A plutonium-238 fuel pellet, glowing with the heat it produces. Credit: U.S. Department of Energy

The fuel in an RPS is plutonium oxide, 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.

Plutonium Production for RPS

After a gap of nearly 30 years, the United States has restarted production of new plutonium oxide heat source material, which is the fuel used in radioisotope power systems (RPS) built by the Department of Energy (DOE) to provide electricity and heat for NASA missions that explore some of the most extreme places in the solar system.

Radioisotope power systems convert heat from the natural radioactive decay of the isotope plutonium-238 (used in a ceramic form of plutonium oxide) into electrical power to operate the computers, science instruments, and other hardware aboard NASA missions such as the Curiosity rover on Mars and the New Horizons spacecraft flyby of Pluto and beyond.

U.S. production of plutonium oxide for space exploration at the Department of Energy’s Savannah River Plant in South Carolina ceased in the late 1980s, and interim purchases of heat source material from Russia have ended.

Since 2011, Congress has provided funds to NASA to support the ability of the DOE Office of Nuclear Energy to resume U.S. domestic production of plutonium for civil space applications, using a series of specialized DOE facilities and staff at Oak Ridge National Laboratory (ORNL) and Idaho National Laboratory (INL).

New plutonium-238 production is part of a broader infrastructure at DOE that provides radioisotope power systems to NASA for use in space missions. Plutonium oxide is produced at ORNL and shipped to Los Alamos National Laboratory (LANL) where it is turned into heat source pellets, which are then shipped to INL to be placed into safe, long-term storage awaiting fueling of future radioisotope power systems there.

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.

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