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RPS: Radioisotope Power Systems
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Explore with radioisotope-powered missions in 3D. EYES on the SOLAR SYSTEM.
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Safety and radioisotope power

Multiple layers of safety features are incorporated into the design of the radioisotope power systems (or RPS) used by NASA. RPS are designed and tested to contain their nuclear fuel during all normal operating conditions and in a wide range of postulated accident scenarios. Testing has demonstrated that they can indeed withstand severe conditions associated with a wide spectrum of credible accidents. (Read more about the design of the General Purpose Heat Source.)

The United States has flown 27 missions with radioisotope power systems and one reactor system over the past five decades, and U.S. RPS have had an outstanding safety and reliability record. They have never caused a spacecraft failure, and over 50 years of effort have been invested in their engineering, safety, analysis, and testing.

NASA has a long history of safe launch and operations of spacecraft carrying radioisotope power systems, and no member of the public or NASA employee has ever been injured by any previous use of an RPS system or a related launch. Safety is a critical priority, and it is integrated into every phase of the design, test, manufacture, and operation of space nuclear systems, including several layers of protective features that minimize the potential for release and dispersion of nuclear material under a wide range of accident conditions.

Previous Launch Accidents Involving an RPS

Photo showing the Nimbus B-1 spacecraft's power source on the seafloor where its fuel was recovered intact, as designed, and reused on NASA's Nimbus III spacecraft.
Three missions using radioisotope power systems have been subject to mechanical failures or human errors unrelated to the power system that resulted in early aborts of the mission. In each instance, the radioisotope power system performed precisely as it was designed to do.

  • The April 1964 launch of the Transit 5-BN-3 navigational satellite was aborted during its ascent to orbit. Its Radioisotope Thermoelectic Generator (RTG) burned up upon reentry, as intended by its design, and dispersed its plutonium fuel in the upper atmosphere.
  • The May 1968 launch of the Nimbus B-1 weather satellite was aborted shortly after launch. Its RTG contained the plutonium fuel as designed. The fuel container was retrieved intact and the fuel was used on a subsequent mission (Nimbus III).
  • An RTG intended to operate science instruments on the surface of the moon as part of Apollo 13 returned to Earth in April 1970 following the aborted mission. The Apollo 13 lunar module, "Aquarius," was used successfully as a lifeboat for the three astronauts following damage to their command module (unrelated to the RTG) on the way to the moon. Following the astronauts' safe return, the lunar module carrying the RTG fell into deep water in Pacific Ocean. No release of radiation from this incident has been detected.

NASA and the Department of Energy (DOE) place the highest priority on assuring the safety of the general public and their workers during activities that utilize radioactive materials, and at related facilities.

About Plutonium-238

Plutonium-238 fuel pellet
A plutonium-238 fuel pellet, glowing with the heat it produces.
Credit: U.S. Department of Energy
The fuel in an RPS is plutonium dioxide, which is a radioactive material that produces alpha particles. Alpha particles are a particular type of ionizing radiation that can be shielded by material as thin as a piece of paper. Plutonium-238 is not the type of plutonium used for nuclear weapons and would not work well as fuel in a nuclear reactor.

To be suitable for space missions, a radioisotope must meet all of the following criteria:

  • Exist in an insoluble form and/or otherwise not be readily absorbed into the body in the unlikely event of a launch accident
  • Exist in a form such that it presents no or minimal chemical toxicity when taken into the body
  • Have relatively low neutron, beta and gamma radiation emissions, so as to not adversely affect spacecraft instruments or require excessively massive shielding
  • Be stable at high temperatures, so its characteristics remain essentially unchanged over many years
  • Have a long enough half-life (at least 15 to 100 years), so that it can generate for many years sufficient heat for transformation into electricity
  • Have a high power density, so a small amount of it can generate a substantial amount of heat

The only radioisotope that has consistently met the basic criteria is plutonium-238, which has a half-life of 88 years and a high power density, and has proven to be a very dependable and safe heat source on more than two dozen U.S. space missions over the past 50 years.

In unlikely event of a mission accident, there is a potential for the release and dispersal of the fuel into the environment, and subsequent exposure to humans. Several layers of safety features designed into an RPS help minimize this potential. For example, the fuel is intentionally formulated and used in a ceramic form, similar to the material in a coffee mug. In this form, it primarily breaks into large pieces rather than being vaporized into fine particles, which can be a health hazard when inhaled. The ceramic form also prevents the material from being absorbed into the body if ingested.

Fact sheet: What is Pu-238? (PDF, 666 KB)

Conceptual design for solar powered Cassini spacecraft
The Cassini mission's environmental impact statement determined that a solar powered spacecraft would require a total array area of more than 500 square meters. Arrays this size would have been too large and heavy for Cassini's launch vehicle -- at that time the biggest and most powerful rocket available.

National Environmental Policy Act (NEPA) and Launch Approval

NASA is subject to the National Environmental Policy Act (NEPA), which requires all Federal agencies to integrate environmental values into their decision making processes by considering the environmental impacts of their proposed actions and the reasonable alternatives to those actions.

NASA and DOE have demonstrated that potential mission risks are small, through ground-based testing and modeling, the NEPA process, and related radiological risk assessments.

Nonetheless, a decision to implement a mission carrying an RPS is made only after evaluating the range of potential environmental impacts associated with a NASA mission, and comparing those potential impacts to other reasonable alternatives (including a "no action" alternative), as part of NASA's compliance with NEPA. Additionally, approval by the Director of the White House Office of Science and Technology Policy is required for launch of any U.S. spacecraft carrying an RPS.

Environmental Impact Statements for NASA programs, including RPS-enabled missions are available here.

Additional information

Fact sheet: Safety of Radioisotope Power Systems (PDF, 881 KB)

Further information about RPS safety is available on the FAQ page.

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Last Updated: 13 Dec 2012