Researcher Provides a Historical Perspective for Plutonium Heat Sources

For more than 30 years, Los Alamos has designed, developed, manufactured, and tested heat sources for radioisotope thermoelectric generators (RTGs). These powerful little "nuclear batteries" produce heat from the decay of radioactive isotopes‹usually plutonium-238‹and can provide electrical power and heat for years in satellites, instruments, and computers.

artist rendering of Rover Pathfinder on Mars from NASA/JPL

Early development efforts from the mid-1960s through the early 1970s focused on fuel forms for space applications. The first, plutonia-molybdenum-cermet (PMC), was used on the Pioneer 10 and 11 missions to Jupiter and Saturn and the Viking Lander missions. PMC was fabricated by hot-pressing microspheres of molybdenum-coated plutonia into hockey-puck-shaped discs, which were then stacked and encapsulated in a refractory alloy.

During the same period, Los Alamos developed a medical-grade fuel for use in cardiac pacemakers and early artificial hearts. The fuel in the artificial hearts was made of 90 percent enriched plutonium-238 and provided up to 50 watts of power. Some of the early pacemakers are still in use; some have been returned to Los Alamos' Plutonium Facility for recovery.
Los Alamos developed a General Purpose Heat Source (GPHS) in the late 1970s to meet the larger power requirements of the Galileo, Ulysses, and Cassini space missions.

As electrical power requirements for spacecraft increased during the 1970s, so did the need to increase the power density of the heat source. Using "inert" materials such as molybdenum proved unattractive. Los Alamos began investigating fuel forms that used pure plutonia, which can generate more power in a smaller, lighter package.

The first pure plutonia fuel form developed at Los Alamos was the Multihundred Watt (MHW) for the RTGs used on the Voyager 1 and 2 missions to Jupiter, Saturn, Uranus, and Neptune. The heat source was a 100-watt hot-pressed sphere of plutonia encapsulated in iridium. Twenty-four heat sources were contained within the RTG. Heat was converted to electrical power by 312 silicon-germanium thermoelectric couples. Each Multihundred Watt RTG provided about 157 watts of power at the beginning of the mission.

To meet the larger power requirements of the Galileo, Ulysses, and Cassini missions in the late 1970s, Los Alamos developed the General Purpose Heat Source (GPHS). This RTG contained 572 silicon germanium thermoelectric couples inside a thermoelectric converter and provided 285 watts of electrical power at the beginning.

The heat source for these RTGs consisted of 18 GPHS modules. Each module had four fueled clads and each cylindrical fueled clad contained a hot-pressed 150-gram pellet of plutonium-238 dioxide encapsulated in an iridium-tungsten container.

Los Alamos fabricated the 216 GPHS fueled clads used on the Cassini mission. Each iridium clad contained a sintered iridium powder frit vent designed to release the helium generated by the alpha particle decay of the fuel. The iridium is compatible with plutonium dioxide at temperatures greater than 1,773 K and melts at 2,698 K.

A General Purpose Heat Source (GPHS) fueled clad.

In addition to developing RTGs to provide electrical power to operate instruments, Los Alamos also designed and fabricated heater units to keep equipment operating in the deep-freeze of space. The Light Weight Radioisotope Heater Unit (LWRHU) provided one thermal watt of heat and was used on the Galileo and Cassini spacecraft and Mars Pathfinder Rover. At the heart of this heater unit was a platinum-30 percent rhodium fueled clad containing a hot-pressed 2.67-gram pellet of plutonium-238 dioxide.

Los Alamos has also fabricated a large number of heat sources for the weapons program. From 1981 to 1990 more than 3,000 Milliwatt Generator (MWG) heat sources were fabricated and shipped to the Pinellas, Fla., plant for assembly into the MWG-RTG. These RTGs were used in devices that reduce the possibility of accidental detonation from a nuclear warhead. Each of the Milliwatt Generator heat sources had a nominal power of 4.0 to 4.5 watts. Los Alamos is currently recovering fuel from excess MWG-RTGs and performing shelf-life and stockpile surveillance on both the MWG heat source and the MWG-RTG. See Actinide Research Quarterly, Winter 1994, "Milliwatt Surveillance Program Ensures RTG Safety and Reliability."

The Mars Pathfinder Rover contained three Light Weight Radioisotope Heater Units (LWRHUs).

Historically, reactors at Savannah River produced all of the United States' plutonium-238 until the late 1980s. With the shutdown of the Savannah River reactors, the United States has had to purchase plutonium-238 dioxide from Russia to supplement U.S. inventories for future NASA space missions. Recently, the DOE has proposed that plutonium-238 be produced at the Advanced Test Reactor (ATR) in Idaho and the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory.

A significant amount of the heat source fuel in the Los Alamos inventory, including the Russian material and fuel from disassembled terrestrial heat sources, must be purified before it can be fabricated into new heat sources. Heat source fuel was previously recycled and purified at Savannah River. Los Alamos is qualifying a plutonium-238 aqueous recovery process in its plutonium facility to provide feed for the heat-source fabrication process.

A Light Weight Radioisotope Heater Unit (LWRHU) before final assembly. The heat source consists of a graphite aeroshell, pyrolytic graphite thermal insulators, and a platinum-rhodium fueled clad. A sintered platinum powder frit vent is electron-beam-welded into the end cap of the fueled clad to release helium generated by the alpha decay of the fuel.

A new glove-box line is being installed for this work and the full-scale aqueous recovery process is expected to become operational this fiscal year, which will have the capacity to purify 5 kilograms of plutonium-238 per year. In addition, Los Alamos has the capability of fabricating more than 5 kilograms of plutonium-238 into heat sources per year.

These improvements to the plutonium facility will ensure that Los Alamos can fully support future NASA requirements. The Europa Orbiter mission, scheduled for launch in 2006, and the Solar Probe mission, scheduled for launch in 2007, will each require approximately 3 kilograms of plutonium-238 if a Stirling radioisotope power system were used. If RTGs are used, approximately 8 kilograms of plutonium-238 will be required for each mission.

Over the next decade, several Mars exploration missions will carry LWRHUs, which will require approximately 0.3 kilogram of plutonium-238 per mission. In the long term‹the next 20 to 35 years‹NASA is expected to need 2 to 5 kilograms of plutonium-238 per year for its space missions.n

This article was contributed by Gary Rinehart (NMT-9).

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