graphite impact shells are packaged within a reentry aeroshell that is also constructed of a carbon-carbon composite. This assembly, which is approximately 4 in. × 4 in. × 2 in., is called a general purpose heat source (GPHS) module. This is the standard RPS fuel module now used in all U.S. RPSs, and it reflects many improvements in materials and packaging that have been introduced over time.5

Testing and analysis must be performed to determine the response of all RPSs to credible accident environments. Testing validates analysis models and establishes and demonstrates the level of safety built into the design. A tremendous amount of testing has been conducted on the GPHS fuel, materials, and hardware since its original design and development in the mid-to-late 1970s.

The efficacy of the U.S. RPS design safety approach was demonstrated during the launch of the Nimbus B-1 meteorological satellite, with two Systems for Nuclear Auxiliary Power (SNAP)-19B2 RPSs on board, from Vandenberg Air Force Base, California, on May 18, 1968. During this launch, range safety destruct of the launch vehicle and upper stage was initiated by the range safety officer because the launch vehicle was ascending erratically. Although the launch vehicle and payload were totally destroyed by the explosion, the RPSs were recovered intact. No release of 238Pu occurred, and the 238PuO2 fuel was used on a later mission.

Nevertheless, the use of RPSs does create some risk that 238Pu could be released into the biosphere, however low that risk may be. To assess this risk, the Unites States has established a flight safety review and launch approval process for RPS-powered missions. This process is structured to ensure that the radiological risk for each mission is characterized in detail and independently evaluated so that an informed launch decision can be made, based on sound risk-benefit considerations.

The U.S. flight safety review and launch approval process for space nuclear power systems is established by Presidential Directive/National Security Council Memorandum 25 (PD/NSC-25, 1977). As part of this process, the DOE prepares a series of increasingly detailed Safety Analysis Reports that characterize the radiological risk for the each mission.

For each NASA mission, the NASA administrator requests establishment of an Interagency Nuclear Safety Review Panel (INSRP) comprised of coordinators from the Department of Defense, the Environmental Protection Agency, NASA, and the DOE, with a technical advisor from the Nuclear Regulatory Commission. The INSRP coordinators and the technical advisor are appointed by senior management from within each agency’s safety oversight office. They are, therefore, independent of the mission and associated RPS development efforts, and they have the responsibility and authority to identify and address issues at any level. Each INSRP is supported by technical experts, as needed, typically in six working groups: Launch Abort, Reentry, Power Systems, Meteorology, Biomedical and Environmental Effects, and Risk Integration and Uncertainty.

The Final Safety Analysis Report is reviewed in great depth by the INSRP, which often performs additional independent analyses. The INSRP then prepares a Safety Evaluation Report. These reports identify and characterize credible accident scenarios, including the probabilities that 238Pu will be released and the postulated health effects for each accident scenario, to determine overall mission risk and the uncertainties associated with that risk.

NASA uses the Final Safety Analysis Report and the Safety Evaluation Report to determine whether it will formally request launch approval from the White House. If it does, both reports are provided to the director of the Office of Science and Technology Policy (within the Executive Office of the President), who may grant launch approval, deny launch approval, or defer the decision to the President.

The entire launch approval process typically takes 3 years (including the resolution of legal challenges that are sometimes raised), although it could take as long as 8 years. The process usually takes longer than average if a mission uses a launch vehicle, upper stage, launch complex, launch trajectory, and/or spacecraft combination that has not previously been characterized and analyzed. In such cases, extra effort is needed to prepare the Launch Vehicle Databook, which identifies and characterizes accident sequences and environments that could occur during pre-launch, launch, ascent, and trajectory insertion.

FINDING. RPS Nuclear Safety. The U.S. flight safety review and launch approval process for nuclear systems comprehensively addresses public safety, but it introduces schedule requirements that must be considered early in the RPS system development and mission planning process.


DOE (Department of Energy). 1991. Memorandum of Understanding between the Department of Energy and the National Aeronautics and Space Administration Concerning Radioisotope Power Systems for Space Missions, as amended. Signed by VADM Richard H. Truly, NASA administrator, and ADM James D. Watkins, secretary of energy, dated July 26, 1991.

Dieckamp, H.M. 1967. Nuclear Space Power Systems. Atomics International. Unpublished book. September.

Glenn, J., Jr., 1983. The Next 25: Agenda for the U.S. IEEE Spectrum (September):91.

NEPA (National Environmental Policy Act). 1970. National Environmental Policy Act of 1969, as amended, 42 USC Sections 4321-4347. Available at

OSTP (Office of Science and Technology Policy). 2006. “U.S. National Space Policy,” National Security Presidential Directive 49, unclassified version released on October 6, 2006. Executive Office of the


It is possible to conceive of an RPS design that uses a different approach to packaging the 238Pu fuel. However, any new approach would require demonstrating, through analysis and testing, that the new approach will be safe during normal operating conditions and credible accident scenarios. This would be very expensive and time-consuming, in part because some of the facilities used to develop the current fuel system no longer exist.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement