Galileo/Ulysses Contingency Planning (v. 9/19/89) NASA Fact Sheet Galileo and Ulysses Contingency Planning Like their Transit, LES, Apollo, Pioneer, Viking, and Voyager predecessors, the Galileo and Ulysses missions will derive their power from plutonium-238 fueled Radioisotope Thermoelectric Generators (RTGs). Because these missions will use a radioisotope fuel, years of extensive testing and analysis have been undertaken to assess their safety. These assessments indicate that the radiological risks from these missions are very small. Because any risk, no matter how small, is of concern to NASA, comprehensive contingency plans have been formulated to ensure that any radiological release can be met with a well developed and tested response. Contingency Plans: The Basic Elements These plans encompass every phase of the missions: prelaunch countdown, ascent, second stage, on-orbit, payload deployment, and Galileo's Venus-Earth-Earth-Gravity-Assist maneuver. For any of these phases, the contingency plans entail the following steps [KHB 1860.1A, Ch. 1, Appendix D, Annex D-2]: (1) determining whether there has been a release of radioactive materials; (2) assessing and characterizing the extent of any release; (3) predicting the propagation and dispersion of the released material; (4) formulating/recommending protective and mitigating actions to safeguard people and property from the consequences of the release; (5) minimizing the effects of a release through control of the contaminated areas and containment of radioactive materials; (6) recovering and disposing of the radioactive material; and (7) decontaminating and recovering affected areas, facilities, equipment and properties. The specifics regarding the implementation of these steps would depend upon the location of an accident in a particular mission phase. For instance, a prelaunch accident would be located at Kennedy Space Center (KSC) whereas an accidental reentry during payload deployment could impact a site outside the U.S. Response Plans for Pre-Launch or Early Ascent Accidents on the Kennedy Space Center Premises In the prelaunch countdown and early ascent phases of the mission, an accident would most likely occur on the KSC premises. In such a case, a specially designed and equipped Radiological Control Center (RADCC) would direct radiological emergency actions on premises. These emergency actions would involve, among other things, monitoring with prepositioned field teams, ground-radiation detectors, aerial monitoring, and, possibly, sheltering or relocating personnel. In such an emergency, a nearby Federal Radiological Monitoring and Assessment Center (FRMAC) would be activated. From this center, the Department of Energy (DOE -- the RTG supplier) would supply the State of Florida with radiological monitoring and assessment data in accordance with the Federal Radiological Emergency Response Plan (FRERP). Utilizing this data, the State of Florida, in conjunction with local governments, would decide upon an appropriate course of action. This action might involve: (1) mobilizing state and local emergency personnel and issuing them recommendations regarding the general population; (2) using a public address system to instruct launch spectators to get into their cars, put their windows up and turn the air conditioning on in the recirculate mode and tune the radio to the local radio station carrying launch commentary; (3) instructing spectators to leave the area; and (4) instructing residents to remain indoors. As more detailed measurements became available, decisions on the addition or recision of precautions would be made by state and local authorities. Long-term monitoring and recovery measures would be the responsibility of the Environmental Protection Agency (EPA), the State, and other federal organizations. Accidental Re-Entry of the Spacecraft or Playload Only (No Shuttle Involvement) In the event the spacecraft failed to deploy properly, it might re-enter the Earth's atmosphere. The RTGs have been designed to withstand the atmospheric heating associated with this type of reentry. They have also been designed to withstand the soil, sand, or water impact that they would most likely suffer after reentry. In such an event, specially equipped DOE air and ground recovery teams would be dispatched to locate and recover the RTGs. If the RTGs were to be located outside the continental U.S. or its territories, the DOS would coordinate diplomatic clearance for the NASA and DOE personnel. In the case of an accidental reentry during a gravity-assisted flyby of the earth, a very low probability event, dispersion of RTG contents could occur. Were such an event to occur, the DOE would deploy an emergency response team with components of air and ground surveillance to locate any RTG components. After determining the location of these components, the DOE would then send in recovery teams to retrieve them in much the same manner as described for handling inadvertent reentry during payload deployment. ------------------------------------------------------------------------------ Galileo and its Nuclear Power Source (v. 9/19/89) NASA Fact Sheet The Galileo Mission and Its Nuclear Power Source There is great interest in Jupiter because scientists believe Jupiter contains much of the material, in its original state, from which the Sun and planets formed some 4.6 billion years ago. The similarity between Jupiter and our own Sun, in addition ot the presence of JupiterUs numerous orbiting satellites, has led to the characterization of the Jovian system as a miniature solar system. NASA's Galileo spacecraft will be the first spacecraft ever to orbit the planet. The spacecraft will also insert a probe into JupiterUs atmosphere to directly investigate the thick cloud layers covering this colossal planet. Over a period of 22 months, the orbiter will fly a series of orbits around the planet that will take it close to the Galilean satellites frequently. During this period, the spacecraft will also make extensive measurements of JupiterUs complex electromagnetic environment. This broad range of investigations can only be done in the immediate vicinity of Jupiter. Galileo will use radioisotope thermoelectric generators (RTGs) to produce the electricity that will power the spacecraft, science instruments and engineering support subsystems. Using thermocouples, RTGs convert into electricity the heat generated from the radioactive decay of plutonium-238 dioxide, a form of plutonium not used in weapons production. RTGs are lightweight, cost-effective, and highly reliable. These generators produce low levels of power, a design characteristic perfectly suited for deep space missions. The RTG's design has no moving parts. This feature greatly contributes to the gen- erator's endurance records. RTGs have been used by NASA for the peaceful exploration of our solar system for over two decades. The Apollo moon missions experiments, Viking Mars Landers, Pioneer, and Voyager missions have all used RTGs. RTGs are currently providing electricity for Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft as each travels through the outermost reaches of the solar system. The Pioneer and Voyager missions have operated flawlessly for over a decade. Voyager 2 transmission of the first close-up images of Neptune would have been impossible without the use of RTGs. While RTG's have never caused a spacecraft failure, they have been involved in three space mission accidents. In all cases, the RTGs performed exactly as designed. Early RTG models carried only small amounts of nuclear material and were built to burn up at high altitude during accidental reentry. This design requirement was met during the malfunction of the Navy's Transit 5BN spacecraft which carried a SNAP 9A RTG. Since 1964, RTGs have been designed to contain their plutonium fuel. In two later mishaps involving a mission abort (Nimbus B-1, 1968) and an earth reentry (Apollo 13, 1970), the RTGs performed according to design requirements and released no plutonium. For the Galileo mission, RTGs are the only feasible option for spacecraft power. For example, because of Jupiter's distance from the Sun and intense radiation environment, Galileo would require a 2000 square foot solar array weighing more than 1000 pounds to supply the same amount of power provided by the two RTGs, each of which weigh 124 pounds. Neither the shuttle/Inertial Upper Stage (IUS) nor the spacecraft propulsion system could accommodate the added weight of a solar array that size. In addition, the weight of batteries that would be required for Galileo's eight-year mission would be far in excess of the Shuttle's lift capability. RTGs are designed to minimize the possibility of releasing the plutonium fuel anytime during the generators' lifetime, particularly in the event of an accident. The very low probability of a plutonium release results from the protective layering design philosophy of Galileo's RTGs. Under this design approach, the plutonium is separated into marshmallow-sized ceramic fuel pellets and each pellet is contained within three layers of protective material. The layers of materials surrounding the ceramic fuel pellets are specifically designed to safeguard the plutonium from fires, explosions, fragment impacts and earth reentry. Finally, the entire assembly composed of the ceramic fuel pellets and the protective layers is surrounded by a three-quarter-inch thick packet of metal and Fiberglas(tm)-like insulation which provides additional protection from external fragments. The Federal government has established a special launch approval process for missions involving potentially hazardous material such as plutonium. This process is documented in Presidential Directive NSC-25 dated December 14, 1977. To ensure its mission's safety, the Galileo mission has undergone in-depth, multi-year safety testing and analysis over the life of the program. This safety program has involved extensive studies of credible mission accidents (where credibility is generally based on a probability of occurrence greater than 1 in a million). Data from previous launch vehicle accidents (including the Challenger) were combined with theoretical models to estimate the conditions which the RTG would experience if an accident occurred. RTGs and/or their components were subjected to fire, explosion, fragment impact, and ground impact tests over a wide range of conditions to understand how RTGs would respond in an accident. The results of this safety analysis concluded that the probability of an accident resulting in a release of plutonium ranges from 1-in-2,500 to 1-in-2 million and that any release from these low probability events would be very small. The safety analyses, including the accident probabilities and potential consequences, have been subjected to a number of reviews -- both externally and by governmental bodies. The results from the safety analysis have been incorporated in NASAUs environmental impact statement (EIS) which was widely distribued to environmental groups and the public for review and comment. Governmentally sponsored reviews have included: 1. The Nuclear Safety ad hoc Working Group, established by NASAUs safety office to provide an independent external assessment of the conduct of the safety analysis; 2. An Interagency Nuclear Safety Review Pabel (INSRP), which is required by the Presidential Directive to conduct an independent evaluation of the safety of the mission; and 3. Executive Branch review through the Office of Science and Technology Policy (OSTP). ------------------------------------------------------------------------------ Use of Nuclear Power Systems in Space (v. 9/19/89) NASA Fact Sheet Use of Nuclear Power Systems in Space Providing appropriate safety precautions are taken, NASA believes that space nuclear power systems are fully acceptable. There are two types of space nuclear power sources; Radioisotope Thermoelectric Generators (RTGs) and Reactor Power Systems. NASA currently only uses RTGs on deep space missions (200 million miles beyond Earth) or planetary science platforms (ALSEP on the Moon; Viking on Mars) where solar electric power systems would be ineffective. RTGs generate low levels of power, and are long-lived and highly reliable. For example, RTGs have operated for more than a decade on the Pioneer and Voyager spacecraft exploring the outer planets. No other systems (e.g., batteries, fuel cells, etc.) can meet these mission requirements. NASA Plans to Develop a Space Nuclear Reactor Power System NASA, the Department of Defense and the Department of Energy are currently working together to develop a Space Nuclear Power (SP-100) program. This program will provide the technology base required for space reactor power systems in the 10 kilowatt to 1 megawatt range. This program will assure sufficient power, at substantially reduced weight, for selected future Earth orbiting spacecraft, a lunar outpost, or piloted Mars missions. The SP-100 reactor power system is designed to be launched radioactively cold. After mission completion, the reactor will be shut down and stored in space for hundreds of years to ensure fission products decay to safe levels. In the event of accidental reentry, the reactor system will enter intact and remain subcritical so that fission products will no longer be generated or released. Safety Precautions Used by NASA NASA nuclear power systems are all based on the design philosophy of complete radioactive material containment. That is, in the event of reentry from earth orbit, the containers are designed to survive reentry without release any radioactivity or unclear material. Past Space Nuclear Power System Reentries The NASA Nimbus B1 satellite was launched from the then Western Test Range at Vandenberg in May 1968 and failed to reach orbit. The Nimbus B1 RTG went into the Santa Barbara Channel. Its radioisotope heat source containing nuclear material was later recovered intact and its fuel was reused on a later RTG-powered mission. There was no release of radioactive material. On Apollo 13, there was an RTG on the Lunar Excursion Module (LEM) used to power the Apollo Lunar Scientific Experiment Package. When the Apollo 13 spacecraft was forced by an accident to return to Earth without landing on the Moon, the LEM was jettisoned as the spacecraft approached the Earth. The RTG reentered over the Pacific and is in the Tonga Trench in the Pacific Ocean. Air and water samples taken in the reentry area indicated there was no release of radioactive material from the reentry of that RTG, which means that the fuel capsule survived reentry as designed. The third reentry of an RTG occurred with a Navy navigational satellite, Transit 5BN. This occurred in 1964, prior to the adoption of the full fuel containment design philosophy. The design philosophy at that time was to allow RTGs to burn up in the atmosphere in the event of reentry. Consistent with that design philosophy, the Transit 5BN RTG did burn up at an altitude of 75 miles in the vicinity of the Mozambique Channel in the Indian Ocean. This reentry released 17,000 curies of radioactive material into the atmosphere at that altitude. Questions about NASA's use of nuclear power in space not answered here should be addressed to NASA Headquarters, Washington D.C. ------------------------------------------------------------------------------ Space Nuclear Power Technology (v. 9/19/89) NASA Fact Sheet Space Nuclear Power Technology For almost three decades, both the United States and the Soviet Union have used nuclear power sources to meet the thermal and electrical energy requirements for some of their spacecraft. These power requirements include operation of the on-board scientific experiments, spacecraft maintenance and monitoring, temperature control, and data transmission. Although each country's operational program has emphasized different design philosphies, both have tested two fundamentally different approaches to the generation of energy for their space missions using nuclear power sources. These are the use of either radioisotope power generators or nuclear reactors. Radioisotope Power Generators Radioisotope power generators convert the heat (thermal energy) generated from the decay of radioisotopes to electricity. The heat may be used to produce electrical power either by a static or a dynamic conversion process. Static processes use non-moving energy conversion devices such as a thermocouple or a thermionic coverter, whereas dynamic processes employ devices with moving parts, such a a turbine or a reciprocating engine. The thermocouple (thermoelectric conversion) is based on the principle that when two dissimilar metals are joined in a closed circuit and the two junctions are kept at different temperatures, an electrical voltage, and subsequent electrical current, is produced. Thermionic converters produce electricity by boiling off electrons from a heated surface or cathode. The electrons are then captured by a collector or anode. The cathode and the anode are enclosed in a tube with conductive vapors that enhances current flows created by the directional movement of the electrons. All of the U.S. radioisotopic power generators launched to date have employed thermoelectric converters. One of the advantages of the thermoelectric generators is their simplicity: because there are no moving parts, they are highly reliable. Their disadvantage, however, is that they are relatively inefficient. Thus, for spacecraft missions that require more than a few kilowatts of power, a dynamic radioisotope generator could be a more cost-effective solution. These systems use the heat generated from the radioactive decay of radioisotopes to warm a gas or a liquid to rotate a turbine. The turbine in turn drives a generator that produces electricity. The selection of a specific radioisotope for a radioisotope power generator involves several considerations, depending upon the requirements of the spacecraft mission. First, in order to minimize fuel weight, the radioisotope selected must generate a large amount of heat from a small amount of fuel material. A second consideration is the radioisotope's longevity or half-life, the amount of time it will take for one-half of the original material to decay to another element, giving off heat in the process. If the majority of a radionuclide decays prior to the completion of the operational lifetime of the spacecraft mission, it is not a suitable fuel source for that mission. A third consideration is the type of radiation released by the radionuclide. Radionuclides that release high levels of gamma rays require careful shielding of the power source to protect launch preparation workers from external exposure to the emitted radiation. Radionuclides whose primary emissions are alpha particles require essentially no shielding. Because alpha particles cannot penetrate the skin, they are not a radiation hazard to workers unless they are absorbed into the body. In the past, because of mission power and longevity requirements, United States mission planners using nuclear power sources have relied exclusively on the use of static radioisotope converters, usually referred to as Radioisotope Thermoelectric Generators (RTGs). All designs have employed thermoelectric elements and have used Plutonium-238 as a fuel source, due to its relatively high heat to mass ratio, long half-life of 87.7 years, and low gamma ray emissions. The USSR RTG program has apparently been less ambitious than that of the United States. Although the specific design details are uncertain, most authorities think the Soviets have used Polonium-210 as a radioisotopic fuel source for most of their RTG-powered missions. Although this radionuclide has a very high heat to mass ratio and low gamma emissions, its relatively short half-life of 138.4 days makes it an attractive candidate only for relatively short-duration spacecraft missions. Nuclear Reactors For spacecraft missions with large power requirements, a nuclear reactor may be a better alternative than a radioisotope system. Nuclear reactors produce heat via the controlled bombardment of the fissile material with neutrons. The nucleus of the bombarded material, generally enriched Uranium-235, then splits into two or more fragments. The heat released by this process can then be converted directly to electricity using thermoelectric elements (static systems) or can be used to heat a gas or a liquid that rotates a turbine (dynamic systems). The turbine in turn provides power to a generator that produces electricity. This process is similiar to that employed by commercial power plants. To date, the United States has only launched one experimental nuclear reactor. The Soviet Union has used reactors for its Radar Ocean Reconnaissance Satellites (RORSATs). Both the U.S. and the Soviet reactor designs have employed thermoelectric conversion elements. Fuel Source Disposal The radioactive material and decay products in both RTGs and reactors require a safe disposal of the fuel source at the end of the mission's lifetime or in the event of a mission abort. U.S. and Soviet spacecraft designers have employed different design and disposal strategies, depending on the mission objectives. With the exception of one launch of an experimental reactor, the United State's space program has employed RTG's for their nuclear powered spacecraft. These spacecraft have been used for deep space, lunar or earth observation satellites. The earth observation satellites were placed in orbits whose altitudes were greater than 700 kilometers. From these altitudes, the natural orbital decay will require several hundred years. During this period most of the original fuel source will decay. Although the Soviets have launched at least two RTGs, their RORSATs use reactors for their power systems. Because these satellites operate at very low altitudes of around 250 kilometers, their natural orbital decay may only take days or weeks. Therefore, at the end of each mission, the power source is boosted to a much higher altitude (900 to 1000 kilometers), where the fuel source is allowed to decay for as much as 1000 years. The design philosophy for fuel containment in the event of a mission abort and earth re-entry differs between the two countries. In the United States, the current RTG design philosophy is fuel containment; that is, if a mission must be aborted the RTGs are designed to retain the fuel material when the spacecraft re-enters the earth's atmosphere. The success of this design philosophy was tested twice during the course of the US space program. In both cases, the RTGs performed as designed and no fuel was released. In the Soviet Union, the current reactor design philosophy is fuel dispersal; that is, if the spacecraft re-enters the atmosphere, the reactor is designed to allow fuel dispersal by vaporization in the atmosphere. In the late 70's the Soviets re-designed their spacecraft to ensure complete fuel vaporization by separating the fuel core from the spacecraft prior to its re-entry. The success of this design philosophy was demonstrated in 1983: a Soviet RORSAT re-entered the atmosphere over the Indian Ocean, followed by the re-entry of its fuel core over the South Atlantic. In neither case were substantial radioactive particulates found in the atmosphere after re-entry. ------------------------------------------------------------------------------ Past Space Nuclear Power System Accidents (v. 9/19/89) NASA Fact Sheet Past Accidental and Incidental Releases of Radioactive Marerial from Space Nuclear Power Sources For almost three decades, both the United States and the Soviet Union have utilized nuclear power sources (NPS) to meet some of the energy requirements of their spacecraft. The U.S. has launched twenty-three nuclear powered spacecraft and Western analysts estimate that the Soviets have launched between twenty and thirty COSMOS series satellites (Ref. 1). During this period, several launch failures, failures to achieve orbit, and accidental re-entries through the Earth's atmosphere have occurred. None of these accidents have caused measurable health effects in the human population, though some environmental contamination has occurred. Both the United States and the Soviet Union have developed nuclear generators--usually referred to as RTG's (Radioisotopic Thermoelectric Generators)--as well as space nuclear reactors (Ref. 1). Accidents Involving U.S. Space Nuclear Power Sources The United States has launched 22 missions with RTG power sources. Three accidents have occurred, though only one has resulted in release of radioactive materials. The U.S. has launched only one experimental space reactor, the SNAP 10-A in 1965. This reactor is currently in a nuclear-safe storage orbit with an estimated life of three-thousand years. The eventual re-entry of SNAP-10A will not occur until the level of radioactivity has decayed to a very low level. In the single instance of radiological release from a U.S. NPS, the RTG performed as designed. The SNAP 9-A RTG (Space Nuclear Auxiliary Power) was launched in 1964 aboard a Department of Defense weather satellite that failed to achieve polar orbit. The SNAP 9-A, designed to burn up and disperse its nuclear inventory in the upper atmosphere during re-entry, performed as planned. The release of radioactive materials was measured by scientists from the Atomic Energy Commission in air and soil sampling efforts. The objective of current U.S. RTG design philosophy is for full fuel containment; that is, in the event of an abort during the launch or on-orbit phase of a mission, the RTGs are designed to retain the fuel material. In two subsequent unplanned incidents involving U.S. RTGs, the new design philosophy successfully prevented the fuel from being released. The first involved two SNAP 19 RTGs in a 1968 meteorological satellite while the other involved one SNAP 27 RTG in the Apollo Lunar Scientific Experiment Package (ALSEP) aboard Apollo XIII in 1970. Neither of these incidents caused release of radioactive materials. The two SNAP 19's were recovered from Santa Barbara Channel five months after the range destruct of the launch vehicle. The nuclear fuel was reprocessed and later re-launched in new RTGs. No release of the fuel was detected. The mission abort maneuver of Apollo XIII separated the Command Service Module from the Lunar Module. The Lunar Module containing the SNAP 27 RTG (as part of the ALSEP) re-entered the atmosphere and impacted in the South Pacific Ocean in the region of the Tonga Trench, where it remains today. Air and water samples taken by the U.S. in the vicinity of the re-entry found no evidence of fuel release. Accidents Involving Soviet Nuclear Power Systems There have been two accidents involving Soviet RTG's, and at least three incidents involving Soviet space nuclear reactors (Ref. 1). In January of 1969, the launch failure of COSMOS 305 lunar mission with a lunar rover presumably powered by RTGs created detectable amounts of radioactivity in the upper atmosphere (Ref. 2, Ref. 3). In the fall of that year, another lunar probe failed to make a translunar injection from Earth orbit. The atmospheric burnup of this RTG also created detectable amounts of radioactivity in the upper atmosphere. Any surviving debris from these incidents is presumed to be on the floor of the ocean (Ref. 3). Soviet incidents of accidental re-entry of nuclear reactors involved COSMOS-series radar ocean reconnaissance satellites (referred to as RORSATs by U.S. analysts) (Ref. 2, Ref. 4). In April 1973, a Soviet RORSAT mission launch failure resulted in the return of the power source in the Pacific Ocean, North of Japan. Radioactive release consistent with the RORSAT mission profile was detected by U.S. air sampling planes (Ref. 2). In 1978, COSMOS 954 failed to boost into a nuclear-safe storage orbit as planned. Nuclear materials survived the fall through the atmosphere and spread over a wide area of Canada's Northwest Territory. A search and recovery effort coordinated by the Canadian government with U.S. help was undertaken after this accident. Since the cleanup operations, no detectable contamination has been found in samples of air, water, or food supplies (Ref. 5, Ref. 6, Ref. 7, Ref. 8). Soviet COSMOS 1402, another of the RORSAT series, failed to boost into a storage orbit in late 1982. The reactor core separated from the remainder of the spacecraft and was the last piece of the satellite to return to Earth in February 1983. The reactor core returned in the South Atlantic Ocean, leaving a radioactive trail through the atmosphere. It is not known whether any radioactive debris eventually reached the Earth's surface: any surviving debris is presumed to have fallen into the ocean (Ref. 8, Ref. 9). In April 1988 the Soviet radar reconnaissance satellite Cosmos 1900 failed to separate and boost the reactor core into a storage orbit. This failure of the basic system raised the possibility that the reactor could re-enter the Earth's atmosphere some time in late summer or early fall. The Soviet Union announced that the satellite was equipped with both a basic system for radiological protection and a redundant system. The redundant system apparently succeeded in separating the nuclear core of Cosmos-1900 on Sept. 30 at which time the reactor core was boosted into a "stable" storage orbit at about 720 km altitude. The intended storage orbit, however, was to have been at more than 800 km altitude. References 1. Johnson, N.L. (1986) "Nuclear power supplies in orbit.", Teledyne Brown Engineering, Colorado Springs, CO (Space Policy ISSN 0265-9646), vol. 2, August. 2. Reese, R.T. and C.P. Vick, "Soviet Nuclear Powered Satellites", Journal of the British Interplanetary Society, Vol 36, pp. 457-462, 1983. 3. Broad, W.J. (1983) "Satellite's Fuel Core Falls 'Harmlessly'", New York Times, February 8, Section C; p. 1, Science Desk. 4. Perry, G.E. (1978) "Russian Ocean Surveillance Satellites", The Royal Air Force Quarterly vol. 18, no. 1 (Spring) pp. 60-67. 5. Gummer, W.K., et al., (1980), COSMOS 954: The Occurrence and Nature of Recovered Debris, Minister of Supply and Services, Canada. 6. U.S. Department of Energy (1978) Operation Morning Light: A non-technical summary of U.S. participation. 7. Eisenbud, M. (1987) Environmental Radioactivity From Natural, Industrial and Military Sources (Academic Press). 8. Moore, A.L. and J.V. Leaphart, "Catch That Falling Star! State Responsibility and the Media in the Demise of Space Objects", Proceedings of the 26th Colloquium on the Law of Outer Space, released by the AIAA. 9. Wilford, J.N. (1983) "U.S. scans Indian Ocean for radiation", New York Times January 25, 1983, Tuesday, Late Cycle, Sec. C; p. 3; Science Desk. ------------------------------------------------------------------------------ Straight Facts About Some RTG Misconceptions (v. 9/19/89) NASA Fact Sheet RTGs Do Not Use Fission To provide power for the instrumentation and experimental equipment on each spacecraft, Radioisotope Thermoelectric Generators (RTGs) use thermocouples to convert the heat generated from the radioactive decay of non-weapons grade plutonium-238 into electricity. Unlike nuclear reactors or nuclear weapons, no fission process is involved in the operation of RTGs, nor is a fission process possible given the type of plutonium used and the design of the RTG. Under no circumstances could RTGs ever explode like a nuclear bomb. No RTG Safety Design Has Ever Failed Systems similar to Galileo's and Ulysses' RTGs have operated flawlessly on the Apollo, Pioneer, Viking and Voyager missions, some of NASA's most productive space missions. Furthermore, a review of the accidents involving RTGs demonstrates that RTGs performed exactly as designed in all cases. Early RTG models contained much less fuel than is used today and were designed to burn up and disperse their fuel at high altitudes. This design proved successful during a 1964 accident involving a SNAP 9A generator. Since 1964, RTGs have been designed to fully contain their plutonium-238 fuel in the event of an accident. A SNAP 19-B2 power generator landed intact in the Pacific ocean in May 1968 after a Nimbus-B launch vehicle failed to reach orbit. The generator was recovered and its fuel was used in a later mission. In April 1970, the Apollo 13 lunar module reentered the atmosphere and its SNAP 27 power generator fell intact in the South Pacific. In both cases, the RTGs performed according to design requirements and released no plutonium during reentry or impact. A Challenger-type Explosion Would Not Compromise the Safety Design of the Radioisotope Thermoelectric Generators Studies of the Challenger accident indicate the pressures in the explosion were well under the pressures required to cause any release of plutonium had RTGs been onboard. Studies of the explosion pressures in the Challenger's cargo bay indicate these pressures were well below the 2000 pounds per square inch (psi) to which RTGs have been tested. In testing the RTGs in a pressure environment, findings are reported using two pressure values representing different measures of the same test environment. The 2000 psi value quoted above is a static shock overpressure (i.e., the change in pressure measured on a surface parallel to the direction of a blastwave as it moves toward the RTG). A 19,600 psi value is the calculated reflected pressure (i.e., the peak pressure experienced at the front of the RTG as it is enveloped by a blastwave). Therefore, when the RTG is tested to 2000 psi static pressure, the front of the RTG test specimen experiences the 19,600 psi reflected pressure. NASA has retracted the 19,600 psi reflected pressure as plausible in various launch pad accident scenarios. Recent tests and analysis to evaluate hydrogen-oxygen explosions have led to the conclusion that the characteristics of pressures used previously for the safety analysis, and generated in tests, are not possible from a hydrogen-oxygen explosion. The worst case static pressure expected from a hydrogen-oxygen explosion is 2,075 psi. The corresponding reflected pressure, considering the low energy density of hydrogen-oxygen explosions, is predicted to be 5,300 psi -- a pressure substantially less than the 19,600 psi previously used for analysis and testing. RTGs Could Not Be Replaced by Other Power Sources for NASA's Deep Space and Planetary Missions Because of Jupiter's distance from the Sun, Galileo would require 2000 square feet of solar cells weighing more than 1000 pounds to supply the same amount of power provided by the two RTGs on the spacecraft, each of which weight 124 pounds. The shuttle could not accommodate the added weight or bulk of a solar array that size. Neither fuel cells nor batteries provide adequate power for missions as lengthy as Galileo and Ulysses. Previous Safety Analyses Conducted for the Galileo and Ulysses Missions Are No Longer Valid For the 1989 shuttle launch, Galileo will now use an Inertial Upper Stage solid rocket to boost the spacecraft on its voyage to Jupiter. Current testing and analysis show this upper stage to be a much less volatile rocket than the Centaur liquid-fueled upper stage planned for the 1986 launch. In fact, films show that the IUS rocket in the Challenger's cargo bay survived the explosion relatively intact, and fell into the ocean without igniting or exploding. The newly configured Galileo and Ulysses missions renders all scenarios in the previous 1985 safety analysis study moot. With the cancellation of the Centaur upper stage, no combination of launch vehicles available to NASA has the capability to place either the Galileo or Ulysses spacecraft on a direct trajectory to their final destination. Because of this, both spacecraft will use gravity-assist trajectories. (Gravity- assist trajectories have been successfully used by NASA in flybys of Venus, Mercury, Jupiter, Saturn and Uranus.) Galileo will first fly by Venus and then by Earth twice to gain sufficient velocity to reach Jupiter. Ulysses will fly to Jupiter to leave the ecliptic plane to explore the polar latitudes of the Sun. For Galileo, NASA is completing an extensive earth-avoidance analysis and mission design to assure there is no credible risk of inadvertent reentry during an Earth flyby. Galileo and Ulysses Both Must Complete Rigorous and Multiple Review Processes Prior to Receiving Launch Approval The Federal Government recognizes that the hazardous nature of plutonium requires a unique launch approval process for spacecraft carrying space nuclear systems. The safety analysis review process is an independent review mandated by Presidential directive involving more than a hundred people from NASA, the Departments of Energy and Defense, and a number of other federal agencies and universities, to assess safety test results and conclusions. Not until the completion of these multiple review processes, finalized by formal approval from the White House Office of Science and Technology Policy or the President, will any NASA mission carrying nuclear materials be launched. ------------------------------------------------------------------------------ SP-100 (v. 9/19/89) NASA Fact Sheet SP-100 The SP-100 is a nuclear reactor power system designed to provide power for a wide range of space missions. The nuclear reactor generates heat which is converted to electric power. The power system can be designed to generate electric power from ten thousand watts, or 10 kilowatts, up to one million watts or more. The ground engineering test program, currently underway, uses a version of the SP-100 designed to generate 100 hundred thousand (100 kWe) of electrical power. This is about the same power level as that of a typical car engine. Also, the average American at home uses about one thousand watts of power continuously, so the SP-100 can generate the energy equivalent to that required by 100 people in their homes. The goal of the program is a space electrical power source that would operate for up to ten years unattended. The power could be used for many applications such as space-based radar to provide continuous location of civil and military aircraft, direct broadcast satellites, or electric propulsion for planetary exploration. The SP-100 project was initiated in 1983 under the management of California Institute of Technology's Jet Propulsion Laboratory (JPL). It is part of a three-phase program planned to culminate in a flight demonstration in the mid-to-late 1990s. The first phase, also managed by JPL, was an evaluation through test and analysis of a large number of reactor-based power system concepts. The results of the Phase 1 work provided the basis for the current ground engineering system approach. Nuclear reactor systems have a number of special features which contribute to their suitability compared to other power systems for accomplishing many space missions. A nuclear reactor system is more compact than a solar power system. The reactor core which generates the heat is no larger than an average office wastepaper basket. The surface area of a solar-powered system is about ten times greater than the nuclear system. Size is an important consideration in choosing a power system because of the tight packaging constraints of launch vehicles. It also is important for some missions where drag caused by the atmosphere will lower a spacecraft's orbit, and on*board propulsion must be used to maintain a proper orbit. Weight is another important consideration. For systems which supply more than about 30 kilowatts, the nuclear reactor power system is lighter and much more compact than solar power systems. The extremely high costs associated with launching satellites into orbit makes weight a critical factor. Large power requirements must be considered. Above 100 kilowatts, solar arrays become less practical because of their large size (about one)quarter of an acre for a 100 kilowatt system. Nuclear reactor systems, however, retain their relatively compact size despite the higher power demand. Large amounts of electrical power will be required for future manned outposts on the Moon and Mars for everything fro scientific experiments to life support. Using the lighter and more compact SP-100 nuclear system would save billions of dollars in launch costs. The SP-100 is made up of a heat energy source, the nuclear reactor; a way to move the heat energy, the pump and coolant loops; a device for converting the heat to electricity, the converter; and a means of shedding waste heat into space, the radiator. A radiation shield and boom separate and isolate the reactor from the rest of the spacecraft and its payload. A "fast neutron spectrum" reactor design was chosen for the SP-100 reactor because it can be made small, lightweight and is basically safe and easy to control. The core of the reactor is designed especially for space applications. It's composed of small desks of enriched uranium nitride fuel contained in sealed tubes. The fuel consists of a mixture of two naturally occurring isotopes of uranium. (An isotope is a variety of the element.) One of these isotopes breaks down when struck by a neutron (fissions), releasing high energy or "fast" neutrons. Some of these neutrons interact with the uranium producing both heat and additional fast neutrons. When there are enough neutrons generated and interacting with the uranium fuel to maintain a continued fission process, a "chain reaction" is produced. In the SP-100 reactor design, there is not enough fuel by itself to sustain a chain reaction. Too many of the neutrons are ejected from the core and lost. Therefore, neutrons must be reflected back into the core by reflectors made of beryllium oxide in order for the reactor to work. When the reflectors are removed from the core, the reactor is turned off and will not sustain a chain reaction. In addition to the reflectors, the reactor is fitted with movable internal safety rods which absorb neutrons. The safety rods are in place during launch and are withdrawn for operation when the SP-100 is at the desired location for start-up in orbit. The SP-100 is inherently subcritical; it can only operate with neutron reflectors in place and safety rods out. The heat generated by the SP-100 reactor is transferred by liquid lithium which is pumped by sealed electromagnetic pumps with no moving parts and a built-in power supply. Lithium is used to transfer the reactor heat because it is relatively unaffected by the neutrons produced by the reactor. It also is a very good conductor of heat and has a low vapor pressure even at the reactor's high operating temperature of 1,350 Kelvin (about 2,000 Fahrenheit). The low pressure, about 22 pounds per square inch, less than the air pressure in a car tire, allows the safe use of lightweight structural materials for the tubes and ducts which conduct the lithium. The liquid lithium conducts the heat to the power conversion unit which changes heat into electrical energy. The power conversion unit uses a series of special semiconductors made of silicon/germanium-gallium phosphide. The semiconductors convert the heat into direct current electricity. A temperature drop of about 500 degrees K (900 F) is maintained across the thermoelectric elements by cooling. This cooling is accomplished with a second liquid lithium loop which transfers the waste heat left over from generating electricity from the converter to a heat-pipe radiator. The heat-pipe radiator dumps the waste heat into space. The radiator is made of hundreds of high temperature heat pipes attached in parallel rows radiating to space. Each heat pipe is a tube of refractory (high temperature) metal about three feet long and one half inch in diameter. A wick runs the length of each pipe and contains a small quantity of potassium. The tubes are sealed at each end. The heat pipe operates simply: The potassium is boiled at the "hot" end of the tube, absorbing a great deal of heat energy. The hot potassium vapor moves along the tube to a cooler part where it condenses and gives up its heat energy to the wall of the tube for radiation into space. After condensing, the liquid potassium is drawn back to the hot end of the tube by the wick and boiled again, and the process is repeated. The heat pipes can conduct heat at a rate many hundreds of times that of solid copper bars. Finally, because each heat pipe is separate and operates by itself, a heat pipe radiator can sustain a great deal of damage (from meteoroids, for example) and still continue to function effectively. In the SP-100 system, there are four to 12 (depending on the power level) reactor cooling system loops each with a heat pipe radiator. The cooling loops operate in parallel and each is capable of functioning independently. This feature along with extra (redundant) heat pipes in each radiator increases the system's reliability. In addition, all of the SP-100 designs of 50 kWe or more include a completely separate and independent reactor auxiliary cooling system which also generates about 1,000 watts of electrical power. This is used to cool the reactor while providing power for basic controls and standby operation in the case of loss of lithium in the reactor. This feature protects the reactor from over-heating and structural damage even if a hit by a meteoroid, or if space debris cause the loss of reactor coolant. The SP-100 includes a radiation shield which, together with a separation boom, reduces the nuclear radiation and heat to the level required by the mission user. The radiation shield is made of lithium hydride/tungsten. Conventional spacecraft techniques are used to distribute the electric power to the user portion of the spacecraft and to convert and regulate the voltage to the meet the user's requirements. The idea of using a nuclear reactor to provide power and propulsion for certain space missions has existed since the start of the space program. An experimental nuclear reactor power system, the SNAP 10A which used thermoelectric power conversion, was launched by the United States in 1965 and worked satisfactorily for 43 days until shut down. It is now in a very high orbit where it will remain for hundreds of years. Except for that one case, the use of nuclear sources for powering spacecraft built and launched by the United States been limited to very low power (less than 1/2 kWe) systems called radioisotope thermoelectric generators (RTG). They do not use nuclear reactor heat sources. RTGs convert the heat generated by the decay of radioisotopes to electricity. The fuel source in all U.S. RTGs has been plutonium-238. Most U.S. spacecraft, however, are powered by solar photovoltaic battery systems which use the sun as the primary source of energy. It is the traditional solution for most spacecraft with generally low energy needs and which operate in the inner solar system where sufficient solar energy is available. During the next fifty years, however, potential U.S. mission applications requiring space reactor power may range throughout the solar system and into interstellar space. The earliest missions may fly in the late 1990s and will probably be earth orbiting military satellites such as space-based radar systems for surveillance for missile and aircraft detection or treaty verification. The first civilian mission will probably be North Atlantic air traffic control radar system to enhance the safety of civilian aviation. Other potential missions are: direct broadcast communications satellites; an unmanned materials processing factory platform; or an outer planet electric propulsion spacecraft for solar system scientific investigation. Also a nuclear electric propulsion orbital transfer vehicle could be developed to more efficiently move payloads from one orbit to another. It is expected that most of the early missions will have unique requirements and/or constraints that will make it necessary to tailor each specific power system. The SP-100 Ground Engineering System technical approach is therefore to develop a generic of "Reference Flight System" design. The performance of this generic system over a range of operating conditions representative of a typical flight mission, is predicted from test and analysis of the individual components and large assemblies making up the system. In this way both the design approach and the analysis predicting system performance are validated. This information can then be used to develop new SP-100 space reactor system designs and configurations using the same basic components with a high degree of confidence that the power system will behave as predicted. The SP-100 is made of modular components which can be assembled to provide electric power at a desired level from 10 kilowatts to hundreds of kilowatts. The modules also can be assembled in different ways to provide the specific system configurations to meet different mission requirements. The SP-100 is designed to be launched with the reactor turned off until it is in the desired orbit. After mission completion, the reactor will be shut down and remain in orbit long enough to ensure that fission products decay to safe levels. In the event of accidental reentry, the reactor system is designed to reenter intact and to remain subcritical (turned off). NASA space power systems are all based on the design philosophy of full fuel containment. That is, in the event of reentry from earth orbit, the containers are designed to survive reentry and land without release of any nuclear material. Of the total of 38 RTGs launched by the U.S. to date, there have been three mission malfunctions involving spacecraft which carried a total of four RTGs. One of these occurred in 1964 before the full fuel containment policy was initiated. This was the SNAP 9A RTG aboard a malfunctioning Navy spacecraft and it burned up in the upper atmosphere as designed. Since 1964, the design philosophy of full fuel containment has performed flawlessly in two mission failures involving RTGs. One landed intact in the Pacific Ocean in 1968 after a Nimbus B weather satellites failed to reach orbit. The two generators were recovered and their fuel used in a subsequent mission. In 1970, the Apollo 13 lunar module reentered the atmosphere and its RTG was jettisoned and fell intact into the Tonga Trench of the Pacific Ocean. In each case air and water samples taken in the reentry area indicate there was no release of radioactive material. The development of the Space Reactor Power System involves a diverse team of industry and government laboratories with funding support and active participation by the Department of Energy, the Department of Defense and NASA.