Laboratory scientists recently met their milestone of purifying and encapsulating 40 heat-source plutonium-fueled clads; enough nuclear fuel to supply one multi-mission radioisotope thermoelectric generator (MMRTG) unit.
The fuel will power NASA’s Mars Science Laboratory rover, the largest and technologically most advanced Mars rover to date. It is scheduled for launch in the fall of 2009.
Laboratory-fabricated fueled clads are currently powering NASA’s Pluto-bound New Horizons probe, launched in 2006, and the Cassini spacecraft, a multinational mission involving NASA and three other space agencies. Los Alamos technology has supported NASA space missions for more than 40 years. Heat source technology fueled the Galileo, Ulysses, and Cassini missions, in addition to the Voyager 1 and 2 missions.
“The MSL rover will be powered by an MMRTG,” said Craig Van Pelt of Pu-238 Science and Technology (PMT-5). The MMRTG relies on general purpose heat source modules or, more colloquially, “nuclear batteries” to generate electricity. Each of the eight modules in the generator unit houses four fueled clads that generate heat from the radioactive decay of plutonium-238. This heat is converted into electricity by solid-state thermoelectric devices.
The plutonium-238 fueled clads used on this mission will power the rover communication and locomotion systems as well as the scientific payload of environmental sensors, radiation detectors, cameras, and several spectrometers, including the Laboratory developed ChemCam, he said. Although the MMRTG unit is designed for a minimum operating life of 14 years, the MSL rover will have a primary mission time of one Martian year (687 Earth days).
Van Pelt’s group collaborates with other Department of Energy laboratories to produce multi-mission radioisotope thermoelectric generator units. Oak Ridge fabricates the iridium material used to encapsulate the plutonium. Los Alamos purifies the plutonium oxide, converts the oxide powder into a ceramic pellet, and then welds the pellets inside the iridium cladding. The fueled clads are sent to Idaho where the generator unit is assembled and tested. Once complete, the MMRTG is delivered to NASA.
Purifying plutonium is not an easy task. “It’s probably one of the most hazardous operations at the plutonium facility—or the entire Lab—because of the very high specific radioactivity of plutonium-238,” Van Pelt said.
Van Pelt said his team continues to struggle with the public’s perceptions of plutonium. “Some people hear plutonium and automatically think ‘bomb,’ ” said Alejandro Enriquez of PMT-5. “It’s important for people to realize there are other, positive applications for the material.”
For example, from the mid-1960s through the early 1970s, the Laboratory developed a medical-grade fuel consisting of 90 percent enriched plutonium-238 for use in early artificial hearts and cardiac pacemakers. Although these devices were fully operational, their bulk caused problems.
Another peaceful application is in space exploration. “Any mission past Mars would be very difficult without plutonium,” said John Matonic of PMT-5.
Using plutonium for space travel makes sense for several reasons, Enriquez said. “It’s safe and provides reliable and uninterrupted heat and electricity,” he said. “And keeping sensitive equipment heated is vital in deep space, where it’s so cold.”
Plutonium heat sources also are low-maintenance. “One of their biggest advantages is that they have no moving parts,” Van Pelt said. “That’s really important, because it’s very difficult to repair, exchange, or clean equipment in outer space.” He said NASA contemplated but then rejected using solar panels as a potential heat source for Mars exploration due to the panels’ large size and difficulty keeping them clean. “Mars is a pretty dusty place,” Van Pelt said.
And, nuclear heat sources are much longer-lived than any conventional battery. "Plutonium has a half-life of 87.7 years, meaning that after 87.7 years, it will still produce 50 percent of its original power,” he said. Long enough to fuel NASA’s Pluto mission, which is expected to reach the former planet by 2014.
Plutonium provides heat in remote and harsh environments where other sources of power are not available or feasible. “There’s just not enough solar insolation past Mars for traditional solar panels to function,” Matonic said. And, nuclear power sources are small and lightweight, Van Pelt said.
Importantly, the power source’s iridium cladding is designed to survive reentry and impact should the nuclear material reenter the Earth’s atmosphere, Enriquez said. “The heat source plutonium oxide, an encapsulated high-fired refractory ceramic, does not dissolve in water, which is essential since any (Earth) reentry event would likely occur over the ocean,” he said.
"The technology developed [at Los Alamos] has shown us the wonders and majesty of creation,” Enriquez said, pointing to breathtaking photos of Saturn taken by Cassini. Van Pelt agreed, “Humans by nature are explorers, and our technology allows them to explore.”
