With the demise of the Clinch River Liquid-Metal Fast Breeder Reactor (LMFBR) project in the closing days of 1982,(1) the American community of nuclear engineers specializing in exotic high-temperature reactors was confronted with the prospective absence of a major project to captivate its attention for the first time in nearly three decades. The end of continuous procession from nuclear aircraft through nuclear rockets to breeder reactors was in sight. Although low-level studies of space power systems had continued through the 1970s, no major development projects were in progress at the time of Clinch River's demise.
However, the end of the Clinch River project was not entirely unanticipated, and efforts were already under way to resuscitate the space nuclear power program that had been terminated in 1972-73. The immediate focus of these efforts was the SP-100, a 100 kWe space nuclear reactor, which became the centerpiece of a joint program of NASA and the Energy and Defense Departments.(2) A liquid metal reactor like Clinch River, the SP-100 substituted lithium for sodium as the primary reactor coolant, reviving many of the concepts previously studied in the SNAP-50 and SPUR projects.
The demise of Clinch River and the renaissance of the SP-100 also occasioned a broad-ranging review of the status of space nuclear power systems at a conference sponsored by the National Academy of Sciences in November 1982.(3) Although this symposium identified a number of reactor concepts that could support power requirements significantly in excess of those associated with the SP-100, such requirements were not in evidence at that time.
A report prepared by a National Research Council panel, based on this conference, concluded the future of space nuclear power was clouded by the fact that:(4)
"... firm requirements do not exist and that one should not expect to discover such requirements as a result of this study... users should recognize that the desired capability will not be forthcoming unless they are more supportive of these initial research and development efforts...
"The important point to remember about space nuclear power is that the time scale for the development is long, 7 - 10 years or perhaps longer for systems in the 100-kW(e) range and more than ten years for higher-powered systems. This time scale is comparable to or longer than the time scale on which requirements may emerge. If begun today, there is the time to carry out a responsible technology program; if delayed, subsequent development efforts may well prove more costly and hurried, with a resulting adverse effect on quality and public acceptability.
"Alternatively, future space programs may be constrained to available nonnuclear power sources, with consequent penalties in mission performance and cost. The benefits of avoiding these penalties could far exceed the costs of the carefully paced research and development program the Committee recommends."
This situation soon changed with the announcement by President Reagan on 23 March 1983 of the Strategic Defense Initiative (SDI). The early version of the SDI program envisioned nearly perfect defenses against very large missile attacks, which would require highly-capable space-based intercept systems. Many of the weapons concepts required very large electrical power levels, and space nuclear reactors were a leading candidate to meet these requirements.
Thus was born the SDIO's Multi-Megawatt (MMW) space power program, which flourished through the end of the decade.
The application of nuclear power systems to the strategic defense was predicated on a number of assumptions which were increasingly subject to question as the program matured:
+ Although some space-based directed and kinetic energy weapons concepts incorporated significant electrical power requirements, other equally attractive space-based directed and kinetic energy weapons concepts did not.
+ While nuclear power sources were capable of being used in conjunction with closed-cycle conversion systems, the large radiators these systems required made open-cycle chemical power systems a more attractive option for electrical power generation (as long as it was assumed that the effluents from open cycle systems would not significantly impact payload operations).
+ It was originally assumed that the goal of the SDI program was a highly robust and extremely (virtually perfect) defense against a large threat, which required very competent weapons systems. As the goals of the program progressively evolved toward more modest ambitions, the requirements for highly capable weapons diminished. Thus the initial focus on space-based directed energy weapons gradually shifted toward interest in ground-based kinetic energy weapons.
Thus the initial enthusiasm for space nuclear power sources was rapidly tempered by uncertain requirements, and the broad range of more attractive competing technical approaches. The MMW program finally succumbed to the same forces that undermined Reagan's initial vision of SDI.
Although the MMW program was ultimately unproductive, it made several lasting contributions to the development of space nuclear power:
The MMW program provided the first opportunity in nearly 15 years to review the status of high-power compact nuclear reactors, reviving interest in concepts that had long lain dormant.
The MMW program established the basis for subsequent work on space nuclear propulsion for the Space Exploration Initiative, which was announced simultaneously with the demise of plans for deployment of large-scale anti-missile systems.
The MMW program was a major factor in the genesis of the Timberwind particle bed reactor system, which was the one major hardware development conducted under the MMW effort.(5)
Indeed, the difficulties faced by proponents of the particle bed reactor in gaining funding support under the MMW program in the face of competition from the more firmly established proponents of the NERVA reactor, not to mention all the previously discussed hurdles faced by the Multimegawatt program more generally, may have contributed to the decision to establish Timberwind as a special access program. This high level of classification provided a measure of insulation for Timberwind from outside examination and criticism. It is probably not coincidental that the Timberwind special access program was established within months of the release of a draft report reviewing the MMW program which was highly critical of the particle-bed reactor concept.
While the full details remain shrouded in secrecy, there is at least some circumstantial evidence that the SDI Multi-Megawatt power program provided both the general impetus for the Timberwind program, as well as influencing the particular decision to conduct this program at a Special Access Required level of classification.
A - PROGRAM BACKGROUND
During most of the 1980s the SDIO's Multi-Megawatt Space Nuclear Power Program was the major focus for work on the high power open and closed cycle reactor concepts. The work on nuclear power systems conducted by the Energy Department under this program was complemented by Air Force investigations of non-nuclear power concepts.
The initial focus of the Multimegawatt program was:(6)
"to identify and develop at least one space nuclear power system concept by 1991 that alone, or in combination with a non-nuclear power system, meets SDI MMW power requirements, and for which technical feasibility issues have been resolved."
Initiated in October 1985, the Program planned a two year Concept Definition phase, leading to a Phase I selection of two or three reactor concepts for more intensive study by October 1987. These Phase I activities included resolution of out-of-reactor fuel compatibility issues, demonstration of fabrication of geometry-specific fuel elements, and evaluation of irradiation tests. This work was to form the basis for Phase II of the Program, selection of a single concept for hardware development, initially slated for October 1991.
SDI power requirements consisted of three different modes (Figure II-1):
Housekeeping (Continuous) Mode, to provide support for operations of space based SDI sensors and weapons platform support systems, including communications and stationkeeping. These would have power requirements that would range from a few kilowatts to a few tens of kilowatts, with requirements for hundreds of kilowatts for large cryogenic infrared sensor systems for up to a decade. These requirements were initially established as extending from hundreds of kilowatts up to a few megawatts for up to seven years.
Alert Mode operations would commence in response to indications and warnings of impending combat operations. Along with peacetime testing requirements, alert mode power levels would have to be maintained for periods of up to one year. Alert mode power levels could range from 100 kWe to 10 MWe, although initial estimates suggested that Alert Mode power levels could range up to 20 megawatts over this period. Prior to May 1986, the Alert mode power was required to be supplied over the entire seven to ten year life of the spacecraft.(7)
Burst Mode operations would extend for hundreds or a few thousand seconds, with tens to thousands of megawatts of electrical power being required to support space-based directed and kinetic energy weapons, and associated fire control systems. Initial estimates for burst mode power requirements ranged from 100 to 1000 MWe for 200 to 2000 seconds.
Definition of Housekeeping Mode requirements were the most firmly established of these three modes, with the primary uncertainty relating to potentially high power needs of large cryogenic infrared sensor systems.
As for Alert Mode requirements, one analysis noted:(8)
"... among the three SDI power modes, the alert-mode requirement is the least clearly defined. The total duration of power needs while in an alert status or during periodic testing might even be a year or more. The power level and duration required for the SDI alert mode appear to depend on a postulated operational cycle that is not easily defined, and may also include power for periodic status-checking."
Burst mode power requirements are driven by the needs of space-based directed and kinetic energy weapons, including excimer and free-electron lasers, neutral particle beam weapons, and space-based electromagnetic launchers. According to one study:(9)
"The choice between chemical and nuclear space power systems depends in large part on the total duration during which power must be provided. On the basis of mass-effectiveness, large durations favor the nuclear reactor space power systems and short durations favor chemical power systems if their effluents can be tolerated. For alert mode requirements at the low-power end of the requirement range stated above, a solar space power system might qualify."
The Program Plan noted:(10)
"Nuclear systems are the only realistic option to provide steady-state (continuous and alert) power needs. Nuclear systems may also compete favorably with chemical power sources for burst power applications, especially for those requiring bursts of more than a few hundred seconds. Potential advantages of nuclear systems include lower mass and volume; enhanced survivability, increased reliability and flexibility, and lower life cycle-costs. Nuclear systems could interface with advanced surveillance and weapon systems without adversely affecting the operational environments for these systems....
"Chemical systems may be able to satisfy SDI needs for high power/short-duration (up to 200-300 seconds) applications, depending on the length of the burst. However, the mass and the size of chemical systems become increasingly large for durations above 200-300 seconds and for high power operations."
Thus the primary focus of the MMW program was the analysis of open and closed cycle systems, and nuclear and chemical power sources. Gas-cooled reactors relative to liquid-metal cooled reactors for SDI MMW electrical power applications included larger and heavier reactors and radiators, but this was offset by the ease with which gas-cooled reactors could use open-cycle cooling. However, open-cycle chemical power systems were generally judged more attractive, subject to the compatibility of exhaust effluents with payload operations. A range of technological alternatives were considered for meeting these requirements (Figure II-2). A similarly broad set of choices was available for nuclear power systems responding to the MMW requirements (Figure II-3). One of the greatest challenges of nuclear power systems remained the high operating temperatures that had bedeviled previous nuclear aerospace propulsion programs (Figure II-4).
1. Miller, Judith, "Critics of a Breeder Reactor Defeat Funds in House Vote," The New York Times, 15 December 1982.
2. The relationship between the Clinch River LMFBR and the SP-100 was noted in the testimony of William A. Harms, Director of Nuclear Reactor Technology Programs at DOE's Oak Ridge National Laboratory in US House of Representatives Committee on Science and Technology, The Space Nuclear Reactor Program, 98th Congress, 1st Session, 24 May 1983, page 30.
3. National Research Council Energy Engineering Board Committee on Advanced Nuclear Systems, Advanced Compact Reactor Systems - Proceedings of a Symposium, 15-17 November 1982, (Washington, National Academy of Sciences Press, 1983).
4. National Research Council, Energy Engineering Board, Committee on Advanced Nuclear Systems, Advanced Nuclear Systems for Portable Power in Space, (Washington, DC, National Academy Press, 1983), pages 5-9.
5. Prior to 1991 there was little public indication of existence of this program. Although program participants frequently published unclassified papers discussing tests conducted under the program, there was no indication of the sponsorship of these tests. The passing reference to "a classified MMW nuclear project," in: General Accounting Office, Nuclear Science -- Challenges Facing Space Reactor Power Systems Development, December 1987, GAO/RCED-88-23, page 35, referred to the Centaurus fission fragment laser system.
6. Department of Energy, Office of Defense Energy Projects and Special Applications, Strategic Defense Initiative Multimegawatt Space Nuclear Power Program - Summary, April 1986, page 1.
7. Brookhaven National Laboratory, Particle Bed Reactor Multimegawatt Concepts, BNL-39495, March 1987, page 1-1.
8. National Research Council, Energy Engineering Board, Committee on Advanced Space Based High Power Technologies, Advanced Power Sources for Space Missions, (Washington, National Academy Press, 1989), page 11.
9. National Research Council, Energy Engineering Board, Committee on Advanced Space Based High Power Technologies, Advanced Power Sources for Space Missions, (Washington, National Academy Press, 1989), page 2.
10. Department of Energy, Office of Defense Energy Projects and Special Applications, Strategic Defense Initiative Multimegawatt Space Nuclear Power Program - Summary, April 1986, page 2.