Lee Mason

Space Nuclear Power

June 12, 2007

Lee Mason gave a very engaging and informative presentation on space nuclear power systems on June 12, 2007. Mason has twenty years of experience working with NASA, the first ten of which were spent in the Space Analysis office and then the technology office. He has always been passionate about his work in power and propulsion, ranging from solar dynamic power systems for the International Space Station to nuclear powered probe missions to Jupiter’s moons. Currently, Mason is working in the Division Surface Power Program, developing power technologies for projects on the moon and Mars. 

Space Nuclear Power Model

Dr. Mason began with the history of space nuclear power. The SNAP-10A was the first big nuclear power program adopted by NASA. It used fission reactions, potentially yielding hundreds of kilowatts. This is the proposed type of power supply for surface power. The other type of nuclear power is an isotope system, which is more commonly used due to reliability, safety, and cost; however, it provides less than a kilowatt of power. In either case, there are numerous benefits to using nuclear power, including its high power, long life, compactness, low mass, reliability, independence on sunlight and robustness.

Fission reactor systems are composed of reactors, power converters, heat rejection systems and Power Management and Distribution (PMAD) systems. Lee Mason’s greatest interest is in power conversion options. There are five options: Brayton, Stirling, Rankine, Thermoelectric, and Thermionic. The former three are dynamic conversion technologies and the latter are static; generally, the dynamic systems have greater efficiency. Static systems are better for redundancy’s sake. Static systems are smaller and cheaper, thus more are installed. There are further tradeoffs associated with each technology outlined in Mason’s notes. The conversion systems range from five to thirty-five percent efficiency. Heat rejection units are necessary because of the heat lost due to inefficiency of the converters. These include heat exchangers, fluid pumps, heat pipes and radiator panels.

There are two main system discriminators that Mason mentioned. The first is Efficiency versus Temperature. Preferably, temperature is low so that advanced materials are not needed; however, remembering the Carnot cycle, the higher temperature, the more efficient the system. The second is Specific Mass versus Power; as power increases, specific mass decreases.

The current five-year plan is to build a fission surface power reactor suitable for the moon and Mars. The proposal is a modular 40kW reactor with a proposed life span of 8 years. With a buried configuration and the radiators deployed above ground, the surface will act as a shield. This would only be 3 x 3 x 7 meters stowed. Any system proposed goes through a rigorous and thorough development process. First is the Technical Demonstration Unit, which can last approximately seven years. Then a development test model is created, followed by the engineering model and then flight model. Finally, a system is approved for launch. This whole process can take up to twenty years.

—Kyle Gaiser