MMRTG

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.

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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

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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

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LIST OF TABLES PAGE NO

1. RTG’s in space 18

2. RTG’s on earth 19

3. Nuclear power system in space 20

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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).

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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.

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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.

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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

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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.

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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).



see table below for values used). Both generators can be simplified to heat



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



see table below for values used). Both generators can be simplified to heat



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

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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


the atmosphere; therefore their use in spacecraft and elsewhere has attracted controversy.


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


radioisotope heater units) during the first 3.5 minutes following



million. If an accident which had the potential to cause contamination occurred during the



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


the atmosphere; therefore their use in spacecraft and elsewhere has attracted controversy.[17]


Huygens probe launched in 1997

estimated the probability of contamination accidents at various stages in the mission. The


first 3.5 minutes following



d the potential to cause contamination occurred during the



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

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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


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



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


s the only time the 1972 UN Liability Convention has been invoked.

Page 16

238 contamination in


on the seabed. The cask is expected to contain the fuel for at least 10 half-lives (i.e.

cted seawater tests and



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


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 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


tals joined in two places, with a temperature difference between the junctions.


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


tals joined in two places, with a temperature difference between the junctions.


. 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


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



a metallic probe of known composition is kept at a constant temperature and held in contact


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


, then the voltage gradient is given

. This simple relationship, which does not depend on

e a temperature difference; an absolute



is kept at a constant temperature and held in contact


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



Fortunately, the magnitude of the effect depends on the metal in use.



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



temperature gradient, the end result is a measurement of the difference in temperature



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



Fortunately, the magnitude of the effect depends on the metal in use.



volts per degree

he two metals of the thermocouple but



measurement of the difference in temperature



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


junctions all reach a uniform internal temperature, and provided an ideal voltmeter is us



, 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.


conductors matter):

Seebeck coefficients of materials A and B as a function of

are the temperatures of the two junctions. The voltages



). Because the two wires give different voltages leading up to the junction,

the resulting measured overall voltage is nonzero.

Thermocouple characteristic function


above formula can be approximated as . In general this


), 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


junctions all reach a uniform internal temperature, and provided an ideal voltmeter is used,



is the Seebeck coefficient at that point, and is

end voltage can be found by


of materials A and B as a function of

are the temperatures of the two junctions. The voltages Vb and Vc



p to the junction,


. In general this is not the


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.


e constant is chosen such that . The measured


(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


. The measured


(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

MMRTG

Documents

heat source

new rtg

nuclear power system

conversion of heat

new generation of power

plutoniums heat energy

galileo mission

watts of electrical