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To The Seminar on
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RADIOISOTOPE THERMOELECTRIC GENERATOR
(RTG)
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INTRODUCTION
Radioisotope Thermoelectric Generator is a electrical generator.
It uses a radioactive material as the fuel and uses the fact that radioactive materials
generate heat as they decay into nonradioactive materials.
This released heat is converted in to electricity by using Seebeck effect using an
array of thermocouples.
The output obtained in a RTG is a steady output voltage and its power capacity is
a few 100 W.
RTG provides an uninterrupted and reliable source of heat and electricity in remote
and harsh environment such as deep space.
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Considered as a type of battery and so used as power sources in satellites,
space probes and unmanned remote facilities.
It provides power and heat for spacecrafts to many years.
Also known as space batteries or nuclear batteries.
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HISTORYThe first RTG launched in space by the United States was in 1961 aboard the
SNAP 3 in Navy Transit 4 A spacecraft.
One of the first terrestrial uses of RTG was in
1966 by US Navy at the uninhabitted Fairway
Rock island in Alaska, where it remained in use
until its removal in 1995.
Used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini,
and New Horizons.
New Horizon in assembly hall
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Used to power two Viking landers and for the scientific experiments left on the Moon
by the Appollo 12.
In addition to spacecraft, the Soviet Union constructed many unmanned lighthouses
and navigation beacons powered by RTGs. There are approximately 1000 such RTGs
in Russia.
However, criticed argue that they could cause environmental and security
problems, as leakage or theft of the radio active material could pass unnoticed
for years (or possibly forever: some of these light houses cannot be found
because of poor record keeping).
Utilized by the United States Air Force to power remote sensing stations.
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In the past, small “plutonium cells” were used in implanted heart pacemakers
to ensure a very long battery life.
Although not strictly RTGs, similar units called radioisotopes heater units are
also used by various spacecraft including the Mars Exploration Rovers,
Galileo and Cassini.
RTGs were also used for the Nimbus, Transit and Les satellites.
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DESIGNwww.etraid.in
The ASRG EU
The EU contains two Advanced Stirling Convertor (ASC)-E convertors secured together through an interconnect sleeve. An electric heat source (EHS), held against each ASC-E heat collector, provides the heat input. The cold-side adapter flanges (CSAFs) conduct heat rejected from the convertors away through the beryllium housing and fins, to be then radiated away in a vacuum environment or convected to air. Argon fills the housing, sealed using various O-rings and gaskets.
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A gas management valve allows access to the argon. The controller is mounted to the outside of the housing. Connectors on the end enclosures, housing, and controller provide electrical interfaces to the alternators, sensors, power input and output, control, and telemetry. The EU is mounted via four mounting tabs on one end of the housing, bolting to a spacecraft interface or directly to its support.
The ASRG EU www.etraid.in
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ASRG EU test facility.
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Based on the standards of nuclear technology.
Main component is a sturdy container, full of radioactive material (fuel).
Walls of the container are pierced by thermocouples.
Other end of the thermocouple is connected to a heat sink.
Passive radioactive decay in radioactive material causes it to produce heat.
Heat flow through thermocouple and out the heat sink, generating electricity
in process.
WORKING PRINCIPLE
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Thermocouple 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.
Commonly used thermoelectric materials are Germanium alloys, Lead telluride
and Tellurides of Antimony, Germanium and Silver.
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Test Facility Requirements
The design process for the test facility began by defining the requirements. Although the EU could be tested in a variety of ways under various operating conditions, the extended operation test as discussed in the introduction focused on 24/7 steady operation for several years. To meet cost and schedule constraints, it was important to define the functionality necessary for extended operation.
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The ASRG EU test facility was based on existing extended-operation test facilities at GRC described in References 4 to 7. Although many features are common with existing facilities, the ASRG EU drove several unique features, described in detail here.
Test Facility Designwww.etraid.in
The ASRG EU is rigidly mounted to the test table as shown in Figure 3. The EU housing includes four mounting tabs at one end to which four standoffs are attached. These standoffs lift the EU above the test table enough to allow access to the electrical connectors on the end of the EU, and they bolt to a 3/4-in- thick steel table top, with Kistler 9251A load cells inserted between the standoffs and table top.
Mechanical Mountingwww.etraid.in
For previous ASC in-air extended-operation convertor test at GRC, heat was removed from the cold end heat exchanger by conduction through a flange and directly to the cooling fluid. In the case of the ASRG EU, heat conducted away from the cold end of the convertor through the CSAF and into the sides of the EU housing and the fins provides the cooling mechanism. From there the heat could have been rejected either into cooling tubes attached to the fins or to the air surrounding the EU. To avoid any risk of damage or marring of the EU surface, it was decided to use the air surrounding the EU for heat rejection.
Heat Rejection Systemwww.etraid.in
ASRG EU mounting to the test tablewww.etraid.in
Test stand transporting the ASRG EU.www.etraid.in
SELECTION OF FUELS
CRITERIA
The half-life must be long enough that it will produce energy at a relatively continuous
rate for a reasonable amount of time. And at the same time, the half-life needs to be
short enough so that it decays sufficiently quickly to generate a usable amount of heat.
The fuel must produce a large amount of energy per mass and volume (density).
It should produce high energy radiation that is easily absorbed and transferred into
thermal radiation, preferably alpha radiation.
ElementElement Half-life Half-life (years) (years)
Watts/g Watts/g (thermal) (thermal)
$/Watt $/Watt (thermal) (thermal)
Polonium-210 Polonium-210 0.378 0.378 141 141 570570
Plutonium-238 Plutonium-238 86.886.8 0.550.55 30003000
Cesium-144 Cesium-144 0.7810.781 2525 1515
Strontium-90 Strontium-90 2828 0.930.93 250250
Curium-242 Curium-242 0.4450.445 120120 495495
AS
RG
EU
in Lexan cage.ASRG EU in Lexan cage. www.etraid.in
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Top view of ASRG EU showing the air-flow of
the auxiliary fans across sides of the EU.
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Argon System
During ground operations, an argon cover gas fills the ASRG housing to prevent oxidation of the graphitic components of the heat source. The test facility’s argon system maintains pressure between 5 and 9 psig whereas the EU requires a pressure between 3.3 and 15.3 psig.
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Facility Electrical PowerAn uninterruptible power supply (UPS)
provides backup power for the ASRG EU test facility so that a loss of building power, even momentarily, does not cause a loss of control and convertor stall. Furthermore, a 50-kW natural-gas-fueled backup generator provides power in the event of a loss of building power. In the unlikely event of loss of power from the backup generator and building power, the system will automatically shut down the ASRG EU. Power draw during shutdown is under 1500 VA, and the UPS can power the system for at least 6 hr during a controlled shutdown.
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Instrumentation, Monitoring, and Safety Systems
Extensive effort went into designing the instrumentation, monitoring, and safety systems for the test facility. These systems are covered in Reference 8, so they are not discussed in detail in this paper. Instrumentation in the ASRG EU and the test facility includes a variety of sensors to monitor performance of both the EU and the test systems. Some of the sensors in the ASRG EU were installed by LM and delivered with the unit. Where possible, multiple sensors are used to provide redundancy and verification.
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Convertor Control for the ASCs
The test facility supports two types of convertor control for the ASCs: AC bus control and ACU control. Reference 8 discusses AC bus control in detail. ASRG EU testing began at GRC with AC bus control in part to verify the test facility functionality before transitioning to ACU control.
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Heat Source
The ASRG EU was tested at LM using an EHS that was dynamically and dimensionally equivalent to a General Purpose Heat Source (GPHS) module (Ref. 2). This was important to obtaining meaningful data from tests like sine transient, random vibration, and shock tests. After internal inspection of the ASRG EU following LM’s tests, it was decided to replace the EHS with a cartridge heater furnished by GRC
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Heat Source Control
Heater power controller schematic.
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Test Facility Maintenance
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Improvements
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Heat Rejection System
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Components and Connections in the Alternator Load Circuit
From the beginning of operation, the alternator power and piston amplitude showed anomalous behavior in that these parameters would increase slightly, sometimes very gradually and sometimes rapidly, and then decrease, usually rapidly back to the initial condition.
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210 PoHas high energy density due to its high radioactive activity.
But limited use due to very short life.
If there is no efficient cooling, self heating power is
sufficient for melting then partly vapourizing itself.
238PuLowest shielding requirements.
Long half life.
Requires less than 2.5 mm of lead shielding to keep radiation.
Used in the form of Plutonium oxide.
Image of a plutonium RTG
pellet glowing red hot.
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241 Am
Has longer half life than 238 Pu.
Energy density is one by fourth that of 238 Pu.
Needs about 18mm of lead shielding.
Its shielding requirements in an RTG are 2nd lowest of all possible isotopes.
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LIFE SPAN
Soviet RTGs in dilapidated and vandalized condition, powered by Strontium-90, 90Sr
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Most RTGs use 238 Pu which decays with a half life of 87.7 years.
RTG using Pu will diminish in power output by 1-.51/87.7 or
.787% of there capacity per year.
23 years after production, such an RTG will have decreased in power by 1-.523/87.7 or
10% that is providing 83.4% of its initial output.
Thus starting capacity of 470W, after 23 years it would have a capacity of
.834 x 470 = 392W.
LIFE SPANwww.etraid.in
EFFICIENCY
Higher efficiency means less radioactive fuel is needed to produce the same
amount of power and therefore a lighter overall weight for the generator. This is a
critically important factor in space flight launch considerations.
Thermocouples used in RTGs are very reliable and long lasting, but are very
inefficient. So efficiency above 10% have never been achieved and most RTGs
have efficiency between 3 – 7 %.
Thermionic converter, the energy conversion device which relies on the principle
of thermionic emission can achieve efficiency between 10 – 20 %, but require
high temperature than at which standard RTGs run.
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Thermophotovoltaic cells have an efficiency slightly higher than thermocouples and
can be overlaid on top of the thermocouples, potentially doubling efficiency.
Theorotical thermophotovoltaic cell designs have efficiency upto 30% but these
have yet to built of conformed.
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SAFETY
Diagram of a stack of general purpose heat source modules used in RTGs
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Nuclear runaway scenario is impossible.
Most RTg designs are inherently immune to nuclear meltdown or other
runaway problems.
Only kind of problems RTGs are subject to use radioactive contamination,
which is harmful to environment.
To minimize the risk of fuel leakage, fuel is stored in individual modular units
with their own heat shielding.
This modular units are surrounded by a layer of Iridium metal and encased in high
strength graphite blocks. These two materials are corrosion and heat resistant.
SAFETYwww.etraid.in
Surrounding the graphite blocks is an aeroshell, designed to protect the entire
assembly against the heat of reentering the earth’s atmosphere.
Fuel is also stored in ceramic form, that is heat resistant, minimizing the
risk of vaporization and aerosolization. The ceramic is highly insoluble.
Inspection of Cassini spacecraft RTGs before launch
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APPLICATIONSUsed as power sources in satellites, space probes and unmanned remote facilities.
Used as power sources for navigation beacons, radio beacons, light houses
and weather stations.
Used at places where solar cells are not viable.
Most desirable power source for unmanned
and unmaintained situations needing a few
100 watts or less of power of durations too
long for fuel cells, batteries and generators.
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Name & Name & Model Model
Used On (# Used On (# of RTGs per of RTGs per User) User)
Maximum Maximum output output
Radio-Radio-isotope isotope
Max Max fuelfuelused used (kg) (kg)
Mass (kg) Mass (kg)
ElectriElectrical cal
((W) )
Heat Heat (W) (W)
SRG* SRG* in prototype in prototype phase, phase, MSL
~110 ~110 (2x55) (2x55)
~500 ~500 238238PuPu ~1 ~1 ~34 ~34
MMRTGMMRTG in prototype in prototype phase, phase, MSL
~110 ~110 ~2000 ~2000 238Pu 238Pu ~4 ~4 <45 <45
GPHS-GPHS-RTGRTG
Cassini (3), Cassini (3), New New Horizons Horizons (1), Galileo (1), Galileo (2), Ulysses(2), Ulysses
300 300 4400 4400 238Pu 238Pu 7.8 7.8 55.9-57.855.9-57.8[13]
MHW-MHW-RTGRTG
LES-8/9, LES-8/9, Voyager 1 Voyager 1 (3), Voyager (3), Voyager 2 (3)2 (3)
160160[13]
24002400[14]
238Pu 238Pu ~4.5 ~4.5 37.737.7[13]
SNAP-3BSNAP-3B Transit-Transit-4A(1)4A(1)
2.72.7[13]
52.552.5[14]
238Pu 238Pu ? ? 2.12.1[13]
SNAP-9ASNAP-9A Transit Transit 5BN1/2 (1)5BN1/2 (1)
25[13] 25[13] 525[14525[14] ]
238Pu238Pu ~1 ~1 12.3[13] 12.3[13]
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Name & Name & Model Model
Used On (# Used On (# of RTGs per of RTGs per User) User)
Maximum output Maximum output Radio-Radio-isotope isotope
Max Max fuelfuelused used (kg) (kg)
Mass (kg) Mass (kg)
ElectricElectrical al
(W) (W)
Heat Heat (W) (W)
SNAP-19 SNAP-19 Nimbus-3 Nimbus-3 (2),Pioneer (2),Pioneer 10 (4), 10 (4), Pioneer 11 Pioneer 11 (4)(4)
40.3[1340.3[13] ]
525 525 238Pu 238Pu ~1 ~1 13.6[13] 13.6[13]
Modified Modified SNAP-19SNAP-19
Viking 1(2), Viking 1(2), Viking 2(2)Viking 2(2)
42.7[1342.7[13] ]
525 525 238Pu 238Pu ~1 ~1 15.2[13] 15.2[13]
SNAP-27SNAP-27 Appolo 12-17 Appolo 12-17 ALSEP (1)ALSEP (1)
7373 14801480 238Pu 238Pu 3.8 3.8 20 20
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DISADVANTAGES
Usually thermocouples are used for conversion of energy, but their efficiency
is very less between 3 to 7 percentage, so it affects the efficiency of the RTG.
If the radioactive material is leaked it will affect the environment harmfully.
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CONCLUSIONEfficiency is an important factor in spaceflight launch cost consideration.
NASA have been developing a next generation radioisotopes - 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 prototype demonstrated an average efficiency of 23%.
Use of non-contacting moving parts, non-degrading flexural bearing in test
units, demonstrated no appreciable degradation over years of operations.
Experimental results demonstrate that an SRG could continue running for
decades without maintenance.
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Vibrations can be eliminated on a concern by implementation of dynamic,
balancing or use of dual – opposed piston movement.
Potential applications of a Stiriling radioisotope power system include exploration
and science missions to deep – space, Mars and the Moon.
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•ACU ASC Control Unit•ASC Advanced Stirling Convertor•ASRGEU Advanced Stirling Radioisotope Generator Engineering Unit•CSAF Cold-Side Adapter Flange•EHS Electric Heat Source•GPHS General Purpose Heat Source•GRC Glenn Research Center•LM Lockheed Martin•NASA National Aeronautics and Space Administration•UPS Uninterruptible Power Supply
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Thank YouThank You
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