A Lunar Nuclear Reactor

Researchers at NASA and the Department of Energy recently tested key technologies for developing a nuclear fission reactor that could power a human outpost on the moon or Mars. The tests prove that the agencies could build a "safe, reliable, and efficient" system by 2020, the year NASA plans to return humans to the moon.

A fission reactor works by splitting atoms and releasing energy in the form of heat, which is converted into electricity. The idea for using nuclear power in space dates back to the late 1950s, when they were considered for providing propulsion through Project Orion. In the 1960s a series of compact, experimental space nuclear reactors were developed by NASA under the Systems Nuclear Auxiliary Power program. But public safety concerns and an international treaty banning nuclear weapons in space stopped development.

Generating power: A power-conversion unit consisting of two Stirling engines, sitting opposite each other, is set up for testing at NASA’s Marshall Space Flight Center. Pumped liquid metal is used to transfer heat from the reactor to the engines, where it is converted to electricity.

Now nuclear power is being considered for lunar and Mars missions because, unlike alternatives such as solar power, it can provide constant energy, a necessity for human life-support systems, recharging rovers, and mining for resources. Solar power systems would also require the use of energy storage devices like batteries or fuel cells, adding unwanted mass to the system. Solar power is further limited because the moon is dark for up to 14 days at a time and has deep craters that can obscure the sun. Mars is farther away from the sun than either the Earth or the moon, so less solar power can be harvested there.

The new nuclear power system is part of a NASA project started in 2006, called Fission Surface Power, that is examining small reactors designed for use on other planets. While nuclear power remains controversial, the researchers say that the reactor would be designed to be completely safe and would be buried a safe distance from the astronauts to shield them from any radiation it would generate.

The recent tests examined technologies that would see a nuclear reactor coupled with a Stirling engine capable of producing 40 kilowatts of energy--enough to power a future lunar or Mars outpost.

Cooling off: Marc Gibson, a NASA test engineer, inspects the radiator panel used to cool the fission power system that is being tested at Glenn Research Center. The panel is six feet by nine feet. Twenty such radiators would be needed for a full-scale system.

"We are not building a system that needs hundreds of gigawatts of power like those that produce electricity for our cities," says Don Palac, the project manager at NASA Glenn Research Center in Cleveland, OH. The system needs to be cheap, safe, and robust and "our recent tests demonstrated that we can successfully build that," says Palac.

To generate electricity, the researchers used a liquid metal to transfer the heat from the reactor to the Stirling engine, which uses gas pressure to convert heat into the energy needed to generate electricity. For the tests, the researchers used a non-nuclear heat source. The liquid metal was a sodium potassium mixture that has been used in the past to transfer heat from a reactor to a generator, says Palac, but this is the first time this mixture has been used with a Stirling engine.

"They are very efficient and robust, and we believe [it] can last for eight years unattended," says Lee Mason, the principal investigator of the project at Glenn. The system performed better than expected, Palac says, generating 2.3 kilowatts of power at a steady pace.

he researchers also developed a lightweight radiator panel to cool the system and dissipate the heat from the reactor. The prototype panel is approximately six feet by nine feet--one-twentieth the size required for a full-scale system. Heat from a water-cooling system is circulated to the radiator where it dissipates.

The researchers tested the radiator panel in a vacuum chamber at Glenn that replicates the lack of atmosphere and the extreme temperatures on the moon--from over 100 degrees Celsius during the day to below 100 degrees Celsius at night. The panel dissipated six kilowatts of energy, more than expected--a "very successfully test," says Palac. On the moon, the panel must also survive the dusty environment cause by the regolith.

Lastly, the researchers tested the performance of the Stirling alternator in a radiation environment at Sandia National Laboratories in Albuquerque, NM. The objective was to test the performance of the motor, ensuring that the materials would not degrade. The alternator was subjected to 20 times the amount of radiation it would expect to see in its lifetime and survived without any significant problems.

Mason says that the tests are very important in showing the feasibility of the system and that the next step is for the researchers to conduct a full system demonstration, by combining a non-nuclear reactor simulator with the Stirling engine and radiator panel. He says that these tests should be completed in 2014.

The researchers are also working on the power transmission and electronics of the system. "A lunar base needs lots of power for things like computers, life support, and to heat up rocks to get out resources like oxygen and hydrogen," says Ross Radel, a senior member of the technical staff and part of the advanced nuclear concepts group at Sandia. His group is working on the systems dynamic analysis, a computer model that predicts how the reactor will perform during testing. "Nuclear is a stepping stone to move further out into manned space exploration," says Radel.

"It is a fascinating project and the only possible method of providing power for a manned trip to Mars," says Daniel Hollenbach, a researcher in the nuclear science and technology division at Oak Ridge National Laboratory, who was not involved in the project.

Mason says that nuclear fission is one of a number of concepts being tested as a power source for human missions to the moon and Mars, and if selected, he says the technology could be deployed by 2020.

By Brittany Sauser


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