From Idaho Business Review on 2/24/2021
By Catie Clark
The Idaho National Laboratory plays an integral role in NASA missions, fueling and testing the radioisotope-powered generators for equipment such as the Mars rover Perseverance. Perseverance gets its electric power based on 20th-century space technology. Now the INL is pivoting to fuel and test 21st-century advances for the new outer-space generators currently in development, using technology which is three times more efficient.
The Mars rover Perseverance uses a device known as a Radioisotope Thermoelectric Generator. The RTG uses a plutonium-238 heat source attached to a solid-state device that uses the Seebeck effect to generate electricity. Unpacking that mouthful into English, the Seebeck effect is where electricity is generated at the interface between two different metals when that interface is heated. The circuit made from the two touching metals is called solid-state because there are no moving parts. The radioactive decay of the plutonium provides the heat.
NASA, its contractors and the INL are working together on the next generation of dynamic power sources to replace the solid-state RTGs. NASA provides the specifications of what it needs, contractors compete to engineer a working generator design and the INL fuels, tests and delivers the competed power source package.
Eric Clarke
“The first RTGs date back to the 1960s,” Eric Clarke told the Idaho Business Review. Clarke is a mechanical engineer and the manager of the Radioisotope Power Systems Department at the INL. “They have around 10% efficiency… (In comparison) the Dynamic Radioisotope Power Systems are at least three times more efficient.” When the plutonium heat source is the only source of energy for a mission, efficiency is important when a piece of equipment has to last more than a decade.
The DRPS technology now in development will use the reliable pre-existing plutonium-238 heat source of the RTGs but will use dynamic power conversion instead of the solid-state thermoelectric Seebeck effect. “The DRPS is dynamic because it has moving parts … The heat energy will be converted into mechanical energy, which will then be converted into electrical energy.” One method being developed uses a Stirling engine, a device that uses the thermal expansion and contraction of a working fluid to convert heat into mechanical motion.
The anticipated efficiency improvements of the DRPS technology are already known because NASA has already researched thermal-to-mechanical energy conversion using methods like the Stirling cycle. The design that NASA eventually selects will be built and sent to the INL to receive its plutonium and be tested.
“What we’re working on is a proto-flight demonstration (of the DRPS) for a lunar mission,” Clarke explained, describing a multi-year project timeline. “The INL is responsible for the nuclear safety aspects of the launch. We’re looking at a 2028 launch schedule, which means that we should have the DRPS at the INL in 2026.”
“Plutonium-238 is the heat source in the General Purpose Heat Sources used by NASA,” Clarke remarked. “The GPHS is a reliable and tested design. GPHS in space get their heat through the nature decay of radioisotopes. They are not miniature reactors, which get their heat from the kinetic energy of a sustainable chain-reaction of atomic fission.
Plutonium-238 is used in GPHSs in space because weight matters in NASA missions. It produces a very high production of watts per gram. Its half-life and decay are also suitable for space. Radioisotopes energetic enough for power generation in space have short half-lives. Plutonium-238 has a half-life of 87.7 years, which is good enough to meet NASA’s criterion of enough energy for 17 years of power generation.
Plutonium-238 decays by alpha emission. It emits only low amounts of beta, neutron and gamma radiation. This means that plutonium-238 does less radiation damage and needs less shielding. Neutrons, beta particles and gamma radiation can travel far enough to do radiation damage unless shielded. Shielding adds weight. Plutonium-238’s low neutron, beta and gamma radiation means less shielding is required. Its alpha particles are energetic but relatively heavy so they don’t travel far and can be shielded with a thin layer of plastic.
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