What is the Radioisotope Power Systems Program?
The Radioisotope Power Systems (RPS) Program, managed by NASA, is an ongoing strategic investment in nuclear power technologies that seeks to maintain NASA's current capabilities for space science, and enable safe and successful future space exploration missions.
NASA, working in collaboration with the Department of Energy (DOE), fosters more capable future space missions by supporting the development of advanced systems and technologies for producing electrical power using heat from the natural decay of plutonium-238.
RPS are ideally suited to provide power for missions that need autonomous, long-duration operations in the most extreme cold, dusty, dark, and high-radiation environments, either in space or on planetary surfaces.
NASA works with the DOE to maintain the capability to produce the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which saw its first use on the Curiosity Mars rover and it invests in new technology, such as more efficient thermoelectric materials and Stirling engines to produce electricity.
Why develop new RPS technologies?
NASA is considering demanding space science missions that may require new RPS capabilities. In addition, new RPS technologies could make more efficient use of current supplies of plutonium-238.
NASA is developing new RPS with multi-mission capability that would operate in the vacuum of deep space as well as on Mars or other planetary bodies that have an atmosphere. The radioisotope thermoelectric generators (RTGs) used for the Cassini and Pluto/New Horizons missions were designed to operate only in space.
Radioisotope power systems are selected for potential use in space only when they enable or significantly enhance the ability of a mission to meet its scientific and operational goals. As new science goals are identified, the RPS program assesses the required power capabilities to identify new technology needs or development efforts.
An RPS has clear potential benefits for outer solar system missions, where environmental conditions may preclude other electrical power sources, and for missions to locations with very limited sunlight, such as a permanently shadowed crater on the Moon or on an asteroid.
Why do RPS use plutonium-238?
Several radioisotopes could theoretically be used as a heat source for RPS. Plutonium-238 (in the form of plutonium dioxide) has been selected by the Department of Energy for several important reasons:
- Compared to other isotopes, it produces mainly alpha-particle radiation (which is relatively easy to shield against), and little gamma radiation, making it safer to handle and work around compared to other isotopes.
- It has properties that allow it to be used safely in a ceramic form that is not easily absorbed by humans, animals, or plants in the event of a release of material.
- The relatively short half-life of Pu-238 (88 years) means it degrades naturally in a manageable manner over the length of a typical space mission, producing a small gradual decline in power output that can be accurately predicted and managed.
- It has what engineers call good power density (equal to watts of heat produced per gram of material), which allows for small, lightweight heat sources.
- It is available and able to be produced in sufficient quantities.
- It has minimal and easily mitigated ionizing radiation effects on spacecraft systems.
What kinds of missions would need an RPS?
Current concepts for missions that could be enabled or significantly enhanced by the use of radioisotope power include missions to Mars, Venus, Jupiter, Europa, Saturn, Titan, Uranus, Neptune, the moon, asteroids and comets.
Potential future missions under study are discussed in the 2011 decadal survey of planetary science by the National Academy of Sciences, and in the annual strategic plan for the NASA Science Mission Directorate.
Nuclear power can enable or enhance missions where sunlight is infrequent, obscured, or dimmed by distance, making solar power impractical.
To learn more about NASA's space science and solar system exploration goals, visit http://science.nasa.gov/about-us/science-strategy/
What is an RTG? What is an MMRTG?
A Radioisotope Thermoelectric Generator, or RTG, is a type of power system for space missions that converts heat from the natural radioactive decay of plutonium-238 into electricity using devices called thermocouples, where heat is applied across a circuit that includes dissimilar metals. This produces an electric current via the Seebeck effect. This process involves no moving parts. Essentially a nuclear battery, an RTG provides power to a spacecraft and its science instruments. On some missions, such as the Curiosity Mars rover, the excess heat from the RTG can also be used to keep spacecraft systems warm in cold environmental conditions.
The Multi-Mission RTG, or MMRTG, is the eighth generation of such power systems, which have been used safely and successfully by the United States for more than 50 years since their first launch in 1961. It is based on the earlier SNAP-19 RTG design used on the two Viking lander missions, and is capable of working both in space and in a planetary atmosphere (such as on the surface of Mars), hence the name "multi-mission" RTG. NASA's Curiosity rover was launched carrying an MMRTG as its source of electrical power on November 26, 2011, and landed successfully on August 6, 2012.
How much plutonium is in the MMRTG? How much power would it supply?
Each MMRTG uses eight General Purpose Heat Source (GPHS) modules, containing a total of 10.6 pounds (4.8 kilograms) of plutonium dioxide, of which Pu238 represents 71% by weight. This compares to 18 GPHS modules in the previous generation of radioisotope thermoelectric generators (the GPHS-RTG, which was able to produce a larger amount of power). The last GPHS-RTG was launched in 2006 on NASA's New Horizons fly-by mission to Pluto on July 14, 2015.
The MMRTG is designed to supply about 110 watts of electrical power at the beginning of a mission. Depending on the power requirements for a mission, multiple MMRTGs could be combined on one spacecraft to operate additional scientific equipment reliably in cold environments.
What is NASA doing with Stirling technology for radioisotope power?
A stirling engine based power convertor would convert heat from the natural radioactive decay of plutonium-238 into electrical power using the back-and-forth motion of a piston utilizing an alternator, rather than the static metallic thermocouples used by an RTG. This technology converts heat to electricity at a higher efficiency than static technologies. With this advantage, Stirling Radioisotope Power Systems can make more efficient use of limited Pu-238, and produce less waste heat for missions where this is an issue.
NASA continues to conduct tests with currently available hardware, and has issued a request for information from industry on the current state-of-the-art. NASA is developing a plan with the goal to mature dynamic conversion based technologies that would prioritize reliability, robustness, and total lifecycle costs over efficiency and mass, looking toward a system that could be ready to fly on space missions in the late 2020s.
Does an RPS supply spacecraft heat in addition to electrical power?
It is possible to reclaim some of the waste heat produced by an RPS to supply heat for spacecraft systems and instruments. And, depending on a mission's thermal needs, a spacecraft could utilize Radioisotope Heater Units (RHUs) for additional thermal control.
How many more missions can be flown given the current limited U.S. supply of plutonium-238?
The Department of Energy has allocated 35 kilograms of plutonium-238 for civil space applications. Of this amount, 17 kilograms complies with the current specifications for use; the remainder could be blended with newer fuel and be available for use in the future.
The number of missions that can be supported with current inventories depends on the power required by the individual missions and the planned power sources. The latest decadal survey for planetary science conducted by the National Academy of Sciences (released in March 2011) strongly recommended several future missions that could be powered by a RPS.
Broadly speaking, the current U.S. inventory of plutonium-238 can support the Mars 2020 rover mission (using one fueled Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) flight unit), and one to two future missions to be determined, such as potentially the next NASA New Frontiers mission (for which an announcement of opportunity is expected to be released in 2017).
Has plutonium production restarted? What has DOE accomplished so far?
NASA and the DOE jointly announced in December 2015 that the first small amount of new plutonium-238 (Pu-238) fuel for future deep-space mission radioisotope power systems had recently been produced, after a gap of nearly 30 years. Oak Ridge National Laboratory (ORNL) produced 50 grams of new Pu-238, toward an initial annual production capability of 300-400 grams within a few years.
The Pu-238 Supply Project is currently demonstrating and validating the processes required for steady production, and then will begin scaling up toward the full average production rate of 1.5 kg plutonium dioxide/yr.
Instead of nuclear space systems, why not invest in better solar cells for use on space missions?
No single type of power system can supply the range of electrical power needs for the wide array of NASA's missions, so the space agency invests in research and development in a variety of power system types, from batteries and fuel cells to solar cells and radioisotope power systems.
The solar power system for NASA's Jupiter-bound Juno spacecraft, launched in 2011, was developed out of research into improving the performance of so-called Low-Illumination, Low-Temperature (LILT) solar cells for operation at the distance of Jupiter. In recent years, NASA has coordinated with the space photovoltaic technology, vendor, and user communities to define technology research and development paths for more efficient and robust LILT solar cells for deep space applications. Even so, the Juno LILT solar arrays degrade in the charged particle radiation environment around Jupiter and its moons, limiting the mission's lifetime and the regions through which the spacecraft can fly.
Each spacecraft has its own power requirements, based on its need to function within the specific environment called for by its mission and the minimum science goals of each mission. In addition to some of NASA's most ambitious and successful past and present missions, some future NASA missions would not be possible without an RPS.
Solar power is not practical where sunlight is infrequent or obscured, such as craters at the lunar poles, and becomes less practical as missions travel farther distances from the Sun. Because of the diminished intensity of sunlight, solar panels can become impractically large, and potentially hinder the spacecraft's ability to maneuver and point accurately at the sun.
NASA research and development dollars are invested to meet the needs of its strategic plans, such as the annual science strategy for NASA's Science Mission Directorate and the decadal surveys for planetary science conducted by the National Academy of Sciences. The decadal survey issued in 2011 calls for NASA to consider a variety of solar, battery and nuclear power systems to enable a wide range of demanding missions.
The latest NASA technology research roadmaps include plans for investments in Space Power and Energy Storage, which forecasts NASA technology development efforts in chemical systems (fuel cells and batteries), solar energy (photovoltaic and thermal systems), radioisotope power systems, fission power, and fusion power. The high priority of radioisotope power systems, particularly new Stirling-based systems, was affirmed in early 2012 by the National Academy of Sciences report "NASA Space Technology Roadmaps and Priorities: Restoring NASA's Technological Edge and Paving the Way for a New Era in Space."
What do taxpayers receive for investments in space nuclear research and technology development?
Investing wisely in space nuclear power enables continued cost-effective exploration of our solar system. Such investment can revolutionize our ability to explore the solar system, which contains important clues about the origin and evolution of the planets and the beginnings of life, including possible life beyond Earth.
Money invested by NASA in space nuclear technology can make future missions more effective and more efficient, and thus produce a significant return on investment. Technology research creates good jobs here on Earth in science and technology. Both of these aspects can also inspire future generations to pursue careers in science and engineering.
NASA's funding priorities are established annually by the President and the U.S. Congress, based on consideration of national needs, the agency's capabilities, and its proposed new activities. NASA in turn develops its budget proposals in accordance with its program goals and priorities, based on its charter to explore space for the peaceful benefit of all humankind, and an open strategic planning process that includes extensive external input and review by the scientific community.
The Mars Exploration Rovers Spirit and Opportunity have lasted years longer than NASA thought they would. Does NASA really need to launch RPS when solar power seems to work better and longer than predicted for the rovers?
Because Spirit and Opportunity rely on solar energy, they were limited in the latitudes, terrain, and seasons that they could land and operate in. Future missions to Mars may have more demanding goals that could require a wider operating range.
In addition to their solar panels, the two Mars Exploration Rovers each were designed to carry and use eight radioisotope heater units (RHUs) as part of their thermal control system. These RHUs have contributed directly to the long and productive lives of the rovers by conserving battery energy that would otherwise have been diverted for heating, especially during unforeseen mission events. The rovers were built to operate for 90 days, but Opportunity continues operating and returning valuable scientific data today, and Spirit remained active for more than six years.
On each of the rovers, the RHUs supply an equivalent of 112 watt-hours of energy during nighttime operations. Solar power supplemented by RHUs was the best fit for Spirit and Opportunity's initial three-month mission operation requirement and power needs.
Without RHUs, the rovers probably would not have continued operating long enough to complete their three-month prime mission, because of the extra draw on the battery that would have been required for heating.
In addition to the RHUs providing critical thermal energy, the rovers have benefitted from random weather events on Mars. For example, local winds and dust devils have unexpectedly boosted solar power to the rovers by clearing Martian dust off of the solar arrays. While these fortuitous winds are welcomed, such natural dust-clearing events are highly unpredictable, and cannot be relied upon in the design process for future miss
Is it safe to launch and use RPS?
Yes. Radioisotope power systems have been used safely and successfully by NASA to explore the solar system for more than 40 years. These power systems even went to the moon with the Apollo astronauts.
Several layers of safety features in an RPS help minimize the release and dispersal of nuclear material under a wide range of possible accident conditions.
The General Purpose Heat Source modules, which contain the nuclear fuel, provide protection for potential ground impact and accidental reentry scenarios. As part of the engineering/design process that aims to improve each new generation of RPS, the heat-source modules in the MMRTG have additional protective material that would provide enhanced protection for potential ground impact and accidental reentry scenarios.
What safety review process does NASA go through for a mission with nuclear materials?
NASA complies with an extensive launch approval process for any space mission planning to utilize Radioisotope Heater Units (RHUs), Radioisotope Power Systems (RPS), or nuclear reactors. Though the primary responsibility for launching and operating the mission safely belongs to NASA, many organizations have responsibilities and/or interests in a spacecraft that uses nuclear systems.
Any NASA mission that proposes to use an RPS, RHU, or nuclear reactor undergoes a comprehensive environmental review, including public meetings and open comment periods during the mission planning and decision-making process as part of NASA's compliance with the National Environmental Policy Act. Additionally, any such mission proposed by NASA would not launch until formal approval for the mission's launch nuclear safety is received from the Office of the President.
What would NASA do if there were an accident involving an RPS?
In advance of launches, NASA works closely with federal, county, and state public information services and emergency management organizations to prepare to respond to any accident in an effective manner, including timely flow of information to the public. Such coordination is part of a comprehensive Radiological Contingency Plan to ensure that NASA, DOE, EPA, and state and local authorities are fully prepared.
Before any launch of a mission carrying an RPS, monitoring teams with a variety of technical expertise and the necessary support equipment would be deployed around the launch site and the surrounding communities to assess any possible release of nuclear material in the event of a launch accident.
To avoid potential exposures, local emergency response officials might ask the public to take precautionary measures following an accident with a mission carrying a fueled RPS or nuclear heat source, such as remaining indoors for a limited period.
What hazards do RPS pose in case of a mission accident?
The fuel in an RPS is plutonium dioxide, which is a radioactive material. In the unlikely event of a mission accident, there is a potential for the release and dispersal of the radioisotope fuel into the environment, and subsequent exposure to humans. This exposure could result in an elevated long-term chance of inducing certain cancers or other related health effects.
To minimize this potential, several layers of safety features have been designed into the heat source. One key safety feature is the ceramic fuel form, which cannot be readily digested. The fuel is formulated to break into large non-respirable sized particles to minimize the potential for inadvertent inhalation. NASA and DOE have demonstrated that potential mission risks are small, through extensive ground-based testing and modeling, and the studies conducted to support the NEPA process for each newly proposed NASA mission.
Have there been any previous failures with space nuclear systems?
The United States has flown 27 missions with radioisotope thermoelectric generators (RTGs) and one nuclear reactor system over the past five decades.
Three past missions carrying spacecraft using radioisotope power systems failed in accidents unrelated to the power system. In each instance, the radioisotope power system performed as it was designed.
- The April 1964 launch of the Transit 5-BN-3 navigational satellite was aborted during its ascent to orbit. The satellite's RTG burned up upon reentry and, as intended by its design, dispersed its plutonium fuel in the upper atmosphere.
- The May 1968 launch of the Nimbus B-1 weather satellite was aborted during its ascent to orbit; its RTG contained the plutonium fuel as designed, the generator was retrieved intact, and the fuel was re-used on a subsequent mission.
- 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 Apollo mission. The Apollo 13 lunar module was used successfully as a "lifeboat" for the three astronauts following unrelated damage to their command module on the way to the Moon. The lunar module carrying the RTG was targeted to fall into the Pacific Ocean near the Tonga Trench. Extensive reconnaissance of the area detected no release of the power system's plutonium dioxide fuel.
Does NASA have any plans to recover the SNAP-27 RPS from the Apollo 13 lunar lander at the bottom of the Pacific Ocean?
Why should the public believe NASA can manage the safety of a nuclear power system's development given its Space Shuttle safety record?
NASA has an outstanding record of safety in launching spacecraft carrying nuclear power systems, with 17 successful launches and no failures over the past three decades.
NASA continues to study, plan and practice how to improve the performance of these processes, applies this extensive experience to every launch, and prepares for likely outcomes. This includes extensive radiological contingency planning, with related resources in place for each launch in the unlikely event of an accident.
NASA and the Department of Energy place the highest priority on assuring the safety of their workers and the general public during activities that utilize radioactive materials, and at related facilities.
What is meant by the plutonium dioxide having a "ceramic form"?
The plutonium dioxide fuel used in an RPS is a specially refined, fire-resistant ceramic material that is manufactured and used in the form of pellets to reduce the possibility of dispersing the fuel in a launch or reentry accident.
This ceramic resists being dissolved in water and reacts little with other chemicals. If fractured, the ceramic—like a broken coffee cup—tends to break into relatively large particles and chunks that pose significantly less hazard than inhalable microscopic particles.
What is the material that the plutonium dioxide fuel is "clad" in?
Multiple layers of protective material protect and contain the fuel and reduce the chance of a release of the plutonium dioxide. The plutonium dioxide pellets are first clad in iridium, a strong, ductile, corrosion-resistant metal with melting point greater than 4820 degrees Fahrenheit (2660 degrees Celsius).
How hot is the fuel clad (iridium) inside an RPS?
Depending on the RPS design and operating conditions, the temperature of a fuel clad could range from 1382 to 2012 °F (750 to 1100 °C).
How long will an RPS continue to produce power?
The Multi-Mission Thermoelectric Generator (MMRTG) has a design life of 14 years plus three years of pre-launch storage life. However, every U.S. planetary exploration mission that has used RPS over the past four decades and counting has been extended beyond its original period of operation, permitting years—and sometimes even decades—of additional scientific data to be returned to Earth.
The longest-operating RPS-powered spacecraft are Voyager 1 and 2, the most distant human-made objects in space. Launched in 1977, these hardy spacecraft are more than 10 billion miles from the sun but, thanks to the power still produced from their RPS, they continue to return valuable data to scientists on Earth about the distant edge of the sun's influence, where our planetary system meets interstellar space.
Is it safe to perform an Earth flyby gravity-assist with a spacecraft carrying nuclear material?
Yes. NASA has flawlessly directed spacecraft to perform several Earth flybys and dozens of similar flybys of other bodies in the solar system (in conditions where navigation is much more challenging), with extremely high accuracy.
In general, Earth flyby maneuvers have been designed so there is less than one in one million chance of a flyby accident that causes the spacecraft to reenter Earth's atmosphere. These precision maneuvers use proven techniques such as keeping the orbital path of a spacecraft pointed well away from Earth until a carefully planned navigation adjustment close to the date of the flyby.
In addition, the enhanced GPHS modules are designed to contain their plutonium dioxide fuel in the unlikely event of an Earth re-entry during a flyby maneuver
What would happen to an RPS in an Earth flyby accident?
All RPS include several layers of protective features, which are designed to contain their plutonium dioxide fuel in a wide range of potential accidents, verified through both a long history of safety testing and advanced supercomputer simulations.
RPS are designed to separate into free-flying GPHS modules in a reentry or flyby accident. The response of each module to thermal and mechanical stresses would vary depending on its specific entry angle and motion. Modules that exhibit any tumbling motion during reentry would not be expected to release any plutonium dioxide. All of the modules contain their fuel inside several layers of hard, heat-resistant material that is highly resistant to impact damage and is designed to contain the fuel upon its landing on the ground or in the ocean.
Extensive safety analysis is performed for each individual mission that proposes to use an RPS, and the probabilities and possible consequences of many different accident scenarios are calculated and cross-checked by independent groups. NASA would not propose to fly a mission that has been determined to be unsafe for the public or its workers.
David Woerner’s paper on “Next-Generation RTGs for NASA” won the American Institute of Aeronautics and Astronautics (AIAA) 2017 Aerospace Power Systems Best Paper Award.
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