Space Radioisotope Power Systems Safety

The United States has flown 27 missions with radioisotope power systems and test, manufacture, and operation of space nuclear systems, including several.
Table of contents

• Radioisotope thermoelectric generator
• Recent Joint Studies Related to the Development of Space Radioisotope Power Systems
• Radioisotope thermoelectric generator - Wikipedia

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RTGs use thermoelectric generators to convert heat from the radioactive material into electricity. Thermoelectric materials in space missions to date have included silicon—germanium alloys, lead telluride and tellurides of antimony, germanium and silver TAGS. Studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations.

Some prototype Po RTGs have used thermionics, and potentially other extremely radioactive isotopes could also provide power by this means, but short half-lives make these unfeasible. Several space-bound nuclear reactors have used thermionics, but nuclear reactors are usually too heavy to use on most space probes. Thermophotovoltaic cells work by the same principles as a photovoltaic cell , except that they convert infrared light emitted by a hot surface rather than visible light into electricity.

Thermophotovoltaic cells have an efficiency slightly higher than thermoelectric modules TEMs and can be overlaid on top of themselves, potentially doubling efficiency. Thermophotovoltaic cells and silicon TEMs degrade faster than metal TEMs, especially in the presence of ionizing radiation. Dynamic generators can provide power at more than 4 times the conversion efficiency of RTGs.

Greater efficiency can be achieved by increasing the temperature ratio between the hot and cold ends of the generator. The use of non-contacting moving parts, non-degrading flexural bearings , and a lubrication-free and hermetically sealed environment have, in test units, demonstrated no appreciable degradation over years of operation. Experimental results demonstrate that an SRG could continue running for decades without maintenance.

Vibration can be eliminated as a concern by implementation of dynamic balancing or use of dual-opposed piston movement.

Potential applications of a Stirling radioisotope power system include exploration and science missions to deep-space, Mars, and the Moon. The increased efficiency of the SRG may be demonstrated by a theoretical comparison of thermodynamic properties, as follows. These calculations are simplified and do not account for the decay of thermal power input due to the long half-life of the radioisotopes used in these generators.

The assumptions for this analysis include that both systems are operating at steady state under the conditions observed in experimental procedures see table below for values used. Both generators can be simplified to heat engines to be able to compare their current efficiencies to their corresponding Carnot efficiencies. The system is assumed to be the components, apart from the heat source and heat sink. This yields a Second Law efficiency of RTGs pose a risk of radioactive contamination: For spacecraft, the main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth, harmful material could be released into the atmosphere; therefore their use in spacecraft and elsewhere has attracted controversy.

Recent Joint Studies Related to the Development of Space Radioisotope Power Systems

However, this event is not considered likely with current RTG cask designs. For instance, the environmental impact study for the Cassini—Huygens probe launched in estimated the probability of contamination accidents at various stages in the mission. The probability of an accident occurring which caused radioactive release from one or more of its 3 RTGs or from its radioisotope heater units during the first 3. To minimize the risk of the radioactive material being released, the fuel is stored in individual modular units with their own heat shielding. They are surrounded by a layer of iridium metal and encased in high-strength graphite blocks.

These two materials are corrosion- and heat-resistant. Surrounding the graphite blocks is an aeroshell, designed to protect the entire assembly against the heat of reentering the Earth's atmosphere. The plutonium fuel is also stored in a ceramic form that is heat-resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highly insoluble. Between —, 28 U.

The plutonium used in these RTGs has a half-life of A consequence of the shorter half-life is that plutonium is about times more radioactive than plutonium i. Since the morbidity of the two isotopes in terms of absorbed radioactivity is almost exactly the same, [32] plutonium is around times more toxic by weight than plutonium The alpha radiation emitted by either isotope will not penetrate the skin, but it can irradiate internal organs if plutonium is inhaled or ingested.

Particularly at risk is the skeleton , the surface of which is likely to absorb the isotope, and the liver , where the isotope will collect and become concentrated. There have been several known accidents involving RTG-powered spacecraft:. One RTG, the SNAPC, was lost near the top of Nanda Devi mountain in India in when it was stored in a rock formation near the top of the mountain in the face of a snowstorm before it could be installed to power a CIA remote automated station collecting telemetry from the Chinese rocket testing facility.

The seven capsules [37] were carried down the mountain onto a glacier by an avalanche and never recovered. It is most likely that they melted through the glacier and were pulverized, whereupon the plutonium zirconium alloy fuel oxidized soil particles that are moving in a plume under the glacier.

The SNAP heat source traveled to the moon in a graphite cask attached to the lander leg from which an astronaut removed it with a handling tool after a successful landing and placed it in the RTG. Several of these units have been illegally dismantled for scrap metal resulting in the complete exposure of the Sr source , fallen into the ocean, or have defective shielding due to poor design or physical damage.

The US Department of Defense cooperative threat reduction program has expressed concern that material from the Beta-M RTGs can be used by terrorists to construct a dirty bomb. RTGs and nuclear power reactors use very different nuclear reactions. Nuclear power reactors use controlled nuclear fission in a chain reaction. The rate of the reaction can be controlled with neutron absorbers, so power can be varied with demand or shut off entirely for maintenance.

However, care is needed to avoid uncontrolled operation at dangerously high power levels. Chain reactions do not occur in RTGs, so heat is produced at an unchangeable, though steadily decreasing rate that depends only on the amount of fuel isotope and its half-life. An accidental power excursion is impossible.

However, if a launch or re-entry accident occurs and the fuel is dispersed, the combined power output of the radionuclides now set free does not drop. In an RTG, heat generation cannot be varied with demand or shut off when not needed. Therefore, auxiliary power supplies such as rechargeable batteries may be needed to meet peak demand, and adequate cooling must be provided at all times including the pre-launch and early flight phases of a space mission. Due to the shortage of plutonium, a new kind of RTG assisted by subcritical reactions has been proposed. This way a long-lived neutron source is produced.

Because the system is working with a criticality close to but less than 1, i. However the essentials are unmodified. RTG have been proposed for use on realistic interstellar precursor missions and interstellar probes. The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes.

A power enhancement for radioisotope heat sources based on a self-induced electrostatic field has been proposed. A typical RTG is powered by radioactive decay and features electricity from thermoelectric conversion, but for the sake of knowledge, some systems with some variations on that concept are included here:. Spacecraft use different amounts of material, for example MSL Curiosity has 4. From Wikipedia, the free encyclopedia. New Horizons in assembly hall. Thermoelectric effect Seebeck effect Peltier effect Thomson effect Seebeck coefficient Ettingshausen effect Nernst effect.

Thermoelectric materials Thermocouple Thermopile Thermoelectric cooling Thermoelectric generator Radioisotope thermoelectric generator Automotive thermoelectric generator. This section does not cite any sources. Please help improve this section by adding citations to reliable sources.

Unsourced material may be challenged and removed. March Learn how and when to remove this template message. List of nuclear power systems in space.

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Stirling Radioisotope Generator Alkali-metal thermal to electric converter Atomic battery Betavoltaics Optoelectric nuclear battery Radioisotope heater units Radioactive isotope Thermionic converter. Retrieved 30 March Archived from the original PDF on 6 August Retrieved 10 October Elswick September 24, Archived from the original on 16 August Desperately seeking plutonium, NASA has 35 kg of Pu to power its deep-space missions - but that will not get it very far". Archived from the original PDF on Retrieved 24 July Retrieved 19 October Archived from the original PDF on 14 May Retrieved 7 May Retrieved January 30, A PDF is a digital representation of the print book, so while it can be loaded into most e-reader programs, it doesn't allow for resizable text or advanced, interactive functionality.

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Spacecraft require electrical energy.

This energy must be available in the outer reaches of the solar system where sunlight is very faint. It must be available through lunar nights that last for 14 days, through long periods of dark and cold at the higher latitudes on Mars, and in high-radiation fields such as those around Jupiter. Radioisotope power systems RPSs are the only available power source that can operate unconstrained in these environments for the long periods of time needed to accomplish many missions, and plutonium Pu is the only practical isotope for fueling them. Plutonium does not occur in nature. The committee does not believe that there is any additional Pu or any operational Pu production facilities available anywhere in the world.

The total amount of Pu available for NASA is fixed, and essentially all of it is already dedicated to support several pending missions--the Mars Science Laboratory, Discovery 12, the Outer Planets Flagship 1 OPF 1 , and perhaps a small number of additional missions with a very small demand for Pu. If the status quo persists, the United States will not be able to provide RPSs for any subsequent missions. The National Academies Press and the Transportation Research Board have partnered with Copyright Clearance Center to offer a variety of options for reusing our content.

You may request permission to:. For most Academic and Educational uses no royalties will be charged although you are required to obtain a license and comply with the license terms and conditions. Click here to obtain permission for Radioisotope Power Systems: An Imperative for Maintaining U. Leadership in Space Exploration. For information on how to request permission to translate our work and for any other rights related query please click here. For questions about using the Copyright. Loading stats for Radioisotope Power Systems: Leadership in Space Exploration E-mail this page Embed book widget.