RTG: prosaic heat and electricity for spacecraft

It so happened that in the series “Peaceful Cosmic Atom” we move from the fantastic to the widespread. Last time we talked about power reactors, the obvious next step is to talk about radioisotope thermoelectric generators. Recently on Habré there was an excellent post about the RTG of the Cassini probe , and we will consider this topic from a wider point of view.

Process physics

Heat production

Unlike a nuclear reactor, which uses the phenomenon of a nuclear chain reaction, radioisotope generators use the natural decay of radioactive isotopes. Recall that atoms are made up of protons, electrons, and neutrons. Depending on the number of neutrons in the nucleus of a particular atom, it may be stable, or may show a tendency to spontaneous decay. For example, the cobalt atom of ⁵⁹ Co with 27 protons and 32 neutrons in the nucleus is stable. Such cobalt has been used by mankind since ancient Egypt. But if we add one neutron to ⁵⁹ Co (for example, by placing “ordinary” cobalt in an atomic reactor), we get ⁶⁰Co, a radioactive isotope with a half-life of 5.2 years. The term “half-life” means that in 5.2 years one atom will decay with a probability of 50%, and about half of the atoms will remain. All “ordinary” elements have their isotopes with different half-lives:


3D map of isotopes, thanks to LJ user crustgroup for the picture.

Choosing a suitable isotope, one can obtain an RTG with the required service life and other parameters:

Isotope	Production method	Specific Power, W / g	Volume power, W / cm³	Half life	Integrated isotope decay energy, kW · h / g	The working form of the isotope
60 Co (cobalt-60)	Reactor Irradiation	2.9	~ 26	5,271 years	193.2	Metal alloy
238 Pu (plutonium-238)	atomic reactor	0.568	6.9	86 years	608.7	Plutonium carbide
90 Sr (strontium-90)	fission fragments	0.93	0.7	28 years	162,721	SrO, SrTiO 3
144 Ce (cerium-144)	fission fragments	2.6	12.5	285 days	57,439	CeO 2
242 Cm (Curium-242)	atomic reactor	121	1169	162 days	677.8	Cm 2 O 3
147 Pm (Promethium-147)	fission fragments	0.37	1,1	2.64 years	12.34	Pm 2 O 3
137 Cs (cesium-137)	fission fragments	0.27	1.27	33 years	230.24	Cscl
210 Po (Polonium-210)	irradiation of bismuth	142	1320	138 days	677.59	alloys with lead, yttrium, gold
244 Cm (Curium-244)	atomic reactor	2,8	33.25	18.1 years	640.6	Cm 2 O 3
232 U (Uranium-232)	thorium irradiation	8,097	~ 88.67	68.9 years	4887,103	dioxide, carbide, uranium nitride
106 Ru (ruthenium-106)	fission fragments	29.8	369,818	~ 371.63 days	9,854	metal alloy

The fact that the decay of isotopes occurs independently means that the RTG cannot be controlled. After loading the fuel, it will heat up and produce electricity for years, gradually degrading. Reducing the amount of fissile isotope means that there will be less nuclear decays, less heat and electricity. Plus, a drop in electrical power will exacerbate the degradation of the electric generator.
There is a simplified version of the RTG in which the decay of the isotope is used only for heating, without generating electricity. Such a module is called a heating unit or RHG (Radioisotope Heat Generator).

Converting Heat to Electricity

As in the case of a nuclear reactor, we get heat at the output, which must be converted into electricity in some way. To do this, you can use:

• Thermoelectric Converter . By connecting two conductors of different materials (for example, chromel and alumel) and heating one of them, you can get a source of electricity.
• Thermionic converter . In this case, an electronic lamp is used. Its cathode is heated, and the electrons get enough energy to "jump" to the anode, creating an electric current.
• Thermophotoelectric Converter . In this case, an infrared photocell is connected to the heat source. A heat source emits photons that are captured by a photocell and converted into electricity.
• Thermoelectric converter on alkali metals . Here, an electrolyte from molten sodium and sulfur salts is used to convert heat into electricity.
• The Stirling engine is a heat engine for converting the temperature difference into mechanical work. Electricity is obtained from mechanical work using any generator.

History

The first experimental radioisotope energy source was introduced in 1913. But only from the second half of the 20th century, with the proliferation of nuclear reactors on which it was possible to produce isotopes on an industrial scale, RTGs began to be actively used.

USA

In the USA, the RTGs were handled by the SNAP organization, which you already knew from your previous post.
SNAP-1 .
It was an experimental RTG on ¹⁴⁴ Ce and with a generator on the Rankine cycle (steam engine) with mercury as a coolant. The generator successfully worked 2500 hours on Earth, but did not fly into space.

SNAP-3 .
The first RTG to fly into space on the Transit 4A and 4B navigation satellites. Energy power 2 W, weight 2 kg, used plutonium-238.


Sentry
RTG for meteorological satellite. Energy power 4.5 W, isotope - strontium-90.

SNAP-7 .
A family of ground-based RTGs for lighthouses, light buoys, weather stations, acoustic buoys and the like. Very large models, weight from 850 to 2720 kg. Energy power - tens of watts. For example, SNAP-7D is 30 W with a mass of 2 tons.

SNAP-9
Serial RTG for Transit navigation satellites. Weight 12 kg, electrical power 25 watts.

SNAP-11
Experimental RTG for lunar landing stations Surveyor. It was proposed to use the Curium-242 isotope. Electric power - 25 watts. Not used.

SNAP-19
Serial RTG, used in many missions - Nimbus meteorological satellites, Pioneer-10 and -11 probes, Viking Martian landing stations. The isotope is plutonium-238, energy power is ~ 40 watts.


SNAP-21 and -23
RTGs for underwater use on strontium-90.

SNAP-27
RTGs for powering Apollo scientific equipment. 3.8 kg plutonium-238 gave an energy power of 70 watts. Lunar scientific equipment was turned off as far back as 1977 (people and equipment on Earth demanded money, but they were not enough). RTGs for 1977 produced from 36 to 60 watts of electrical power.


MHW-RTG
The name stands for "multiswt RTG". 4.5 kg plutonium-238 produced 2,400 watts of thermal power and 160 watts of electrical power. These RTGs were located on the Lincoln Experimental Satellites (LES-8.9) and have been supplying Voyagers with heat and electricity for 37 years. For 2014, RTGs provide about 53% of their initial capacity.


GPHS-RTG
The most powerful of the space RTGs. 7.8 kg of plutonium-238 produced 4400 watts of thermal power and 300 watts of electrical power. Used on the Ulysses solar probe, Galileo, Cassini-Huygens probes and flies to Pluto on the New Horizons.


MMRTG
RTG for Curiosity. 4 kg of plutonium-238, 2000 watts of thermal power, 100 watts of electrical power.

Warm tube plutonium cube.


US RTGs with time reference.

Summary table:

Title	Media (number on the unit)	Maximum power		Isotope	Fuel weight kg	Gross weight
		Electric, W	Thermal, W
MMRTG	MSL / Curiosity Rover	~ 110	~ 2000	238 Pu	~ 4	<45
GPHS-RTG	Cassini (3) , New Horizons (1) , Galileo (2) , Ulysses (1)	300	4400	238 Pu	7.8	55.9–57.8
Mhw-rtg	LES-8/9 , Voyager 1 (3) , Voyager 2 (3)	160	2400	238 Pu	~ 4.5	37.7
SNAP-3B	Transit-4A (1)	2.7	52.5	238 Pu	?	2.1
SNAP-9A	Transit 5BN1 / 2 (1)	25	525	238 Pu	~ 1	12.3
SNAP-19	Nimbus-3 (2), Pioneer 10 (4) , Pioneer 11 (4)	40.3	525	238 Pu	~ 1	13.6
SNAP-19 modification	Viking 1 (2), Viking 2 (2)	42.7	525	238 Pu	~ 1	15.2
SNAP-27	Apollo 12-17 ALSEP (1)	73	1,480	238 Pu	3.8	20

USSR / Russia

In the USSR and Russia, there were few space RTGs. He became the first experimental generator RTG "Lemon-1" of polonium-210, created in 1962:
.

The first space RTGs were Orion-1 with an electric power of 20 W at polonium-210 and the Cosmos-84 and Cosmos-90 launched on the connected satellites of the Strela-1 series. The heating units were on the Lunokhods -1 and -2, and the RTG was on the Mars-96 mission:


At the same time, RTGs were very actively used in lighthouses, navigation buoys and other ground equipment - the BETA, RTG- series IEU ”and many others.

Design

Almost all RTGs use thermoelectric converters and therefore have the same design:

Prospects

All flying RTGs are distinguished by a very low efficiency - as a rule, electric power is less than 10% of thermal. Therefore, at the beginning of the XXI century, NASA launched the ASRG - RTG project with a Stirling engine. It was expected to increase the efficiency to 30% and 140 watts of electric power at 500 watts of heat. Unfortunately, the project was stopped in 2013 due to exceeding the budget. But, theoretically, the use of more efficient converters of heat into electricity can seriously increase the efficiency of RTGs.

Advantages and disadvantages

Advantages:

1. Very simple design.
2. It can work for years and decades, degrading gradually.
3. It can be used simultaneously for heating and power supply.
4. It does not require management and supervision.

Disadvantages:

1. Rare and expensive isotopes are required as fuel.
2. Fuel production is complex, expensive and slow.
3. Low efficiency.
4. Power is limited to hundreds of watts. The RTG of kilowatt electric power is already poorly justified, megawatt - it makes almost no sense: it will be too expensive and heavy.

The combination of such advantages and disadvantages means that RTGs and heating units occupy their niche in space energy and will save it further. They allow you to simply and efficiently heat and power interplanetary spacecraft with electricity, but you should not expect any energy breakthrough from them.

Sources

In addition to Wikipedia, the following were used:

• Document "Space nuclear energy: opening the last horizon" .
• The topic “Domestic RTGs” on “Cosmonautics News”.
• Domestic RTGs .
• Teledyne heritage .
• RTG on the NASA website .