Upload
shrishti
View
21
Download
3
Tags:
Embed Size (px)
DESCRIPTION
MMRTG is power supply of NASA's Curiosity rover i.e., Mars Science Laboratory
Citation preview
VRECPage 1
ABSRACT
The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is essentially
a nuclear battery that will operate the rover’s instruments, robotic arm, wheels, computers
and radio. It is fueled with plutonium-238 that gives off heat as it naturally decays. No
moving parts are required to generate electricity. The system uses thermocouples to create
voltage from the temperature difference between the nuclear material and the cold Martian
exterior.
The system can generate 110 watts of electrical power continuously for years. The
Mars Science Lab mission is scheduled to operate an entire Martian year (687 Earth days,
nearly 23 months) once it lands in August 2012. convert this heat into electricity
Generator is fueled with a ceramic form of plutonium dioxide encased in multiple
layers of protective materials. INL operators remotely place plutonium-filled iridium capsules
into 16 graphite impact shells. They then assemble two shells each into high-strength carbon
blocks to make eight fuel modules. These modules are then stacked and loaded into the
Multi-Mission Radioisotope Thermoelectric Generator.
RTGs work by converting heat from the natural decay of radioisotope materials into
electricity. RTGs consist of two major elements: a heat source that contains plutonium - 2 3
8dioxide and a set of solid-state thermocouples that convert the plutonium’s heat energy to
electricity. Conversion of heat directly into electricity is not a new principle. It was
discovered 150 years ago by a German scientist named Thomas Johann Seebeck . He
observed that an electric voltage is produced when two dissimilar, electrically conductive.
VRECPage 2
CONTENTS
LIST OF FIGURES
LIST OF TABLES
1. INTRODUCTION
2. RADIOISOTOPE THERMOELECTRIC GENERATOR
3. PLUTONIUM-238
4. SEEBECK EFFECT
5. THERMOCOUPLE
6. MULTI-MISSION RADIOISOTOPE THERMOELECTRIC GENERATOR
7. ADVANTAGES AND DISADVANTAGES
8. APPLICATIONS
9. CONCLUSION
10. FUTURE SCOPE
11. BIBLIOGRAPHY
VRECPage 3
LIST OF FIGURES PAGE NO
1. RTG in Cassini spacecraft 7
2. RTG in New Horizon spacecraft 7
3. RTG in Galileo mission 10
4. GPHS module 13
5. SNAP-RTG 15
6. PU-238 pallet 21
7. Seebeck effect 22
8. Thermocouple 26
9. Characteristics of thermocouple 28
10. MMRTG 30
VRECPage 4
LIST OF TABLES PAGE NO
1. RTG’s in space 18
2. RTG’s on earth 19
3. Nuclear power system in space 20
VRECPage 5
1.INTRODUCTION
Space exploration missions require safe, reliable, long-lived power systems to
provide electricity and heat to spacecraft and their science instruments. A uniquely capable
source of power is the radioisotope thermoelectric generator (RTG) – essentially a nuclear
battery that reliably converts heat into electricity.
The Department of Energy and NASA are developing a new generation of power
system that could be used for a variety of space missions. The new RTG, called a Multi-
Mission Radioisotope Thermoelectric Generator (MMRTG), is being designed to operate on
planetary bodies with atmospheres such as Mars, as well as in the vacuum of space. In
addition, the MMRTG is a more flexible modular design capable of meeting the needs of a
wider variety of missions as it generates electrical power in smaller increments, slightly
above 100 watts. The design goals for the MMRTG include ensuring a high degree of safety,
optimizing power levels over a minimum lifetime of 14 years, and minimizing weight.
Conversion of heat directly into electricity is not a new principle. It was discovered
150 years ago by a German scientist named Thomas Johann Seebeck. He observed that an
electric voltage is produced when two dissimilar, electrically conductive materials are joined
in a closed circuit and the two junctions are kept at different temperatures. Such pairs of
junctions are called thermoelectric couples (or thermocouples). The power output is a
function of the temperature of each junction and the properties of the thermoelectric
materials. The thermocouples in RTGs use heat from the natural radioactive decay of
plutonium-238 to heat the hot junction of the thermocouple, and use the cold of outer space to
produce a low temperature at the cold junction of the thermocouple.
RTGs are not a new part of the U.S. space program. In fact, they have enabled the
National Aeronautics and Space Administration (NASA) to explore the Solar System for
many years. The Apollo missions (to the Moon), the Viking missions (to Mars), and the
Pioneer, Voyager, Ulysses, Galileo, Cassini and Pluto New Horizons (outer Solar System)
missions all used RTGs.
VRECPage 6
2.RADIOISOTOPE THERMOELECTRIC GENERATOR
A radioisotope thermoelectric generator (RTG, RITEG) is an electrical generator
that uses an array of thermocouples to convert the heat released by the decay of a suitable
radioactive material into electricity by the Seebeck effect.
RTGs have been used as power sources in satellites, space probes and such
unmanned remote facilities as a series of lighthouses that the former Soviet Union erected
inside the Arctic Circle. RTGs are usually the most desirable power source for robotic or
unmaintained situations that need a few hundred watts (or less) of power for durations too
long for fuel cells, batteries, or generators to provide economically and in places where solar
cells are impractical. Safely using RTGs requires containing the radioisotopes long after the
productive life of the unit.
In the same brief letter where he introduced the communications satellite, Arthur C.
Clarke suggested that, with respect to spacecraft, "the operating period might be indefinitely
prolonged by the use of thermocouples.”
RTGs were developed in the US during the late 1950s by Mound Laboratories in
Miamisburg, Ohio under contract with the United States Atomic Energy Commission. The
project was led by Dr. Bertram C. Blanke.
The first RTG launched into space by the United States was SNAP 3 in 1961, aboard
the Navy Transit 4A spacecraft. One of the first terrestrial uses of RTGs was in 1966 by the
US Navy at uninhabited Fairway Rock in Alaska. RTGs were used at that site until 1995.
A common RTG application is spacecraft power supply. Systems for Nuclear
Auxiliary Power (SNAP) units were used for probes that traveled far from the Sun rendering
solar panels impractical. As such, they were used with Pioneer 10, Pioneer 11, Voyager 1,
Voyager 2, Galileo, Ulysses, Cassini, New Horizons and the Mars Science Laboratory. RTGs
were used to power the two Viking Landers and for the scientific experiments left on the
Moon by the crews of Apollo 12 through 17 (SNAP 27s).
VRECPage 7
Because the Apollo 13 moon landing was aborted, its RTG rests in the South Pacific
Ocean, in the vicinity of the Tonga Trench. RTGs were also used for the Nimbus, Transit and
LES satellites. By comparison, only a few space vehicles have been launched using full-
fledged nuclear reactors: the Soviet RORSAT series and the American SNAP-10A.
In addition to spacecraft, the Soviet Union constructed many unmanned lighthouses
and navigation beacons powered by RTGs. Powered by strontium-90 (90Sr), they are very
reliable and provide a steady source of power. Critics argue that they could cause
environmental and security problems as leakage or theft of the radioactive material could
pass unnoticed for years, particularly as the locations of some of these lighthouses are no
longer known due to poor record keeping. In one instance, the radioactive compartments
were opened by a thief. In another case, three woodsmen in Georgia came across two ceramic
RTG heat sources that had been stripped of their shielding. Two of the three were later
hospitalized with severe radiation burns after carrying the sources on their backs. The units
were eventually recovered and isolated.
There are approximately 1,000 such RTGs in Russia. All of them have long
exhausted their 10-year engineered life spans. They are likely no longer functional, and may
be in need of dismantling. Some of them have become the prey of metal hunters, who strip
the RTGs' metal casings, regardless of the risk of radioactive contamination.
The United States Air Force uses RTGs to power remote sensing stations for Top-
ROCC and Save-Igloo radar systems predominantly located in Alaska.
In the past, small "plutonium cells" (very small 238Pu-powered RTGs) were used in
implanted heart pacemakers to ensure a very long "battery life". As of 2004, about 90 were
still in use.
Design
The design of an RTG is simple by the standards of nuclear technology: the main
component is a sturdy container of a radioactive material (the fuel). Thermocouples are
placed in the walls of the container, with the outer end of each thermocouple connected to a
heat sink. Radioactive decay of the fuel produces heat which flows through the
thermocouples to the heat sink, generating electricity in the process.
VRECPage 8
A thermocouple is a thermoelectric device that converts thermal energy directly into
electrical energy using the Seebeck effect. It is made of two kinds of metal (or
semiconductors) that can both conduct electricity. They are connected to each other in a
closed loop. If the two junctions are at different temperatures, an electric current will flow in
the loop.
Fuels
Fig: Inspection of Cassini spacecraft RTGs before launch
Fig: New Horizons in assembly hall
Criteria
The radioactive material used in RTGs must have several characteristics:
1. It should produce high energy radiation. Energy release per decay is proportional to
power production per mole. Alpha decays in general release about 10 times as much
energy as the beta decay of strontium-90 or cesium-137.
VRECPage 9
2. Radiation must be of a type easily absorbed and transformed into thermal radiation,
preferably alpha radiation. Beta radiation can emit considerable gamma/X-ray
radiation through bremsstrahlung secondary radiation production and therefore
requires heavy shielding. Isotopes must not produce significant amounts of gamma,
neutron radiation or penetrating radiation in general through other decay modes or
decay chain products.
3. Its half-life must be so long that it will release energy at a relatively continuous rate
for a reasonable amount of time. The amount of energy released per time (power) of a
given quantity is inversely proportional to half-life. An isotope with twice the half-life
and the same energy per decay will release power at half the rate per mole. Typical
half-lives for radioisotopes used in RTGs are therefore several decades, although
isotopes with shorter half-lives could be used for specialized applications.
4. For spaceflight use, the fuel must produce a large amount of power per mass and
volume (density). Density and weight are not as important for terrestrial use unless
size is also restricted. The decay energy can be calculated if the energy of radioactive
radiation or the mass loss before and after radioactive decay is known.
Selection of isotopes
The first two criteria limit the number of possible fuels to fewer than 30 atomic
isotopes within the entire table of nuclides. Plutonium-238, curium-244 and strontium-90 are
the most often cited candidate isotopes, but other such isotopes as polonium-210,
promethium-147, caesium-137, cerium-144, ruthenium-106, cobalt-60, curium-242,
americium-241 and thulium isotopes have also been studied.
238Pu, 90Sr
Plutonium-238 has the lowest shielding requirements and longest half-life; its power
output is 0.54 kilowatts per kilogram. Only three candidate isotopes meet the last criterion
(not all are listed above) and need less than 25 mm of lead shielding to block the radiation. 238Pu (the best of these three) needs less than 2.5 mm, and in many cases no shielding is
needed in a 238Pu RTG, as the casing itself is adequate. 238Pu has become the most widely
used fuel for RTGs, in the form of plutonium (IV) oxide (PuO2). 238Pu has a half-life of 87.7
years, reasonable power density, and exceptionally low gamma and neutron radiation levels.
VRECPage 10
Strontium-90 also requires little shielding, as it decays by β emission, with negligible
γ emission. While its half life of 28.8 years is much shorter than that of 238Pu, it also has
much lower decay energy. Thus its power density is only 0.46 kilowatts per kilogram.
Because the energy output is lower it reaches lower temperatures than 238Pu, which results in
lower RTG efficiency. 90Sr is a high yield waste product of nuclear fission and is available in
large quantities at a low price.
210Po
Some prototype RTGs, first built in 1958 by the US Atomic Energy Commission, has
used polonium-210. This isotope provides phenomenal power density because of its high
radioactive activity, but has limited use because of its very short half-life of 138 days. A
kilogram of pure 210Po in the form of a cube would be about 48 mm (about 2 inches) on a side
and emit about 140 kW.
242Cm, 244Cm, 241Am
Curium-242 and curium-244 have also been studied as well, but require heavy
shielding for gamma and neutron radiation produced from spontaneous fission.
Americium-241 is a potential candidate isotope with a longer half-life than 238Pu: 241Am has a half-life of 432 years and could hypothetically power a device for centuries.
However, the power density of 241Am is only 1/4 that of 238Pu, and 241Am produces more
penetrating radiation through decay chain products than 238Pu and needs about 18 mm of lead
shielding. Even so, its shielding requirements in an RTG are the second lowest of all possible
isotopes: only 238Pu requires less. With a current global shortage of 238Pu, a closer look is
being given to 241Am.
Life span
Most RTGs use 238Pu, which decays with a half-life of 87.7 years. RTGs using this
material will therefore diminish in power output by 1−0.51/87.74 = 0.787% of their capacity per
year. 23 years after production, such an RTG will have decreased in power by 16.6%, i.e.
providing 83.4% of its initial output. Thus, with a starting capacity of 470 W, after 23 years it
would have a capacity of 392 W. However, the bi-metallic thermocouples used to convert
thermal energy into electrical energy degrade as well; at the beginning of 2001, the power
VRECPage 11
generated by the Voyager RTGs had dropped to 315 W for Voyager 1 and to 319 W for
Voyager 2. Therefore in early 2001, the RTGs were working at about 67% of their original
capacity instead of the expected 83.4%.[12]
Fig: RTG for terrestrial application
This life span was of particular importance during the Galileo mission. Originally
intended to launch in 1986, it was delayed by the Space Shuttle Challenger accident. Because
of this unforeseen event, the probe had to sit in storage for 4 years before launching in 1989.
Subsequently, its RTGs had decayed somewhat, necessitating re-planning the power budget
for the mission.
Efficiency
RTGs use thermoelectric couples or "thermocouples" to convert heat from the
radioactive material into electricity. Thermocouples, though very reliable and long-lasting,
are very inefficient; efficiencies above 10% have never been achieved and most RTGs have
efficiencies between 3–7%. Thermoelectric materials in space missions to date have included
silicon–germanium alloys, lead telluride and telluride’s 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.
VRECPage 12
A thermionic converter—an energy conversion device which relies on the principle
of thermionic emission—can achieve efficiencies between 10–20%, but requires higher
temperatures than those at which standard RTGs run. Some prototype 210Po RTGs have used
thermionic, 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 thermionic, but nuclear reactors are usually too heavy to use on most space probes.
Thermo photovoltaic 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. Thermo photovoltaic cells have efficiency slightly higher than thermocouples and
can be overlaid on top of thermocouples, potentially doubling efficiency. Systems with
radioisotope generators simulated by electric heaters have demonstrated efficiencies of
20%,[13] but have not been tested with actual radioisotopes. Some theoretical thermo
photovoltaic cell designs have efficiencies up to 30%, but these have yet to be built or
confirmed. Thermo photovoltaic cells and silicon thermocouples degrade faster than
thermocouples, especially in the presence of ionizing radiation.
Dynamic generators can provide power at more than 4 times the conversion
efficiency of RTGs. NASA and DOE have been developing a next-generation radioisotope-
fueled power source called the Stirling Radioisotope Generator (SRG) that uses free-piston
Stirling engines coupled to linear alternators to convert heat to electricity. SRG prototypes
demonstrated an average efficiency of 23%. 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
VREC
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.
The thermal efficiency, denoted η
Where primes ( ' ) denote the time derivative.
From a general form of the First Law of Thermodynamics, in rate form:
Assuming the system is operating at steady state and
ηth, then, can be calculated to be 110
SRG). Additionally, the Second Law efficiency, denoted η
Where ηth,rev is the Carnot efficiency, given by:
In which Theat sink is the external tempera
for the MMRTG (Multi-Mission RTG)
the temperature of the MMRTG,
yields a Second Law efficiency of 14.46% for the MMRTG
SRG).
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
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
The thermal efficiency, denoted ηth, is given by:
Where primes ( ' ) denote the time derivative.
From a general form of the First Law of Thermodynamics, in rate form:
Assuming the system is operating at steady state and ,
, then, can be calculated to be 110 W / 2000 W = 5.5% (or 140 W / 500 W = 28% for the
SRG). Additionally, the Second Law efficiency, denoted ηII, is given by:
is the Carnot efficiency, given by:
is the external temperature (which has been measured to be 510
Mission RTG)[which?] and 363 K for the SRG) and T
the temperature of the MMRTG,[which?] assumed 823 K (1123 K for the SRG). This
yields a Second Law efficiency of 14.46% for the MMRTG[which?] (or 41.37% for the
Page 13
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
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
W = 28% for the
ture (which has been measured to be 510 K
K for the SRG) and Theat source is
K for the SRG). This
(or 41.37% for the
VREC
Safety
Diagram of a stack of general purpose heat source
Radioactive contamination
RTGs pose a risk of radioactive contamination
leaks, the radioactive material may contaminate the environment.
For spacecraft, the main concern is that if an accident were to occur during launch
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.
However, this event is not considered likely with current RTG cask designs. For
instance, the environmental impact study for the Cassini
estimated the probability of contamination accidents at various stages in the mission. T
probability of an accident occurring which caused radioactive release from one or more of its
3 RTGs (or from its 129 radioisotope heater units
launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were
1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a
million. If an accident which ha
launch phases (such as the spacecraft failing to reach orbit), the probability of contamination
actually being caused by the RTGs was estimated at about 1 in 10. In any event, the launch
was successful and Cassini–Huygens reached
general purpose heat source modules as used in RTGs
radioactive contamination: if the container holding the fuel
leaks, the radioactive material may contaminate the environment.
For spacecraft, the main concern is that if an accident were to occur during launch
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.
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 1997
estimated the probability of contamination accidents at various stages in the mission. T
probability of an accident occurring which caused radioactive release from one or more of its
radioisotope heater units) during the first 3.5 minutes following
launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were
1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a
million. If an accident which had the potential to cause contamination occurred during the
launch phases (such as the spacecraft failing to reach orbit), the probability of contamination
actually being caused by the RTGs was estimated at about 1 in 10. In any event, the launch
Huygens reached Saturn.
Page 14
: if the container holding the fuel
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.[17]
However, this event is not considered likely with current RTG cask designs. For
Huygens probe launched in 1997
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
first 3.5 minutes following
launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were
1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a
d the potential to cause contamination occurred during the
launch phases (such as the spacecraft failing to reach orbit), the probability of contamination
actually being caused by the RTGs was estimated at about 1 in 10. In any event, the launch
VRECPage 15
The plutonium-238 used in these RTGs has a half-life of 87.74 years, in contrast to
the 24,110 year half-life of plutonium-239 used in nuclear weapons and reactors. A
consequence of the shorter half-life is that plutonium-238 is about 275 times more radioactive
than plutonium-239 (i.e. 17.3 curies (640 GBq)/g compared to 0.063 curies (2.3 GBq)/g). For
instance, 3.6 kg of plutonium-238 undergoes the same number of radioactive decays per
second as 1 ton of plutonium-239. Since the morbidity of the two isotopes in terms of
absorbed radioactivity is almost exactly the same, plutonium-238 is around 275 times more
toxic by weight than plutonium-239.
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:
1. The first one was a launch failure on 21 April 1964 in which the U.S. Transit-5BN-3
navigation satellite failed to achieve orbit and burnt up on re-entry north of
Madagascar. The 17,000 Ci (630 TBq) plutonium metal fuel in its SNAP-9a RTG was
injected into the atmosphere over the Southern Hemisphere where it burnt up, and
traces of plutonium-238 were detected in the area a few months later.
2. The second was the Nimbus B-1 weather satellite whose launch vehicle was
deliberately destroyed shortly after launch on 21 May 1968 because of erratic
trajectory. Launched from the Vandenberg Air Force Base, its SNAP-19 RTG
containing relatively inert plutonium dioxide was recovered intact from the seabed in
the Santa Barbara Channel five months later and no environmental contamination was
detected.[24]
3. In 1969 the launch of the first Lunokhod lunar rover mission failed, spreading
polonium 210 over a large area of Russia
4. The failure of the Apollo 13 mission in April 1970 meant that the Lunar Module
reentered the atmosphere carrying an RTG and burnt up over Fiji. It carried a SNAP-
27 RTG containing 44,500 Ci (1,650 TBq) of plutonium dioxide which survived
reentry into the Earth's atmosphere intact, as it was designed to do, the trajectory
being arranged so that it would plunge into 6–9 kilometers of water in the Tonga
VREC
trench in the Pacific Ocean
atmospheric and seawater sampling confirmed the assumption that the cask is intact
on the seabed. The cask is expected to contain the fuel for at least 10 half
870 years). The US Department of Energy has condu
determined that the graphite casing, which was designed to withstand reentry, is stable
and no release of plutonium should occur. Subsequent investigations have found no
increase in the natural background radiation in the area. The
represents an extreme scenario because of the high re
returning from cis-lunar space
This accident has served to validate the design of later
safe.
Fig: A SNAP-27 RTG deployed by the astronauts of
reentry of Apollo 13
There were also five failures involving Soviet or Russian spacecraft which were
carrying nuclear reactors rather than RTGs between 1973 and 1993 (see
Cosmos 954 accidentally reentered Earth's atmosphere, strewing radioactive uranium 235
over 124,000 kilometers in northern Canada, and exposing several people to harmful
radiation. This was the only time the
Pacific Ocean. The absence of plutonium-238 contamination in
atmospheric and seawater sampling confirmed the assumption that the cask is intact
on the seabed. The cask is expected to contain the fuel for at least 10 half
870 years). The US Department of Energy has conducted seawater tests and
determined that the graphite casing, which was designed to withstand reentry, is stable
and no release of plutonium should occur. Subsequent investigations have found no
increase in the natural background radiation in the area. The Apollo 13 accident
represents an extreme scenario because of the high re-entry velocities of the craft
lunar space (the region between Earth's atmosphere and the Moon).
is accident has served to validate the design of later-generation RTGs as highly
27 RTG deployed by the astronauts of Apollo 14 identical to the one lost in the
also five failures involving Soviet or Russian spacecraft which were
carrying nuclear reactors rather than RTGs between 1973 and 1993 (see RORSAT
accidentally reentered Earth's atmosphere, strewing radioactive uranium 235
over 124,000 kilometers in northern Canada, and exposing several people to harmful
s the only time the 1972 UN Liability Convention has been invoked.
Page 16
238 contamination in
atmospheric and seawater sampling confirmed the assumption that the cask is intact
on the seabed. The cask is expected to contain the fuel for at least 10 half-lives (i.e.
cted seawater tests and
determined that the graphite casing, which was designed to withstand reentry, is stable
and no release of plutonium should occur. Subsequent investigations have found no
Apollo 13 accident
entry velocities of the craft
(the region between Earth's atmosphere and the Moon).
generation RTGs as highly
identical to the one lost in the
also five failures involving Soviet or Russian spacecraft which were
RORSAT). In 1978,
accidentally reentered Earth's atmosphere, strewing radioactive uranium 235
over 124,000 kilometers in northern Canada, and exposing several people to harmful
has been invoked.
VRECPage 17
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 aero shell, 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, minimizing the risk of
vaporization and aerosolization. The ceramic is also highly insoluble.
The most recent accident involving a spacecraft RTG was the failure of the Russian
Mars 96 probe launch on 16 November 1996. The two RTGs onboard carried in total 200 g of
plutonium and are assumed to have survived reentry as they were designed to do. They are
thought to now lie somewhere in a northeast-southwest running oval 320 km long by 80 km
wide which is centered 32 km east of Iquique, Chile.
Many Beta-M RTGs produced by the Soviet Union to power lighthouses and beacons
have become orphaned sources of radiation. Several of these units have been illegally
dismantled for scrap metal resulting in the complete exposure of the Sr-90 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.
NUCLEAR FISSION
RTGs and nuclear power reactors use very different nuclear reactions. Nuclear power
reactors use controlled nuclear fission. When an atom of U-235 or Pu-239 fuel fissions,
neutrons are released that trigger additional fissions in a chain reaction at a rate that can be
controlled with neutron absorbers. This is an advantage in that power can be varied with
demand or shut off entirely for maintenance. It is also a disadvantage in that care is needed to
avoid uncontrolled operation at dangerously high power levels.
While running, nuclear reactors create high levels of particularly dangerous radiation,
like high-energy neutrons. After shutdown of a reactor, power levels drop quickly to a few
percent of the rated power, and drop further to around one per mille within one year. If a
reactor is still off ("cold") at launch, even, if is destroyed in a launch accident, the amounts of
radiation released will be rather low, as only unused fuel will be set free. Even, if a space
VRECPage 18
reactor is destroyed after having operated for some time on orbit in a reentry accident, the
amount of long-term radiation released is much less compared to an equal power rating RTG,
due to the aforementioned quick power drop.
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 now radionuclides 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.
Plutonium-238 is a fissionable material. While it cannot be used in a conventional
nuclear reactor, as plutonium-238 is not fissile with thermal (or slow) neutrons, it is
fissionable with fast neutrons, as they occur during the chain reaction of a nuclear bomb, or
inside proposed "fast" neutron reactors. The critical mass of plutonium-238 is similar to that
of plutonium-239, the fuel of the Nagasaki nuclear bomb. Some properties of plutonium-238,
namely its high decay heat and its (as compared to plutonium-239) high neutron production
rate, make building a Pu-238-bomb rather complex. Nonetheless, even a low-yield Pu-238
bomb would release much more intense mid-term (with half-lives between one year and one
hundred years) radiation, as even a high-yield Pu-239 bomb with the same amount of
Plutonium would do.
While a Pu-238 bomb would likely be a Fizzle with respect to its equivalent TNT
yield, it would likely be a very effective dirty bomb. While plutonium-238 is quite safe
outside of the human body due to the short reach of the α radiation, it becomes very unsafe
when it enters the human body, for example by inhalation of particulates: Due to the short
reach of the α-rays, radiation damage to the tissue surrounding such particulates is very high,
increasing the risk of cancer.
VRECPage 19
Models
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:
Space
Name & Model
Used On (# of RTGs per User)
Maximum output Radio-isotope
Max fuelused (kg)
Mass (kg)Electrical
(W)Heat (W)
ASRG* prototype design (not launched), Discovery Program
~140 (2x70)
~500 238Pu ~1 ~34
MMRTG MSL/Curiosity rover ~110 ~2000 238Pu ~4 <45GPHS-RTG
Cassini (3), New Horizons (1), Galileo (2), Ulysses (1)
300 4400 238Pu 7.8 55.9–57.8[33]
MHW-RTG
LES-8/9, Voyager 1 (3), Voyager 2 (3)
160[33] 2400[34] 238Pu ~4.5 37.7[33]
SNAP-3B Transit-4A (1) 2.7[33] 52.5 238Pu ? 2.1[33]
SNAP-9A Transit 5BN1/2 (1) 25[33] 525[34] 238Pu ~1 12.3[33]
SNAP-19 Nimbus-3 (2), Pioneer 10 (4), Pioneer 11 (4)
40.3[33] 525 238Pu ~1 13.6[33]
modified SNAP-19
Viking 1 (2), Viking 2 (2)
42.7[33] 525 238Pu ~1 15.2[33]
SNAP-27 Apollo 12–17 ALSEP(1)
73 1,480 238Pu[35] 3.8 20
Buk (BES-5)**
US-As (1) 3000 100,000 235U 30 ~1000
SNAP-10A***
SNAP-10A (1) 600[36] 30,000 Enriched uranium
431
* The ASRG is not really a RTG, it uses a stirling power device that runs on radioisotope (see stirling radioisotope generator)
** The BES-5 Buk (БЭС-5) reactor was a fast breeder reactor which used thermocouples based on semiconductors to convert heat directly into electricity.[37][38]
VRECPage 20
*** The SNAP-10A used enriched uranium fuel, zirconium hydride as a moderator, liquid sodium potassium alloy coolant, and was activated or deactivated with beryllium reflectors.[36] Reactor heat fed a thermoelectric conversion system for electrical production.[36]
Terrestrial
Name &
Model
Use Maximum output Radioisotope Max
fuel
used
(kg)
Mass
(kg)Electrical
(W)
Heat
(W)
Beta-M Obsolete Soviet
unmanned
lighthouses &
beacons
10 230 90Sr 0.26 560
Efir-MA 30 720 ? ? 1250
IEU-1 80 2200 ? ? 2500
IEU-2 14 580 ? ? 600
Gong 18 315 ? ? 600
Gorn 60 1100 90Sr ? 1050
IEU-2M 20 690 ? ? 600
IEU-1M 120 (180) 2200
(3300)
? ? 2(3) ×
1050
Sentinel
25[39]
Remote U.S. arctic
monitoring sites
9–20 SrTiO3 0.54 907–
1814
Sentinel
100F[39]
53 Sr2TiO4 1.77 1234
Nuclear power systems in space
Known spacecraft nuclear power systems and their fate. Systems face a variety of
fates, for example, Apollo's SNAP-27 was left on the Moon. Some other spacecraft also have
small radioisotope heaters, for example each of the Mars Exploration Rovers have a 1 watt
radioisotope heater. Spacecraft use different amounts of material, for example MSL Curiosity
has 4.8 kg of plutonium-238 dioxide, while the Cassini spacecraft has 32.7 kg.
VRECPage 21
Name and/or model Launched Fate/location
MSL/Curiosity rover MMRTG (1) 2011 Mars surface
Apollo 12 SNAP-27 ALSEP 1969 Lunar surface (Ocean of Storms)[40]
Apollo 13 SNAP-27 ALSEP 1970 Earth re-entry (over Pacific nr Fiji)
Apollo 14 SNAP-27 ALSEP 1971 Lunar surface (Fra Mauro)
Apollo 15 SNAP-27 ALSEP 1971 Lunar surface (Hadley–Apennine)
Apollo 16 SNAP-27 ALSEP 1972 Lunar surface (Descartes Highlands)
Apollo 17 SNAP-27 ALSEP 1972 Lunar surface (Taurus–Littrow)
Transit-4A SNAP-3B? (1) 1961 Earth orbit
Transit 5A3 SNAP-3 (1) 1963 Earth orbit
Transit 5BN-1 SNAP-3 (1) 1963 Earth orbit
Transit 5BN-2 SNAP-9A (1) 1963 Earth orbit
Transit 9 1964 Earth orbit
Transit 5B4 1964 Earth orbit
Transit 5B6 1965 Earth orbit
Transit 5B7 1965 Earth orbit
Transit 5BN-3 SNAP-9A (1) 1964 Failed to reach orbit[43]
Nimbus-B SNAP-19 (2) 1968 Recovered after crash
Nimbus-3 SNAP-19 (2) 1969 Earth re-entry 1972
Pioneer 10 SNAP-19 (4) 1972 Ejected from Solar System
Pioneer 11 SNAP-19 (4) 1973 Ejected from Solar System
Viking 1 lander modified SNAP-19 1976 Mars surface (Chryse Planitia)
Viking 2 lander modified SNAP-19 1976 Mars surface
Cassini GPHS-RTG (3) 1997 Orbiting Saturn
New Horizons GPHS-RTG (1) 2006 Leaving the Solar System
Galileo GPHS-RTG (2), 1989 Jupiter atmospheric entry
Ulysses GPHS-RTG (1) 1990 Heliocentric orbit
LES-8 MHW-RTG 1976 Near geostationary orbit
LES-9 MHW-RTG 1976 Near geostationary orbit
Voyager 1 MHW-RTG(3) 1977 Ejected from Solar System
Voyager 2 MHW-RTG(3) 1977 Ejected from Solar System
VRECPage 22
3. PLUTONIUM-238
Plutonium-238 (also known as Pu-238 or 238Pu) is a radioactive isotope of
plutonium that has a half-life of 87.7 years.
Plutonium-238 is a very powerful alpha emitter and – unlike other isotopes of
plutonium – it does not emit significant amounts of other, more penetrating and thus more
problematic radiation. This makes the plutonium-238 isotope suitable for usage in
radioisotope thermoelectric generators (RTGs) and radioisotope heater units – one gram of
plutonium-238 generates approximately 0.5 watts of thermal power.
Fig: PU-238 pallet
Plutonium-238 was the first isotope of plutonium to be discovered. It was synthesized
by Glenn Seaborg and associates in 1941 by bombarding uranium-238 with deuterons,
creating Neptunium-238, which then decays to form plutonium-238. Plutonium-238 decays to
uranium-234 and then further along the radium series to lead-206.
The main application of Pu-238 is as the heat source in radioisotope thermoelectric
generators (RTGs). RTG technology was first developed by Los Alamos National Laboratory
during the 1960s and 1970s to provide radioisotope thermoelectric generator power for
cardiac pacemakers. Of the 250 plutonium-powered pacemakers Medtronic manufactured,
twenty-two were still in service more than twenty-five years later, a feat that no battery-
powered pacemaker could achieve. This same RTG power technology has been used in
spacecraft such as Voyager 1 and 2, Cassini–Huygens and New Horizons, and in other
devices, such as the Mars Science Laboratory, for long-term nuclear power generation.
VREC
A thermoelectric circuit composed of materials of different Seebeck coefficient (p
doped and n-doped semiconductors), configured as a
resistor at the bottom is replaced with a
sensing thermocouple.
The Seebeck effect is the conversion of
electricity and is named after the
1821, discovered that a compass needle would be deflected by a closed loop formed by two
different metals joined in two places, with a temperature difference between the junctions.
This was because the metals responded differently to the temperature difference, creating a
current loop and a magnetic field
involved, so he called the phenomenon the thermo
Christian Orsted rectified the mistake and coined the term "thermoelectricity".
Fig: seebeck
The Seebeck effect is a classic example of an
measurable currents or voltages in the same way as any other emf. Electrom
modify Ohm's law by generating currents even in the absence of voltage differences (or vice
versa); the local current density is given by
4.SEEBECK EFFECT
A thermoelectric circuit composed of materials of different Seebeck coefficient (p
doped semiconductors), configured as a thermoelectric generator
resistor at the bottom is replaced with a voltmeter the circuit then functions as a temperature
is the conversion of temperature differences directly into
and is named after the Baltic German physicist Thomas Johann Seebeck
1821, discovered that a compass needle would be deflected by a closed loop formed by two
tals joined in two places, with a temperature difference between the junctions.
This was because the metals responded differently to the temperature difference, creating a
magnetic field. Seebeck did not recognize there was an electric current
involved, so he called the phenomenon the thermo magnetic effect. Danish physicist
rectified the mistake and coined the term "thermoelectricity".
Fig: seebeck effect
The Seebeck effect is a classic example of an electromotive force (emf) and leads to
measurable currents or voltages in the same way as any other emf. Electrom
by generating currents even in the absence of voltage differences (or vice
is given by
Page 23
A thermoelectric circuit composed of materials of different Seebeck coefficient (p-
thermoelectric generator. If the load
the circuit then functions as a temperature-
differences directly into
Thomas Johann Seebeck, who, in
1821, discovered that a compass needle would be deflected by a closed loop formed by two
tals joined in two places, with a temperature difference between the junctions.
This was because the metals responded differently to the temperature difference, creating a
. Seebeck did not recognize there was an electric current
magnetic effect. Danish physicist Hans
(emf) and leads to
measurable currents or voltages in the same way as any other emf. Electromotive forces
by generating currents even in the absence of voltage differences (or vice
VREC
where is the local voltage and
described locally by the creation of an electromotive field
where is the Seebeck coefficient
material, and is the gradient in temperature
The Seebeck coefficients generally vary as function of temperature, and depend
strongly on the composition of the conductor. For ordinary materials at room temperature, the
Seebeck coefficient may range in value from
If the system reaches a steady state where
simply by the emf:
conductivity, is used in the thermocouple
temperature may be found by performing the voltage measurement at a known reference
temperature. A metal of unknown composition can be classified by its thermoelectric effect if
a metallic probe of known composition
with the unknown sample that is locally heated to the probe temperature. It is used
commercially to identify metal alloys. Thermocouples in series form a
Thermoelectric generators are used for creating power from heat differentials.
The Seebeck effect is used in thermoelectric
engines, but are less bulky, have no moving parts, and are typically more expensive and less
efficient. They have a use in power plants for convert
power (a form of energy recycling
generators (ATGs) for increasing fuel efficiency. Space probes often use
thermoelectric generators with the same mechanism but using radioisotopes to generate the
required heat difference.
and is the local conductivity. In general the Seebeck effect is
described locally by the creation of an electromotive field
Seebeck coefficient (also known as thermopower), a property of the local
in temperature .
coefficients generally vary as function of temperature, and depend
strongly on the composition of the conductor. For ordinary materials at room temperature, the
Seebeck coefficient may range in value from −100 μV/K to +1,000 μV/K
steady state where , then the voltage gradient is given
. This simple relationship, which does not depend on
thermocouple to measure a temperature difference; an absolute
temperature may be found by performing the voltage measurement at a known reference
temperature. A metal of unknown composition can be classified by its thermoelectric effect if
a metallic probe of known composition is kept at a constant temperature and held in contact
with the unknown sample that is locally heated to the probe temperature. It is used
commercially to identify metal alloys. Thermocouples in series form a
are used for creating power from heat differentials.
The Seebeck effect is used in thermoelectric generators, which function like
, but are less bulky, have no moving parts, and are typically more expensive and less
efficient. They have a use in power plants for converting waste heat into additional electrical
energy recycling), and in automobiles as automotive thermoelectric
(ATGs) for increasing fuel efficiency. Space probes often use
with the same mechanism but using radioisotopes to generate the
Page 24
. In general the Seebeck effect is
), a property of the local
coefficients generally vary as function of temperature, and depend
strongly on the composition of the conductor. For ordinary materials at room temperature, the
, then the voltage gradient is given
. This simple relationship, which does not depend on
e a temperature difference; an absolute
temperature may be found by performing the voltage measurement at a known reference
temperature. A metal of unknown composition can be classified by its thermoelectric effect if
is kept at a constant temperature and held in contact
with the unknown sample that is locally heated to the probe temperature. It is used
commercially to identify metal alloys. Thermocouples in series form a thermopile.
generators, which function like heat
, but are less bulky, have no moving parts, and are typically more expensive and less
into additional electrical
automotive thermoelectric
(ATGs) for increasing fuel efficiency. Space probes often use radioisotope
with the same mechanism but using radioisotopes to generate the
VRECPage 25
5. THERMOCOUPLE
A thermocouple is a temperature-measuring device consisting of two dissimilar
conductors that contact each other at one or more spots. It produces a voltage when the
temperature of one of the spots differs from the reference temperature at other parts of the
circuit. Thermocouples are a widely used type of temperature sensor for measurement and
control, and can also convert a temperature gradient into electricity. Commercial
thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and
can measure a wide range of temperatures. In contrast to most other methods of temperature
measurement, thermocouples are self powered and require no external form of excitation. The
main limitation with thermocouples is accuracy; system errors of less than one degree Celsius
(°C) can be difficult to achieve.
Any junction of dissimilar metals will produce an electric potential related to
temperature. Thermocouples for practical measurement of temperature are junctions of
specific alloys which have a predictable and repeatable relationship between temperature and
voltage. Different alloys are used for different temperature ranges. Properties such as
resistance to corrosion may also be important when choosing a type of thermocouple. Where
the measurement point is far from the measuring instrument, the intermediate connection can
be made by extension wires which are less costly than the materials used to make the sensor.
Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius;
practical instruments use electronic methods of cold-junction compensation to adjust for
varying temperature at the instrument terminals. Electronic instruments can also compensate
for the varying characteristics of the thermocouple, and so improve the precision and
accuracy of measurements.
Thermocouples are widely used in science and industry; applications include
temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial
processes. Thermocouples are also used in homes, offices and businesses as the temperature
sensors in thermostats, and also as flame sensors in safety devices for gas-powered major
appliances.
VREC
In 1821, the German–Estonian
when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now
known as the thermoelectric effect
necessarily involves connecting another conductor to the "hot" end. This additional conductor
will then also experience the temperature gradient, and develop a voltage of its own which
will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use.
Using a dissimilar metal to complete the circuit creates a circuit in which the two legs
generate different voltages, leaving a small difference in voltage available for measurement.
That difference increases with temperature, and is between 1 and 70 micro
Celsius (µV/°C) for standard metal combinations.
Fig: Thermocouple measuring circuit
The voltage is not generated at the junction of t
rather along that portion of the length of the two dissimilar metals that is subjected to a
temperature gradient. Because both lengths of dissimilar metals experience the same
temperature gradient, the end result is a
between the thermocouple junction and the reference junction. As long as the junction is at a
uniform temperature, it does not matter how the junction is made (it may be brazed, spot
welded, crimped, etc.), however it is crucial for accuracy that the
Estonian physicist Thomas Johann Seebeck discovered that
ctor is subjected to a thermal gradient, it will generate a voltage. This is now
thermoelectric effect or Seebeck effect. Any attempt to measure this voltage
necessarily involves connecting another conductor to the "hot" end. This additional conductor
will then also experience the temperature gradient, and develop a voltage of its own which
Fortunately, the magnitude of the effect depends on the metal in use.
Using a dissimilar metal to complete the circuit creates a circuit in which the two legs
generate different voltages, leaving a small difference in voltage available for measurement.
t difference increases with temperature, and is between 1 and 70 micro volts per degree
Celsius (µV/°C) for standard metal combinations.
measuring circuit
The voltage is not generated at the junction of the two metals of the thermocouple but
rather along that portion of the length of the two dissimilar metals that is subjected to a
temperature gradient. Because both lengths of dissimilar metals experience the same
temperature gradient, the end result is a measurement of the difference in temperature
between the thermocouple junction and the reference junction. As long as the junction is at a
uniform temperature, it does not matter how the junction is made (it may be brazed, spot
ver it is crucial for accuracy that the leads of the thermocouple
Page 26
discovered that
ctor is subjected to a thermal gradient, it will generate a voltage. This is now
effect. Any attempt to measure this voltage
necessarily involves connecting another conductor to the "hot" end. This additional conductor
will then also experience the temperature gradient, and develop a voltage of its own which
Fortunately, the magnitude of the effect depends on the metal in use.
Using a dissimilar metal to complete the circuit creates a circuit in which the two legs
generate different voltages, leaving a small difference in voltage available for measurement.
volts per degree
he two metals of the thermocouple but
rather along that portion of the length of the two dissimilar metals that is subjected to a
temperature gradient. Because both lengths of dissimilar metals experience the same
measurement of the difference in temperature
between the thermocouple junction and the reference junction. As long as the junction is at a
uniform temperature, it does not matter how the junction is made (it may be brazed, spot
of the thermocouple
VREC
maintain a well-defined composition. If there are variations in the composition of the wires in
the thermal gradient region (due to contamination, oxidation, etc.), outside the junction, thi
can lead to changes in the measured voltage
Derivation from Seebeck effect
Upon heating, the Seebeck effect will initially drive a current. However, provided the
junctions all reach a uniform internal temperature, and provided an ideal voltmeter is us
then the thermocouple will soon reach an equilibrium where no current will flow anywhere (
). As a result, the voltage gradient at any point in the circuit will be given simply by
, where
the temperature gradient at that point. The total measured end
adding up the voltage contributions all along the wires.
This leads to a measured voltage difference independent of many details (e.g. neither
the size nor the length of the conductors matter):
where and are the Seebeck coefficients
temperature, and and are the temperatures of the two junctions. The voltages
are measured at the cold ends of materials A and B, respectively (see figure). The emf is not
generated at the junctions, but rather in the wires leading between the hot and cold junctions
(where ). Because the two wires give different voltages leading u
the resulting measured overall voltage is nonzero.
Thermocouple characteristic function
If the Seebeck coefficients are effectively constant for the measured temperature range, the
above formula can be approximated as
case, however it is possible to completely characterize the thermocouple with a
characteristic function E(T), defined as:
defined composition. If there are variations in the composition of the wires in
the thermal gradient region (due to contamination, oxidation, etc.), outside the junction, thi
can lead to changes in the measured voltage
Derivation from Seebeck effect
Upon heating, the Seebeck effect will initially drive a current. However, provided the
junctions all reach a uniform internal temperature, and provided an ideal voltmeter is us
then the thermocouple will soon reach an equilibrium where no current will flow anywhere (
). As a result, the voltage gradient at any point in the circuit will be given simply by
, where is the Seebeck coefficient at that point, and
ure gradient at that point. The total measured end-to-end voltage can be found by
adding up the voltage contributions all along the wires.
This leads to a measured voltage difference independent of many details (e.g. neither
conductors matter):
Seebeck coefficients of materials A and B as a function of
are the temperatures of the two junctions. The voltages
are measured at the cold ends of materials A and B, respectively (see figure). The emf is not
generated at the junctions, but rather in the wires leading between the hot and cold junctions
). Because the two wires give different voltages leading up to the junction,
the resulting measured overall voltage is nonzero.
Thermocouple characteristic function
If the Seebeck coefficients are effectively constant for the measured temperature range, the
above formula can be approximated as . In general this
case, however it is possible to completely characterize the thermocouple with a
), defined as:
Page 27
defined composition. If there are variations in the composition of the wires in
the thermal gradient region (due to contamination, oxidation, etc.), outside the junction, this
Upon heating, the Seebeck effect will initially drive a current. However, provided the
junctions all reach a uniform internal temperature, and provided an ideal voltmeter is used,
then the thermocouple will soon reach an equilibrium where no current will flow anywhere (
). As a result, the voltage gradient at any point in the circuit will be given simply by
is the Seebeck coefficient at that point, and is
end voltage can be found by
This leads to a measured voltage difference independent of many details (e.g. neither
of materials A and B as a function of
are the temperatures of the two junctions. The voltages Vb and Vc
are measured at the cold ends of materials A and B, respectively (see figure). The emf is not
generated at the junctions, but rather in the wires leading between the hot and cold junctions
p to the junction,
If the Seebeck coefficients are effectively constant for the measured temperature range, the
. In general this is not the
case, however it is possible to completely characterize the thermocouple with a
VREC
This function characterizes the thermocouple completely and is uniquely defined up to a
constant of integration. Often the constant is chosen such that
voltage can be found by consulting a precomputed table of values of the characteristic
function at two places (the hot temperature and the cold temperature). In the example above,
.
Fig: characteristics of thermocouple
Thermocouple manufacturers and metrology standards organizations such as
tables of the function calculated over a range of temperatures, for particular
thermocouple types These tables are computed from
mathematical functions (typically
true characteristic function.
This function characterizes the thermocouple completely and is uniquely defined up to a
e constant is chosen such that . The measured
voltage can be found by consulting a precomputed table of values of the characteristic
(the hot temperature and the cold temperature). In the example above,
Fig: characteristics of thermocouple
Thermocouple manufacturers and metrology standards organizations such as NIST
calculated over a range of temperatures, for particular
These tables are computed from reference functions which are simple
mathematical functions (typically piecewise polynomials) fitted to closely approximate the
Page 28
This function characterizes the thermocouple completely and is uniquely defined up to a
. The measured
voltage can be found by consulting a precomputed table of values of the characteristic
(the hot temperature and the cold temperature). In the example above,
NIST provide
calculated over a range of temperatures, for particular
which are simple
) fitted to closely approximate the
VRECPage 29
6.MULTI-MISSION RADIOISOTOPE THERMOELECTRIC
GENERATOR
The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is a type
of Radioisotope Thermoelectric Generator developed for NASA space missions[1] such as the
Mars Science Laboratory (MSL), under the jurisdiction of the United States Department of
Energy's Office of Space and Defense Power Systems within the Office of Nuclear Energy.
The MMRTG was developed by an industry team of Aero jet Rocket dyne and Teledyne
Energy Systems.
Background
Space exploration missions require safe, reliable, long-lived power systems to
provide electricity and heat to spacecraft and their science instruments. A uniquely capable
source of power is the Radioisotope Thermoelectric Generator (RTG) – essentially a nuclear
battery that reliably converts heat into electricity.
Function
RTGs convert the heat from the natural decay of a radioisotope into electricity. The
MMRTG's heat source is plutonium-238 dioxide. Solid-state thermoelectric couples convert
the heat to electricity. Unlike solar arrays, the RTGs are not dependent upon the sun, so they
can be used for deep space missions.
History
In June 2003, the Department of Energy (DOE) awarded the MMRTG contract to a
team led by Aero jet Rocket dyne. Aero jet Rocket dyne and Teledyne Energy Systems
collaborated on an MMRTG design concept based on a previous thermoelectric converter
design, SNAP-19, developed by Teledyne for previous space exploration missions.[4] SNAP-
19s powered Pioneer 10 and Pioneer 11 missions as well as the Viking 1 and Viking 2
Landers.
VRECPage 30
Design and specifications
The MMRTG is powered by 8 Pu-238 dioxide GPHS modules, provided by the
Department of Energy. Initially, these 8 GPHS modules generate about 2 kW thermal power.
The MMRTG design incorporates PbTe/TAGS thermoelectric couples (from
Teledyne Energy Systems). The MMRTG is designed to produce 125 W electrical power at
the start of mission, falling to about 100 W after 14 years. With a mass of 45 kg the MMRTG
provides about 2.8 W/kg of electrical power at beginning of life.
The MMRTG design is capable of operating both in the vacuum of space and in
planetary atmospheres, such as on the surface of Mars. Design goals for the MMRTG
included ensuring a high degree of safety, optimizing power levels over a minimum lifetime
of 14 years, and minimizing weight.
Usage in space missions
Fig: The Multi-Mission Radioisotope Thermoelectric Generator of Mars Science Laboratory.
VRECPage 31
Radioisotope power has been used on 8 Earth orbiting missions, 8 missions travelling
to each of the outer planets as well as each of Apollo missions following 11 to Earth's moon.
Some of the outer Solar System missions are the Pioneer, Voyager, Ulysses, Galileo, Cassini
and Pluto New Horizons missions. The RTGs on Voyager 1 and 2 have been operating since
1977. Similarly, Radioisotope Heat Units (RHUs) were used to provide heat to critical
components on Apollo 11 as well as the first two generations of Mars rovers. In total, over
the last four decades, 26 missions and 45 RTGs have been launched in the United States.
MMRTG specifically
Curiosity, the MSL rover that was successfully landed in Gale Crater on August 6,
2012, uses one MMRTG to supply heat and electricity for its components and science
instruments. Reliable power from the MMRTG will allow it to operate for at least one Mars
year (687 Earth days).
On Nov. 20, 2013, NASA reported suspending operations on the Mars Curiosity rover
in order to diagnose an electrical problem first observed on Nov. 17. Apparently, an internal
short in the rover's power source, the MMRTG caused an unusual and intermittent decrease
in a voltage indicator on the rover, though power output was unaffected. On Nov. 23, 2013,
the short had cleared, and Curiosity resumed full science operations, with no apparent loss of
capability.
MMRTG is specially designed to power CUROISITY rover which is a car-sized
robotic rover exploring Gale Crater on Mars as part of NASA's Mar Science Laboratory
mission (MSL).
Curiosity was launched from Cape Canaveral on November 26, 2011, at 10:02 EST
aboard the MSL spacecraft and successfully landed on Aeolis Palus in Gale Crater on Mars
on August 6, 2012, 05:17 UTC. The Bradbury Landing site was less than 2.4 km (1.5 mi)
from the center of the rover's touchdown target after a 563,000,000 km (350,000,000 mi)
journey.
VRECPage 32
7. ADVANTAGES AND DISADVANTAGES
Advantages:
Long life span, works more than 20 years efficiently.
Dust formation can’t effect power generation.
Works continuously without decreasing output.
Only heat source must be provided, no need of heat sink.
Very less risk of contamination of radioactive material
Disadvantages:
If radioactive material is inhaled, it damages body parts.
It is very good only for unmanned missions.
VRECPage 33
8. APPLICATIONS
Till now 44 RTG’s are used in entire 22 missions in space.
A common RTG application is spacecraft power supply. Systems for Nuclear
Auxiliary Power (SNAP) units were used for probes that traveled far from the Sun rendering
solar panels impractical.
RTG’s powered the following missions from past 50 years,
Pioneer 10
Pioneer 11
Voyager 1
Voyager 2
Galileo
Ulysses
Cassini
New Horizons
Mars Science Laboratory
RTGs were used to power the two Viking Landers and for the scientific experiments left on
the Moon by the crews of Apollo 12 through 17 (SNAP 27s)
VRECPage 34
9. CONCLUSION
MMRTG and RTG’s are best suited for spacecrafts and rover power applications
compared to solar panels. As we go far from sun radiation will be fading and we can’t use
solar panels as they a\ generate electricity depending on solar radiation.
RTG are very good as power sources in satellites, space probes and such unmanned
remote facilities as a series of lighthouses.
Further developments are taking place to increase efficiency of radioisotope
thermoelectric generators by using stirling techniques.
VRECPage 35
11. FUTURE SCOPE
To further increase efficiency NASA trying to develop advanced Stirling
radioisotope generator
It would have used a Stirling power conversion technology, to convert radioactive-
decay heat into electricity for use on spacecraft.
Development was undertaken under joint sponsorship by the United States
Department of Energy and NASA for potential future space missions.
The development was cancelled in 2013 after the cost had risen to over 260 million
US dollars, 110 million more than originally expected.
The higher conversion efficiency of the Stirling cycle compared with that of
radioisotope thermoelectric generators (RTGs) used in previous missions (Viking,
Pioneer, Voyager, Galileo, Ulysses, Cassini, New Horizons, and Mars Science
Laboratory) would have offered an advantage of a fourfold reduction in PuO2 fuel, at
half the mass of an RTG.
VRECPage 36
11. BIBLIOGRAPHY
� Peacetime Uses for V2 2 (2). Wireless World. February 1945. p. 58.
� Peacetime Uses for V2: scanned image of the original Letter to the Editor 2 (2). Wireless World. February 1945.
� "Nuclear Battery-Thermocouple Type Summary Report". United States Atomic Energy Commission (published 15 January 1962). 1 October 1960.
� "General Safety Considerations" (pdf lecture notes). Fusion Technology Institute, University of Wisconsin–Madison. Spring 2000. p. 21.
� "Radioisotope Thermoelectric Generators". Bellona. 2 April 2005. Retrieved 2013-05-07.
� "IAEA Bulletin Volume 48, No.1 – Remote Control: Decommissioning RTGs". Malgorzata K. Sneve. Retrieved 11 July 2009.
� "Report by Minister of Atomic Energy Alexander Rumyantsev at the IAEA conference
"Security of Radioactive Sources," Vienna, Austria. March 11th 2003 (Internet Archive
copy)". Archived from the original on 6 August 2003. Retrieved 10 October 2009
By
Shrishti Raas
VRECPage 37