In the decades following the first spacefaring rockets, solar power and radioisotope thermoelectric generators have been responsible for providing power to probes, spacecraft, rovers, and more. In the process, these technologies have been fine-tuned and perfected to make them sturdier, more efficient, and more compact. While these advancements may not necessarily bring about a glorious future of space imperialism, they may contribute to a brighter, cleaner future here on Earth. Since the 70s, NASA has driven innovation in solar panel technology, including designing self-cleaning panels that can keep off dust and dirt. As greenhouse gases build up in our atmosphere and existing fossil fuels become depleted, innovations in sustainable energy will be our best bet for keeping our home planet habitable. But what about Mars? We’ve already established that solar energy won’t cut it—a poorly timed dust storm could bury our colonists alive and cut them off from the sun. The radioisotope thermoelectric generators, while more consistent in the long run, couldn’t possibly produce enough energy for a full-fledged colony—unless we happen to find a huge mine of plutonium hidden within Mars’ crust. What we need is a power source that is durable, versatile, efficient, and scalable. For years, NASA knew the answer to that riddle was hidden within the capabilities of nuclear power. A nuclear fission reactor can efficiently generate the power needed to run entire cities, but building such a reactor for reliable and safe use in space has proven too complex. Until recently, that is. In 2018, NASA, in collaboration with the National Nuclear Security Administration (NNSA), announced a new space power solution—the Kilopower reactor using Stirling technology (KRUSTY). After promising ground tests, the Kilopower reactor may soon be the first functional nuclear fission reactor in space.
The Kilopower reactor won’t be the first nuclear fission reactor ever launched into space though. In the 50s and 60s, following the success of the Manhattan Project, the government sponsored a new endeavor, called the System for Nuclear Auxiliary Power (SNAP) program, to develop compact and reliable nuclear reactors for use in space. The closest this program got to a successful space reactor was SNAP-10A. The SNAP-10A reactor used the splitting of uranium-235 to generate thermoelectric energy that was then converted to electrical energy by thermocouples (the same energy conversion system that’s used in radioisotope thermoelectric generators).
Of all the SNAP reactors, SNAP-10A was the first to pass all the design and flight tests and in 1965, it was launched into a polar orbit (an orbit that goes through both poles) 500 miles above Earth. SNAP-10A operated for 43 days until an electrical failure caused the reactor to shut down. It still orbits Earth with a near-circular orbit that will bring it crashing back down in roughly 3,000 years. By then, most of the radioactive material will be gone, but for now, the biggest concern is material brought down to earth through debris collisions. In addition to SNAP-10A, over 30 other defunct nuclear fission reactors from the USSR orbit Earth. A collision between any of these reactors and orbital debris could send radioactive material crashing to Earth.
The reason why SNAP-10A, and many subsequent reactor attempts, failed was really due to technical complexity. Complex electrical and thermonuclear setups translate to weaknesses that can trigger reactor failure. That’s why, starting in 2012, scientists at the Los Alamos National Laboratory (LANL) designed a new type of nuclear fission reactor built around the principle of simplicity. It started with the Demonstration Using Flattop Fission (DUFF—side note, as the video below notes, LANL names all of their nuclear experiments after Simpsons characters). The goal of DUFF was to create a lightweight system that could passively convert nuclear fission heat into electrical energy. The DUFF experiment was a fast success and it led directly into the KRUSTY endeavor, which sought to adapt the DUFF reactor into a high-power space reactor.
The Kilopower reactor that developed out of the KRUSTY program consists of two main systems—a nuclear fission reactor and a Stirling power converter—connected by heat-conducting, passive metal pipes. The entire structure resembles a flat top umbrella that can be retracted for compact storage aboard spacecraft. The nuclear reactor core is in the base of the “umbrella,” and it consists mainly of uranium-235 fuel.
The nuclear fission of uranium-235 occurs when the fairly stable nucleus (U-235 has a half-life of 700 million years) gets bombarded by a stray neutron. The neutron temporarily creates uranium-236, which is highly unstable and immediately splits apart into barium-139 and krypton-94. This process also releases a large amount of heat energy and three more free neutrons, which bombard surrounding uranium-235 atoms, creating a chain reaction. In power generators, the key with nuclear fission is control—you want the chain reaction to self-perpetuate, but not too quickly. To control the reaction, the core has to contain a control rod that absorbs neutrons. When the control rod is pulled up, the reaction begins, but the continued presence of the control rod prevents the reaction from realizing its explosive potential. A fission reaction without a control rod grows exponentially as soon as it starts—the basis of a nuclear bomb. The Chernobyl reactor explosion in 1986 was partly due to control rods that were tipped with graphite. When the emergency shutoff was triggered, instead of slowing down the reaction, the graphite-tipped rods initially sped up the reaction. The reaction was so strong that it cracked the control rods and exploded the top off of the reactor.
In addition to the control rod, the core is surrounded by a neutron reflector that prevents the reaction from petering out completely. Together, the control rod and the neutron reflector keep the reaction going at a manageable rate. The heat coming off of the core travels up the “umbrella” structure through pipes filled with liquid sodium metal. This heat is transferred through the pipes to eight Stirling engines in the center of the flat-top disk. Stirling engines, like thermocouples, translate temperature differentials into usable energy (if you remember, the blog post on entropy from a few months ago had a video with a practical demonstration of a hand-powered Stirling engine). A basic Stirling engine consists of two plates with a piston in between them. The whole system is sealed off so that a set amount of gas is inside with the piston. As one plate heats up, the gas around that plate expands and pushes the piston. But as the piston moves, it exposes more of the gas to the colder plate, causing the gas to contract again and pull the piston back to its original position. As long as one plate is hot and the other is cold, this cycle will continue, and bigger differences in temperature cause the piston to move faster. In this way, the Stirling engines convert the thermal energy coming off the fission reactor into kinetic energy, which can be translated into electrical energy.
In 2018, the Kilopower reactor underwent a series of brutal tests meant to judge its safety, reliability, and efficiency in a variety of difficult conditions. It passed these tests with flying colors and proved that it could consistently provide up to 10 kilowatts of usable power—roughly equivalent to the energy required by 8 average American households. Experts at NASA estimate that just four of these reactors would be enough to establish a small colony on the moon—or Mars—and the reactors could easily be delivered and set-up prior to human arrival. Right now, NASA’s long-term plans include establishing a small lunar colony around 2024 that would provide the data and experience necessary to potentially enable a Martian colony in the 2030s.
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