Space Nuclear Power
Through the cooperative efforts of the U.S. Department of Energy (DOE),
formerly called the Atomic Energy Commission, and NASA, the United
States has used nuclear energy in its space program to provide electrical
power for many missions, including science stations on the Moon,
extensive exploration missions to the outer planets—Jupiter, Saturn,
Uranus, Neptune, and beyond—and even to search for life on the surface
of Mars.
For example, when the Apollo 12 mission astronauts departed from
the lunar surface on their return trip to Earth (November 1969), they left
behind a nuclear-powered science station that sent information back to
scientists on Earth for several years. That science station, as well as similar
stations left on the Moon by the Apollo 14 through 17 missions, operated
on electrical power supplied by plutonium-238-fueled, radioisotope thermoelectric
generators (RTGs). Since 1961, nuclear-power systems have
helped assure the success of many space missions, including the Pioneer
10 and 11 missions to Jupiter and Saturn; the Viking 1 and 2 landers on
Mars; the spectacular Voyager 1 and 2 missions to Jupiter, Saturn, Uranus,
Neptune, and beyond; the Ulysses mission to the Sun’s polar regions; the
Galileo mission to Jupiter, and the Cassini mission to Saturn.
Energy supplies that are reliable, transportable, and abundant represent
a very important technology in the development of solar-system
civilization. Space nuclear-power systems will play an ever-expanding role
in supporting more ambitious deep space–exploration missions by robots
and in supporting human spaceflight beyond Earth orbit, when astronauts
return to the Moon to build a permanent settlement and then visit Mars
to establish a surface base.
Space nuclear-power supplies offer several distinct advantages over
the more traditional solar and chemical space-power systems. These
advantages include compact size, modest mass requirements, very long
operating lifetimes, the ability to operate in extremely hostile environments
(such as intense trapped-radiation belts, the surface of Mars, the
moons of the outer planets, and even interstellar space), and the ability to
operate independent of distance from, or orientation to, the Sun.
Space nuclear-power systems use the thermal energy or heat released
by nuclear processes. These processes include the spontaneous (but
predictable) decay of radioisotopes, the controlled fission or splitting
of heavy atomic nuclei (such as fissile uranium-235) in a self-sustained
neutron chain reaction, and (eventually) the joining together, or fusing,
of light atomic nuclei (such as deuterium and tritium) in a controlled
thermonuclear reaction. This nuclear-reaction heat is converted directly
or through a variety of thermodynamic (heat-engine) cycles into electric
power. Until controlled thermonuclear fusion capabilities are achieved,
space nuclear-power applications will be based on the use of either radioisotope
decay or nuclear fission reactors.
The radioisotope thermoelectric generator consists of two main
functional components: the thermoelectric converter and the nuclear
heat source. The radioisotope plutonium-238 has been used as the heat
source in all U.S. space missions involving radioisotope power supplies.
Plutonium-238 has a half-life of about 87.7 years and therefore supports
a long operational life. (The half-life is the time required for one-half the
number of unstable nuclei present at a given time to undergo radioactive
decay.) In the nuclear decay process, plutonium-238 emits primarily alpha
radiation that has very low penetrating power. Consequently, only a small
amount of shielding is required to protect the spacecraft from its nuclear
emissions. A thermoelectric converter uses the thermocouple principle to
directly convert a portion of the nuclear (decay) heat into electricity.
A space fission-power system is a device designed and engineered to
generate power for space applications using a nuclear reactor to fission (or
split) uranium atoms. During the fission process, a neutron strikes a uranium
atom, causing it to release energy as it splits into smaller atoms, called
fission products. The released thermal energy (heat) is then converted into
electricity through a conversion system to power the spacecraft. This fission
process can be sustained and controlled to provide power at needed
levels in a continuous manner in a reactor system.
Space reactors are designed differently from terrestrial reactors. The
space reactors are much smaller, typically about the size of a 5-gallon can
of paint. Aerospace safety engineers also design space reactors to remain
in a cold, inactive state until arriving at a designated startup location in
space. Once at this designated location the reactor receives the command
signal to initiate operation. This design feature enhances system launch
and operations safety.
Although the design of a space fission-power system is quite complicated,
the basic theory on which it operates is fairly simple. To generate
electric power, there are only three basic subsystems: a controlled fission
reactor core to produce heat, a cooling loop or mechanism that removes
heat from the core, and a power conversion subsystem that receives the
heat from the cooling loop and converts a portion of the input heat into
electric power. The principles of thermodynamics govern that not all of
the input heat can be converted into useful electric energy, so some of the
input heat must be rejected to the environment (outer space). Engineers
use radiators to remove this excess (or waste) thermal energy from the
space power system and reject it to outer space.
Different power-conversion technologies can be used to convert heat
from the reactor into electricity. The final choice of a power-conversion
technology depends on the requirements of the mission and compatibility
with the rest of the spacecraft, including scientific payload. Engineers also
use a radiation shield to protect electronic components and other sensitive
equipment from the radiation emitted from the reactor during operation.
The Russian space program has flown several space nuclear reactors
(most recently a system called Topaz). The United States has flown only
one space nuclear reactor, an experimental system called the SNAP-10A,
which was launched and operated on-orbit in 1965. The objective of the
SNAP-10A program was to develop a space nuclear-reactor power unit
capable of producing a minimum of 500 watts-electric for a period of one
year, while operating in space. The SNAP-10A reactor was a small (about
the size of a garden pail) zirconium hydride (ZrH) thermal reactor fueled
by uranium-235. The SNAP-10A orbital test was successful, although the
mission was prematurely (and safely) terminated on-orbit by the failure of
an electronic component outside the reactor.
Since the United States first used nuclear power in space, great emphasis
has been placed on the safety of people and the protection of the
terrestrial environment. A continuing major objective in any new space
nuclear-power program is to avoid undue risks. In the case of radioisotope
power supplies, this means designing the system to contain the radioisotope
fuel under all normal and potential accident conditions. For space
nuclear reactors, such as the SNAP-10A and more advanced systems, this
means launching the reactor in a “cold” (non-operating) configuration
and starting up the reactor only after a safe, stable Earth orbit or interplanetary
trajectory has been achieved.