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Nuclear Power in Space


SPACE NUCLEAR POWER SOURCES

Most of the unmanned craft sent into space by this country have been powered
by arrays of photovoltaic cells, or solar arrays. Some missions, however,
call for another type of power source. They might be long voyages to the
outer solar system, where sunlight is weak. Or perhaps the large, delicate
solar arrays could not survive the conditions the craft will encounter --
such as passage near the head of Halley's comet.

In such cases, nuclear power sources usually fill the bill. These sources
fall into two main categories: The Radioisotope Thermoelectric Generator (RTG)
and the nuclear reactor. This report briefly describes them, and summarizes
the development of nuclear power sources by the U. S. for use in space.

The RTG consists of a central core containing the radioisotope, surrounded by
a layer of Thermo-Electric Generator (TEG) elements. The TEGs are basically
semiconductor junctions. When a temperature difference exists across the
junction, an electric current is produced. Obviously, the greater the
temperature difference, the more the power that can be obtained. But this
is limited by the strengths of the materials used. The present state of
the art uses silicon-germanium TEGs operating at 1300 deg. K on the hot
side, and perhaps 500 to 600 K on the other. This gives an efficiency of
9 or 10%.

TABLE 1. Radioisotopes Usable in Nuclear-Fueled Generators

Isotope Fuel Power Density Half- Maximum Supply Cost,
Form ------------ Life, Operating kW/Yr. $/Watt
W/cm3 W/gm Years Temp., C
--------------------------------------------------------------------------
Curium 242 Cm2O3 1050 98 0.45 1600 1.5 ?
Polonium 210 GdPo 820 82 0.38 1600 70 150
Curium 244 Cm2O3 28.6 2.6 18.0 1600 8 3650
Plutonium 238 PuO2 3.7 0.41 86.4 1000 11 750
Promethium 147 Pm2O3 2.1 0.28 2.6 1000 2 1000
Strontium 90 SrO 1.2 0.24 28 1000 31 90

Table 1 shows some of the isotopes useful for power generation. While other
radioisotopes have been used in experiments, all the RTGs flown by the U. S.
have been activated by plutonium-238. This material has a half-life of about
88 years, so it can produce a reasonably constant flow of heat over the
duration of a mission. Early designs used the pure metal; today, compacted
plutonium oxide is preferred. Some spacecraft use Radioisotope Heat Units
(RHU) to ward off the cold. The Apollo 11 astronauts emplaced an Early
Apollo Scientific Experiment Package on the moon. This EASEP device was
powered by solar energy; but two 15-Watt RHUs kept it from freezing
during the long Lunar night.

The next five Apollo missions each left an ALSEP -- Apollo Lunar Scientific
Experiment Package -- powered by a 70-Watt RTG. Designed for a maximum
life of 2 years, these remotely-operated laboratories were all functioning
when NASA shut them down on Sept. 30, 1977. Most RTGs we have sent into space
have exceeded both their power and lifetime requirements -- as witness the
performance of the two Viking landers and the Pioneer and Voyager probes.

Table 2. Radioisotope Thermoelectric Generators launched by U. S. 1961-1982

Spacecraft Launch Power Qty Rating Mission Status
Date Site Source Type Code
---------------------------------------------------------------------------
Transit 4A 6/29/61 (ETR) SNAP-3B7 1 2.7W NAVSAT 2
Transit 4B 11/15/61 (ETR) SNAP-3B8 1 2.7W NAVSAT 5
Transit 5BN-1 9/28/83 (WTR) SNAP-9A 1 >25.2W NAVSAT 5
Transit 5BN-2 12/05/63 (WTR) SNAP-9A 1 26.8W NAVSAT 3
Nimbus III 4/14/69 (WTR) SNAP-19B 2 28.2W WEATHSAT 6
Apollo 12 11/14/69 (KSC) SNAP-27 1 73.6W ALSEP 4
Apollo 14 1/31/71 (KSC) SNAP-27 1 72.5W ALSEP 4
Apollo 15 7/26/71 (KSC) SNAP-27 1 74.7W ALSEP 4
Pioneer 10 3/02/72 (ETR) SNAP-19 4 40.7W SOLSYSEX 1
Apollo 16 4/16/72 (KSC) SNAP-27 1 70.9W ALSEP 4
TRIAD 9/02/72 (WTR) Transit-RTG 1 35.6W NAVSAT 1
Apollo 17 12/07/72 (KSC) SNAP-27 1 75.4W ALSEP 4
Pioneer 11 4/05/73 (ETR) SNAP-19 4 39.9W SOLSYSEX 1
Viking 1 8/20/75 (ETR) SNAP-19 2 42.3W MARSLNDR 4
Viking 2 9/09/75 (ETR) SNAP-19 2 43.1W MARSLNDR 4
LES-8 3/14/76 (ETR) MHW-RTG 2 153.7W COMSAT 1
LES-9 3/14/76 (ETR) MHW-RTG 2 154.2W COMSAT 1
Voyager 2 8/20/77 (ETR) MHW-RTG 3 159.2W SOLSYSEX 1
Voyager 1 9/05/77 (ETR) MHW-RTG 3 156.7W SOLSYSEX 1
Galileo TBA GPHS
Ulysses TBA GPHS

Launch Sites: ETR = Eastern Test Range
KSC = Kennedy Space Center
WTR = Western Test Range

Mission Codes: NAVSAT = Navigational satellite
WEATHSAT = Weather satellite
ALSEP = Apollo Lunar Scientific Experiments Package
SOLSYSEX = Solar system exploration
MARSLNDR = Mars lander
COMSAT = Communications satellite

Status Codes: 1 = Still operating
2 = Shut down but operational
3 = Payload failed, power source operational
4 = Totally shut down
5 = Transmission ceased
6 = No longer monitored

Other Acronyms: LES = Lincoln Experimental Satellite
MHW = Multi-Hundred Watt
GPHS = General-Purpose Heat Source
SNAP = Systems for Nuclear Auxiliary Power
.PA
Table 2 shows the history of RTG-powered spacecraft flown by the U. S.
The last two spacecraft, still to be launched, wil be powered by the DOE-
developed General-Purpose Heat Source. This is based on a "brick" which
contains two isotope capsules and can be stacked for a range of power levels.
Table 3, below, summarizes the operating characteristics of these RTG designs.

Table 3. Characteristics of Radioisotope Thermoelectric Generators

Type Thermal Elect. System Design TEG type Notes
Output, Output, Mass, Life,
Watts Watts kg. Years
----------------------------------------------------------------------------
SNAP-3B 52.5 2.7 2.1 5 PbTe
SNAP-9A 525.0 25.0 12.3 6 PbTe
SNAP-19B 525.0 25.0 ? 1 PbTe Nimbus
SNAP-19 525.0 25.0 ? 1 TAGS-SnTe Pioneer
SNAP-19 525.0 35.0 ? 3 TAGS-SnTe Viking
SNAP-27 1480.0 63.5 19.7 1 PbTe Apollo 12+
SNAP-27 1480.0 69.0 19.7 1 PbTe Apollo 17
TRANSIT 855.0 36.2 13.6 5 PbTe
MHW-RTG 2400.0 125.0 39.6 5 SiGe LES-8,9
MHW-RTG 2400.0 128.0 37.7 5 SiGe Voyagers
GPHS-RTG 4410.0 285.0 55.5 N/A SiGe No flt. yet

Acronym: TAGS stands for the four ingredients of this TEG material:
Tellurium, antimony, germanium, and silver.

Think the metallurgy of these compounds gets complex?
Better believe it!!

Nuclear Reactor Power Sources

Like the ones that drive large nuclear power plants, the reactors built for
space depend on the fissioning of Uranium-235. These power sources differ
from RTGs in two very important ways. RTG systems depend on the heat produced
by natural decay of the radioisotope to produce electricity. Since the decay
cannot be turned on and off, the RTG unit is active from the moment when the
radioisotopes are inserted into the assembly. It must be cooled and shielded
constantly. The reactor, on the other hand, CAN be turned on when desired.
It is cool and only mildly radioactive until this happens, so is inherently
safer during launch.

The other major difference between RTGs and reactors is their growth
potential. Radioisotopes are very expensive, and to use them in a generator
producing more than about 10kW would be prohibitively costly. Reactors are
more expensive than RTGs on the low-power end of the scale, but have a very
low delta: a 100kW system is not much bigger or costlier than a 10kW one.

Reactor Development

Table 4, below, shows the various projects undertaken in the 1960s for the
U. S. Space Power Reactor Program. Despite this early effort, only one
nuclear reactor has been launched by the U. S. SNAP-10A functioned on orbit
for 43 days in 1965, producing 500 Watts of power until the failure of a
voltage regulator caused it to shut down. Space reactor development has
been on hold since 1973. Part of this is due to the fact that we were able
to shrink the power demanded by our spacecraft, so that where solar arrays
were unsuitable, small, affordable RTG systems met the need. In 1973, no
near-term need was seen for the high power levels which reactors can supply.
Too, nuclear power had acquired a bad name in this country. Despite the many
potential advantages of nuclear reactors for space power production, funding
for development projects was hard to obtain -- especially for projects which
had no immediate application. Consequently, the early development programs
gradually faltered and faded away due to lack of funding and political
support. Today, the U. S. lags the Soviet Union in this area, and still would
need perhaps 8 years and hundreds of millions of dollars for the development
of a high-power reactor system for use on space missions.

Within the past two years, fortunately, there has been a resurgence of interest
in spaceborne reactors. For the long-term, deep-space probe missions of the
near future, and for the large power levels that will be demanded by the
next generation of spacecraft, nuclear reactors are the best source we know
how to build. A number of civil and military missions have been identified
as eenhanced or enabled by nuclear reactors. These range from air-traffic
control satellites to "space tugs" and a probe to the rings of Saturn.
Government and industry are cooperating on the SP-100 Program to create the
technology base for reactors in the 100kW to 300kW range, and a number of
concept development efforts are under way for multi-megawatt reactors and for
"mixed-mode" devices which can supply both electrical and propulsive power.

A Space Nuclear Power Systems Symposium has been held for two years running
in Albuquerque, NM, and Congressional support for funding of reactor devel-
opment is growing. It appears that the obstacles to space nuclear power
are at present largely technological rather than political.
.pa
Table 4. Nuclear Reactors Built for U. S. Space Power Program

Type Thermal Elect. Startup Shutdown Notes
Power, Power, Date Date
kW kW
------------------------------------------------------------------------
SNAP-2 50 N/A 1959 1960 Experimental reactor
SNAP-2 50 N/A 1961 1963 Development System
SNAP-TSF 10 N/A 1967 1973 Shielding experiment
SNAP-8 600 N/A 1962 1965 Experimental reactor
SNAP-8 600 ? 1968 1969 Developmental reactor
SNAP-10A 39 0.5 1964 1964 Flt. System Gnd Test 1
SNAP-10A 39 0.5 1964 1966 Flt. System Gnd test 3
SNAP-10A 39 0.5 -- -- Spare for flt. unit
SNAP-10A 39 0.5 4/03/65 5/16/65 S10FS-4, operated on orbit
SNAPTRAN-1 39 0.5 1964 1964 Transient test 1
SNAPTRAN-2 39 0.5 1965 1966 Transient test 2
SNAPTRAN-3 39 0.5 1968 1971 Transient test 3
SP-100 ? 100+ Under development


REFERENCES

1. Bennett, Gary L.
Lombardo, James J.
Rock, Bernard J.
"U. S. Radioisotope Thermoelectric Generator Space Operating
Experience (June 1961 -- December 1982)"
Proceedings of 18th Intersociety Energy Conversion Engineering Conference
August 21-26, 1983

2. Bennett, Gary L.
Lombardo, James J.
Rock, Bernard J.
"Development and Use of Nuclear Power Sources for Space Applications"
Journal of the Astronautical Sciences
Vol. XXIX, No. 4, pp. 321-342, Oct.-Dec. 1981

3. Angrist, Stanley W.
Direct Energy Conversion, 3rd Edition
Allyn and Bacon 1976

4. Mohamed S. El-Genk, Mark D. Hoover
Space Nuclear Power Systems 1984, Vol. 1
Orbit Book Co. 1985

 
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