Each concept was also evaluated by the Technology Capabilities Panel for mission benefit, technical risk, cost and safety.(1) Each concept was ranked on a scale of 1 to 5, with 3 representing the value for the NERVA-derived baseline, 5 representing a rating significantly superior to the baseline, and 1 representing a rating significantly worse than the baseline. In addition, each concept was evaluated according to its current Technology Readiness Level, as well as each concept's safety TRL. Estimates were developed for the time and cost to bring each concept to TRL 6, the current status of the NERVA baseline. Competitive costs estimates were presented by Concept advocates, as well as by the Technology Readiness Panel.
Almost all concepts were found to provide improved mission benefit over the NERVA baseline, with the greatest improvement in performance deriving from the Open Cycle Gas Core reactor. The Low Pressure Solid Core and Particle Bed reactors were also considered to offer major mission benefits. In contrast, the Dumbo and Foil reactors, as well as the Droplet Core and Liquid Annular configurations did not appear to provide major mission benefits over the NERVA baseline.
All concepts were considered to pose significantly greater technical risk than the NERVA baseline, with the exception of the NERVA-derived ENABLER. Although it is not widely recognized, significant development efforts for both the CERMET and Wire Core reactors were conducted under the Aircraft Nuclear Propulsion program in the 1950s and 1960s, and much of this engineering heritage may be recoverable. The favorable rating of the Particle Bed Reactor is tempered by the requirement for carbon-carbon components in its turbopump, which pushes the state of the art. Advantages of the Low Pressure reactor include the absence of turbopumps and control drums. A cluster of seven 49 kN engines can be selectively throttled for steering, providing additional simplicity by elimination of gimbal mounting. But the increased efficiency claimed for this concept through H2 molecular disassociation and recombination was considered to require experimental validation (though such experiments were not viewed as particularly challenging).
In general, the Technology Capabilities Panel viewed the more exotic liquid and gas core reactors as posing major technical risks in comparison to most solid core reactor concepts. The exception was the relatively risky Foil reactor, where there were significant questions about the feasibility of this concept due to uncertainties about the efficiency of fission fragment heating of propellants. All concepts were considered to pose significant fuel and coating development challenges, and significant uncertainty was expressed about the effects of molecular H2 disassociation. The Liquid Droplet core reactor was viewed as having very serious problems with fuel retention, a problem shared by the Open Cycle Gas Core reactor. Major technical uncertainties of the Lite Bulb gas core reactor, the most promising of the exotic systems, include problems with seeding the Hydrogen propellant, fuel containment, nozzle heating, and material selection.
Not surprisingly, initial Concept Focal Point advocate estimates of the cost to bring each concept to the TRL-6 of the NERVA baseline were found to be optimistic by the Technology Capabilities Panel. While initial cost estimates ranged from $0.3 billion for the Droplet Core to $4.8 billion for the Foil core, Panel estimates for these systems were $6.7 billion and $7.0 billion, respectively, with other Panel estimates ranging from $2.7 billion for the ENABLER to $9.8 billion for the Open Cycle Gas Core reactor. In general, the Technical Capabilities Panel concluded that reactor development costs would range from $3 to $10 billion, and that the development program would require both Nuclear Furnace tests of fuel elements, as well as a System Ground Test Facility for reactor prototypes.
Safety evaluations generally concluded that liquid and gas core concepts raised serious safety concerns, with only the Pellet Bed and ENABLER ranking higher than NERVA. With the exception of the ENABLER, CERMET and Particle Bed Reactor, all concepts were at very low technology readiness levels. The Technology Capabilities Panel concluded that major unresolved safety issues include the need for further study of water immersion criticality risks, as well as development of policies on reentry accidents, redundancy, and
TRL
Basic 1 Basic Principles Observed and Reported
Research 2 Technology Concept/Application Formulated
3 Analytical and Experimental Critical Function and/or Characteristic Proof-of-Concept
Focused 4 Component and/or Breadboard Validated in Laboratory
Technology 5 Component and/or Breadboard Validated in Relevant Environment
6 System Validation Model Demonstrated in Relevant/Simulated Environment
7 System Validation Model Demonstrated in Actual Environment
System
Hardware
Development 8 Technology Applied to construction of Component and/or Breadboard of Expected Flight Hardware Configuration
9 Capability of Full Scale Subsystem Prototype demonstrated in ground tests
10 Capability of Full Scale Subsystem Prototype demonstrated in Actual Environment
11 Full Scale System Prototype
12 Capability demonstrated in Flight Test of Flight Hardware
13 Capability demonstrated by Operational Flight Experience
radioactive emissions.
The long term strategy contemplated by the Technology Capabilities Panel envisioned an evolutionary progression in which technology development in the 1990s would lead to component and flight system testing in the following decade, with the initial system supporting Lunar and Mars flight operations in the 2010-2020 period. System upgrades to the initial system would support more ambitious Mars operations in the subsequent decade, with continued development leading to the introduction of more advanced systems, such as Gas Core or Fusion in the post-2030 period.
1 - Safety
The range of test failures in the earlier ROVER and subsequent programs is indicative of the need for careful review of safety and reliability issues. The PEEWEE reactor, which was a testbed for experimental fuel elements experienced a terminal failure as a result of improper fabrication of testbed hardware.(2)
A review of safety issues was presented at the Houston Feedback meeting of 15 November 1990.(3) The major safety and quality assurance issues facing nuclear propulsion include policy and requirements, communication, potential design impacts, testing, and public perception of risk.
A Nuclear Safety Policy Working Group for SEI Space Nuclear Propulsion has been constituted to recommend nuclear safety policy. Chaired by a DOE representative with members from NASA, DOD, DOE and National Laboratories, the group first met 19-20 November 1990.
The initial step in the nuclear safety process is development of national policy on nuclear propulsion safety, which will determine system requirements. This national policy, which must be developed on an inter-agency basis, will have to address questions of fission product release, reentry and impact safety, orbital space debris considerations, reactor shielding, system and reactor disposal, redundancy management, and international issues. NASA Code Q has recommended that NASA, DOE and DOD develop a joint agency policy on these issues during 1991.
The next stage in the safety process is communication on safety and quality assurance. This will require strong safety advocacy, which will develop a culture in which safety requirements are clearly communicated to and interpreted for system developers. NASA Code Q recommendations call for inter-agency development of generic safety-driven requirements in 1991, definition of the safety review process in 1991 and 1992, and development of technical specifications in 1992.
Safety and quality assurance can have a variety of impacts on system design. Manrating of shielding must take into account both reactor and natural ambient space radiation hazards. Other hazards that must be managed include orbital debris which could disable the reactor, contamination of the space vehicle by exhausted fission products, and dynamics of Earth return and disposal of the reactor following mission completion. Operational safety will impose constraints on ground testing facilities, launch facilities for integration with the launch vehicle, launch operations, including ascent, abort and landing, as well as on-orbit assembly and pre-flight verification.
Safety assessments will require an appropriate mix of analysis, simulation, sub-scale testing and full scale testing. The precise mix will depend on the need for continuity of documentation throughout the process. Requirements for verification of safety margins will be satisfied through materials selection and operational philosophy. Safety testing will include start-up and shutdown operations, as well as non-steady state operations.
Public perception of risk is complicated by the difference between real and perceived hazards. Education and public awareness efforts will need to prepare answers and develop explanations on nuclear safety issues. Code Q has recommended investigation of an expedited Environmental Impact Statement (EIS) process during the ground test facilities design process.
A number of issues remain open. One technical matter that was raised at the Houston Feedback meeting was the question of the policy on fission product releases as a result of fuel coating failures. It is unclear whether some emissions will be permitted due to this failure mode, since this would raise questions as to why coatings would be required to begin with. It is also unclear the extent to which this is an issue for operations in space versus testing on the ground.(4)
2 - Ground Testing
Los Alamos National Laboratory (LANL) has conducted a major review of the status of nuclear propulsion test facility requirements, based on prior LANL work with Rover and NERVA. Test facilities were required for verification of design codes, components, and full systems. Component testing would include electric furnace testing of reactor fuel elements and module support structures, as well as reactor and engine system components and instrumentation. A System Test Facility would be required for engine assembly tests, reactor zero-power critical experiments, cold and hot (power) testing, as well as post-test examination of the reactor assembly.
This System Test Facility would require
"nuclear exhaust and containment systems for altitude simulation and which provides for exhaust cleanup and containment during the maximum credible accident... based on NERVA experience and today's political climate, there are several issues that are important to an effective ground test program. It will be necessary to provide for adequate cleanup of engine effluents during power testing as well as materials released during credible malfunctions... In general, the maximum fission product release during such malfunction conditions will be less than 50%... One of the final nuclear reactor tests at NTS (the Nuclear Furnace) included an exhaust scrubber system... (which) was very successful in handling effluent flow rates of around 1.7 Kg/s. Such a system may well be scalable to the 10-12 kg/s range... An option for consideration is to exhaust the engine into a large holding pipe. Such pipes are routinely used in the tunnel tests at the Nevada Test Site for containment of nuclear weapon devices."(5)
According to Steve Howe, who coordinates space nuclear technology at LANL, the cost of developing an NTR system would be in the range of $3 to $5 billion, and "the country could have a human-rated nuclear rocket by 2005 to support a Mars mission. That's the fastest you could do it."(6)
At the Houston Feedback meeting, Howe suggested that construction of the Nuclear Furnace Facility and the System Test Facility required for the NTR will pose major challenges. He suggested that construction of the Nuclear Furnace Facility for testing fuel elements would require about 6 years, and that construction of the System Test Facility would need about 8 years, which would support a NTR ILC no earlier than 2003. These facilities will require new technology, since instrumentation for the 3,000 K operating environment currently doesn't exist. He concluded that funding profile requirements for these facilities will probably mandate postponing system testing for NEP systems until the conclusion of NTR system testing. Richard Bohl, who served as a test director during the NERVA program observes that designing environmentally acceptable test facilities will probably pose a greater challenge than building the engines themselves.(7)
The Nuclear Regulatory Commission (NRC) assumed the safety and licensing functions of the Atomic Energy Commission (AEC) in 1975, when the energy development functions of the AEC were subsumed by the Energy Research and Development Administration (ERDA) which was replaced by the Department of Energy in 1977. The NRC is responsible for licensing American nuclear reactors. But thus far all American nuclear power sources launched into space have been exempt from licensing by the NRC. The first five American nuclear power sources launched into space involved Defense Department payloads, and the Atomic Energy Act exempts DoD from licensing procedures for military applications of nuclear materials or facilities. Although NASA does not benefit from a similar legislative exemption, the launch of NASA nuclear power sources has nonetheless not yet been subject to NRC licensing to date. This practical exemption has proceeded from the theory that these civil nuclear power sources are experimental research devices developed under the research mandate of the DOE, and that DOE retains legal title to these power sources, even after they have been launched into space to power NASA payloads (such as the Pioneer 10 and 11, Viking and Voyager planetary probes).
While these practices have gone unchallenged, according to one friendly observer:
"One can, however, envision a time when the technology will have become so mature and so widely used that there may be repetitive production by DOE of essentially identical models, or changes that represent little more than production refinements not significantly advancing the state of the art. Under such circumstances, it could be argued that DOE would no longer be engaged in the research and development on these devices and that this basis for a licensing exemption for users other than DoD or DOE itself would no longer be valid... According to one view, the fact that NASA assumes operational control of the nuclear power systems at the moment of launch and retains it thereafter during all the period of risk has the effect of giving NASA possession under the terms of the Atomic Energy Act and would make the systems subject to licensing were it not for the research and development consideration."(8)
An early report on space nuclear power by a National Research Council panel rejected the notion of Nuclear Regulatory Commission licensing:(9)
"The Committee believes, for a variety of reasons, that NRC licensing is not appropriate for space nuclear launches. First, many of the launches will invovle national security or research and development purposes under direct governement sponsorship. The formal NRC licensing procedures are not particularly adaptable to such activities. It would be more productive to design and implement an effective federal safety program specifically for the project. Second, the space launches will involve, for the foreseeable future, individually tailored launches, not repeated commercial launches, that are difficult to subject to a defined licensing procedure. Third, the central purpose of the Nuclear Regulatory Commission (NRC) is to ensure the safety of the commerical nuclear fuel cycle. The NRC does not have much of the expertise required to regulate nuclear power in space."
However, the panel did suggest that:(10)
"... it would be appropriate to add to this structure a fourth level that preently does not exist. It has been suggested tha a Space Nuclear Power Systems Safety Board ( SNSSB ) be established, similar to the Advisory Committee on Reactor Safeguards ( ACRS ), to provide an independent public advisory opinion on the adequacy of the safety procedures and risk assessment that has been put forward."
NRC licensing of the NTR System Test Facility was controversial at the November Houston Feedback meeting. Some conceded that NRC licensing may be needed as a means to manage public safety and environmental concerns, as was done with the Clinch River Breeder Reactor. Many other participants were very resistant to NRC involvement.
3 - Launch Approval
The launch of a space reactor is currently subject to approval by the President of the United States on a case-by-case basis, based on safety recommendations forwarded to the President by the President's Science Advisor.(11) This recommendation is based on the evaluation by the Inter-agency Nuclear Safety Review Panel (INSRP), which is composed of representatives appointed by the three user agencies (NASA, DoD and DOE), with participation of representatives of other agencies such as the Nuclear Regulatory Commission (NRC), Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA).
The launch safety analysis process proceeds through three stages, from preparation by the proponent agency of a Preliminary Safety Analysis Report, to an Updated Safety Analysis Report, then to the Final Safety Analysis Report, each of which is submitted to the INSRP for review. Each draft of the Safety Analysis Report contains a description of the planned mission, an analysis of potential accidents, including both vehicle and nuclear system failure modes, and a probabilistic assessment of the radiological hazard posed by potential accidents. Based on its evaluation of the FSAR, the Panel issues a Safety Evaluation Report (SER) stating its findings as to whether the launch and operation of a space nuclear power source is free of "undue risk," in accordance with standards such as those enumerated in DOE 5480.1A "Environmental Protection, Safety and Health protection Programs for DOE Operations," and 10 CFR 50 Appendix A. "General Design Criteria for Nuclear Power Plants", as well as standards agreed to by the United Nations International Commission on Radiological Protection.
The INSRP does not make an actual recommendation concerning launch. Rather, the SER is an advisory document for the user agency heads, who submit to the Science Advisor letters of concurrence with the launch request by the proponent agency. The request is then acted on by the President.
1. Clark, John S.,(NASA Lewis Research Center), "Nuclear Thermal Propulsion, a Summary of Concepts Presented at the NTP Workshop July 10-12, 1990," NEP/NTP Workshop Feedback Meeting, (Hilton Nassau Bay Hilton, Houston, TX, 15 November 1990).
2. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989).
3. Sawyer, J. Charles (NASA Headquarters, Code Q), "Nuclear Propulsion Project Safety Panel," NEP/NTP Workshop Feedback Meeting, (Hilton Nassau Bay Hilton, Houston, TX, 15 November 1990).
4. This issue was raised by John Dearien of INEL.
5. Bohl, R.J., and Boudreau, J.E., "Direct Nuclear Propulsion: A White Paper," Los Alamos National Laboratory, January 1987.
6. David, Leonard, "Nuclear Power Backers Contend Mars Vessels Possible by 2005," Space News, 27 August 1990.
7. David, Leonard, "Nuclear Power Backers Contend Mars Vessels Possible by 2005," Space News, 27 August 1990.
8. Muntzing, L.M., "Future Regulation of Space Nuclear Power Systems," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983).
9. 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 28.
10. ibid, page 29.
11. Kerr, Thomas B., "Procedures for Securing Clearance to Launch Reactors," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983).