flow may either be axial or radial. This fast spectrum reactor includes Beryllium reflector surrounding the pressure vessel, as well as a central axial control rod with Beryllium reflector and poison sections, the position of which is used to control reactor power levels.
A number of advantages have been identified for the Wire Core reactor:
+ The Wire Core provides nearly five time the heat transfer area of the NERVA-derived concept, with 19 cm2/cm3 compared to 4 cm2/cm3.
+ The radial flow divergence of the hydrogen propellant moves heat transfer to the outer hotter wires, and cancels gas pressure loads.
+ The separation of fuel and structure eases structural design and fabrication, since the Wire Core uses the wire cladding for structural support, rather than the fuel elements.
+ The short heat path in the wire fuel element reduces temperature differences between the center and surface of the fuel element.
+ The UN fuel, Tungsten cladding, and H2 propellant are compatible at the reactors elevated operating temperatures.The metallic construction is inherently resistent to thermal shock, providing attractive restart capabilities.
10 - Foam Fuel
Babcock & Wilcox is a proponent of the Foam-Fuel Reactor, which has been evaluated for SDI power applications.(28) This reactor concept is currently poorly defined, and little relevant hardware development has been conducted.(29) This reactor is similar in concept to the Particle Bed Reactor, with the propellant passing through a porous graphite and ZrC coated foam containing UC2 filling a pressure vessel. This concept would offer further reductions in system mass over the Particle Bed Reactor by further increasing the heat transfer surface area.
11 - Gas Core Reactors
The University of Florida Innovative Nuclear Space Power Institute (INSPI), Gainesville, Florida, was chartered and sponsored by the SDI Organization Innovative Science and Technology office in September 1985, as a consortium of university and industrial research on advanced nuclear power concepts. Institutions working on the SDI Multi-Megawatt (MMW) gas core reactor project include:
AVCO California State University, Long Beach
GA Technologies University of California, Los Angeles
Maxwell University of Florida, Gainesville
Pacific Sierra
RTS Inc. Los Alamos National Lab
Space Power Inc.
SRI International
The primary concept developed in this effort is a UF4/KF vapor core MHD system with a close Rankine cycle for electrical power generation. Major conclusions from these studies include:
+ UF4 is the preferred fuel for temperatures below 5,000 K, while Uranium metal droplets is an option at temperatures between 3,000 K and 7,000 K, with Uranium vapor appropriate for temperatures above 6,000 K.
+ W, Mo, Re, C and their alloys and carbides are compatible with UF4 at temperatures above 1,800 K.
+ The neutronic stability of externally moderated Gas Core Reactors increases as the fuel density distribution approached the cavity wall. For stable centrally-peaked distributions, the volume of the fuel must exceed 85% of the volume of the core.
Parameters of the Vapor Core Reactor studied for SDI MMW power requirements include:(30)
Parameter Unit Value Reactor Power MWe 200 Average Core Temperature K 3000 Core Pressure atmospheres 50
The gas-core reactor can be coupled with an MHD conversion device to improve overall system efficiency (Figure II-18).(31)
Advantages of the Gas Core reactor include:
+ Power density is not limited by thermal mechanical and fluid flow constraints of fuel or structural components.
+ Vapor fuel does not require fuel fabrication, testing and verification.
+ Inherent hot spot equalization and inherent stability due to expanding fuel.
+ High fuel utilization, with fuel burnup rates of about 200,000 MWD/MT.
Major problems identified with the Gas Core reactor include:
- Efficient fuel confinement, containment, and recirculation are major challenges.
- Reliability of reactor materials in contact with UF4 will require further analysis.
- Problems with fuel recirculation systems include hazards from inadvertent fuel criticality outside the core, additional shielding requirements, and a range of component materials limitations.
- Major safety issues include reactor transient response times, fuel plateout, and recirculation tube rupture hazards.
12 - Fusion
Advanced fusion concepts have also been considered for potential application to far-term SDI power requirements. Notable among these is the Space Orbiting Advanced Fusion Reactor (SOAR).(32) This design is intended to provide burst-mode power of up to 1,000 MWe for 600 seconds. The reactor is based on an advanced Deuterium-Helium-3 fusion cycle, which liberates about 96% of its energy in the form of charged particles which are electrostatically converted into electrical power at high efficiencies (78%). The use of this fusion cycle greatly reduces (two orders of magnitude) the neutron flux to which the spacecraft and payload would be subjected by conventional Deuterium-Tritium fusion reactions. A number of attractive features of this approach have been identified:(33)
"Sufficient terrestrial 3He reserves exist for the use of "low neutron" D-3He fuel,
"Radiation shielding mass is reduced,
"Radiators can be eliminated by dissipating waste heat adiabatically in the shield,
"Highly efficient direct electrostatic conversion of energy to electricity is possible by the use of low mass direct converters,
"No "criticality" potential exists during a launch phase accident,
"No radioactivity is present until operation,
"Low radioactivity and afterheat levels are induced during operation and only short half life isotopes remain for waste disposal,
"Vacuum pumping and cryogenic cooling systems are reduced in mass and complexity, and
"Response time from cold start is rapid ( 10 s)."
Despite these advantages, such fusion reactors will require several decades of development. Research on the tandem mirror magnetic confinement systems needed for reactors using this fusion cycle were discontinued over a decade ago, when the fusion energy research community decided to concentrate its efforts on D-T Tokomak reactor research. The D-3He fusion reaction has ignition temperature and confinement parameters that are significantly more challenging that those for D-T reactions. However, the D-T cycle results in copious production of neutrons, which makes it unattractive for propulsion applications.
13 - Non-Nuclear Concepts
In addition to these nuclear power systems, chemical energy sources have also been evaluated for SDI multimegawatt power requirements. These include both static fuel cell systems, as well as dynamic chemical combustion technologies.
i - Fuel Cells
Fuel cells have been a primary source of power for American piloted space missions for nearly three decades. Fuel cells have also been evaluated for application to SDI multimegawatt power requirements (Figure II-20).(34) One analysis concluded that for space-based directed and kinetic energy weapons, fuel cells:(35)
"... offer three significant benefits to the weapons missions under consideration:
"(1) Static power conversion for enhanced platform stability;
"(2) Modularity that allows the fuel cell to be integrated directly with the weapon;
"(3) System flexibility that permits the fuel cell to be compatible with a wide range of system cooling options as required by the mission."
This study observed that the fuel cell has:
"... inherently minimal torques, vibrations, and thrusts. These characteristics are a benefit for inherent platform stability, an essential consideration in meeting the stringent pointing and tracking requirements for the SDI weapons platforms, particularly the FEL platform."
This analysis further noted:
"The modularity of the fuel cell provides the platform designer with exceptional latitude in integrating the fuel cell power subsystem within the weapon platform. The modular nature of the fuel cell permits the platform designer to either mass the power subsystem in a single area on the platform, or to distribute the fuel cells to best advantage along the platform... This distributed concept provides inherent redundancy at the system level and lead to graceful degradation of weapon performance (as opposed to single point failure)... Secondary benefits of a modular design include ease of module replacement and flexibility for the platform designer in accommodating changing weapons power requirements by varying the number of power modules. A programmatic advantage of the small fuel cell module size is that the small modules reduce the time, money, and risk for development, scale-up, and demonstration of the technology."
Additional operational advantages of fuel cells include the extensive space technology experience base, high theoretical energy storage densities, and the ease of closed cycle operations. The low temperatures and pressures of fuel cell operations minimize material and structural concerns.
ii - Chemical Combustion
Chemical combustion systems typically consist of a rocket motor combustion chamber in which the exhaust is passed either through turbogenerator machinery or an MHD converter. The primary advantages of this approach include:
+ Low system mass;
+ Fast start capability, typically a few seconds;
+ Modular arrangement permits design flexibility and redundancy;
+ Low turbine inlet temperatures for turbomachinery conversion systems;
+ Hydrogen fuel may be provided as waste from the payload thermal management system;
+ For hydrogen-fueled systems, the only effluent is H2, which minimizes contamination concerns.
Drawbacks of combustion systems include:
- Limited capability for on-orbit testing, and the need for propellant resupply to support testing;
- Momentum canceling is required to compensate for turbomachinery torque and vibration;
- Thrust nulling is required to compensate for exhaust thrust vectors.
- The requirement to scrub exhaust effluent of water vapor to reduce impact on spacecraft payload operations is a particularly stressing challenge.
- MHD conversion systems will include contaminants such as Cesium which may have significant impacts on payload operations.
Nonetheless, the advantages of this approach render it a strong candidate for meeting SDI multimegawatt power requirements.
1. Department of Energy, Office of Defense Energy Projects and Special Applications, Strategic Defense Initiative Multimegawatt Space Nuclear Power Program - Summary, April 1986, page 14.
2. ibid, pages 14, 10.
3. "Space Power," Military Space, 19 June 1989, page 4.
4. Chapline, George, et al, "Fission Fragment Rockets -- A New Frontier," 50 Years With Nuclear Fission, National Academy of Sciences and National Institute of Standards & Technology conference, 25-28 April 1989, (American Nuclear Society, La Grange Park, IL, 1990), pages 601-605.
5. "Space Nuclear Power," Military Space, 15 February 1988, page 8.
6. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989).
7. "Rockwell/Westinghouse Team for SEI Nuclear Thermal Propulsion," Defense Daily, 27 September 1990, page 494.
8. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989).
9. Reynolds, H.L., Tory II C Reactor Test Report, UCRL 12069;
Walter, C.E., Technology Development Plan for Multi-Megawatt Space Power Systems,
Lawrence Livermore National Laboratory, UCID-20883, October 1986;
Walter, C.E., et al, Gas-Cooled Reactor Power Systems for Space -- Concept Definition Study Final Report, Lawrence Livermore National Laboratory, 27 March 1987.
10. Hatch, L.P. et al, "Fluidized Solids as Nuclear Fuel for Rocket Propulsion," ARS (American Rocket Society) Journal, vol. 31, num. 4, April 1961, pages 547-548.
11. Boudreau, J.E., and Buden, D., "A New Generation of Reactors for Space Power," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983).
12. Powell, J.R., and Botts, T.E., "Particle-Bed Reactors and Related Concepts," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983).
13. Department of Energy, Office of Defense Energy Projects and Special Applications, Strategic Defense Initiative Multimegawatt Space Nuclear Power Program - Summary, April 1986.
14. Powell, J.R., and Botts, T.E., "Particle-Bed Reactors and Related Concepts," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983).
15. Kania, M.J. "Coated Particle Fuel Performance (Modular HTGR Fuels)," NASA Nuclear Propulsion Workshop, NASA Lewis Research Center, Cleveland OH, July 10-12, 1990.
16. Snyder, H.J., "Some High-Temperature Reactor Technologies," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983).
17. Department of Energy, Office of Defense Energy Projects and Special Applications, Strategic Defense Initiative Multimegawatt Space Nuclear Power Program - Summary, April 1986, page 24.
18. Brookhaven National Laboratory, Particle Bed Reactor Multimegawatt Concepts, BNL-39495, March 1987.
19. ibid, page 7.
20. ibid, pages 1-4 - 1-6, 5.
21. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989).
22. Buden, David, "A Pellet Bed Reactor for Multi-Modal Space Power," Transactions of the Third Symposium on Space Nuclear Power Systems, Albuquerque, NM, 13-16 January 1986, pages MM-2.1 - 3.
23. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989).
24. Boudreau, J.E., and Buden, D., "A New Generation of Reactors for Space Power," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983).
25. General Electric Corporation, A Bimodal Cermet Fueled Refractory Metal Reactor for MMW Applications, 15 October 1986;
General Electric Corporation, 710 High Temperature Gas Reactor Program Summary Report, Volume I through Volume V Summary, GEMP-600;
Cowan, C.L., et al, A Bimodal, Cermet Fueled, Nuclear Power System for Strategic Defense Applications Final Report, Volume 1 Executive Summary, Volume 2 Technical Presentation, General Electric Company, GEFR-00803, March 1987.
26. Barnett, John, "Nuclear Electric Propulsion Technologies: Overview of the NASA/DOE/DOD Nuclear Electric Propulsion Workshop," Proceedings of the Eighth Symposium on Space Nuclear Power Systems, Albuquerque, NM, 1991, pages511-523.
27. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989).
28. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989).
29. Weatherford, L.G., "Ultra-High Temperature Gas-Cooled Reactor With Porous Refractory Foam Fuel," (Letter from Babcock & Wilcox to US DOE San Francisco Operations Office), April 1986;
Short, B.J., Ultra High Temperature Gas Cooled Reactor with Porous Refractory Foam Fuel, Preliminary Feasibility Assessment Report, Phase I, (Babcock and Wilcox), February 1987.
30. Paniker, Matthew, et al, "Static and Dynamic Neutronics Analysis of a Bi-Modal Gaseous Core Reactor System for Space Power," Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, Reno, NV, 12-17 August 1990, volume 1, pages 150-155.
Kahook, Samer, et al, "Neutronic Analysis of the Uranium Tetra-Fluoride, Ultrahigh Temperature Vapor Core Reactor System," Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, Reno, NV, 12-17 August 1990, volume 1, pages 156-161.
Maya, Isaac, et al, "Graphic System Code Analysis of the Ultrahigh Temperature Vapor Core Reactor Nuclear Space Power System," Proceedings of the 25 Intersociety Energy Conversion Engineering Conference, Reno, NV, 12-17 August 1990, volume 1, pages 168-172.
31. Rhee, Hyop, et al, "Gigawatt, Closed Cycle, Vapor-Core-MHD Space Power System," chapter 23 in El-Genk, M.S., editor, Space Nuclear Power Systems 1988, (Malabar, FL, Orbit Book Company, 1989), pages 139-146.
32. Santarius, J.F., et al, "Critical Issues for SOAR: The Space Orbiting Advanced Fusion Power Reactor," chapter 19 in El-Genk, M.S., editor, Space Nuclear Power Systems 1987, (Malabar, FL, Orbit Book Company, 1988), pages 167-176.
Santarius, J.F., et al, "Critical Issues for SOAR: The Space Orbiting Advanced Fusion Power Reactor," chapter 26 in El-Genk, M.S., editor, Space Nuclear Power Systems 1988, (Malabar, FL, Orbit Book Company, 1989), pages 161-167.
33. ibid, 1987, page 167.
34. Verga, Richard, and Buden, David, "Five Years of SDIO Power Development Progress," Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, Reno, NV, 12-17 August 1990, volume 1, pages 6-12.
35. Martin Marietta Astronautics Group, Space Power Architecture Study (SPAS), Final Report, Report Number TR-87-16, June 1988, page 277.