Space Fission Power Post #6:Entry-Level Option: Fission Surface Power (FSP) for Mars/Moon

Fission Surface Power (FSP) systems are well suited to be the workhorse of human exploration infrastructure on the Moon, Mars, or other potential destinations (e.g. Titan, large asteroid).  Some potential surface power electrical loads include habitats, in-situ resource utilization plants, rechargeable rovers, construction equipment, and science experiments.  The power output of a single workhorse surface system might be in the range of 20 to 50 kWe, with a lifetime of ~5 to 10 years. In addition, the relatively low-power and “high” mass allowance of a surface reactor might make it the easiest space fission system to develop.  For a human surface exploration mission, fission power has so many advantages over other power alternatives that the mass requirements will not be as stringent.  This allows “mundane” reactor technologies to be used (i.e. stainless-steel, UO₂) at very benign power/flux levels, which makes development simple.  Plus, as part of a “heavy” mission architecture, there should be less programmatic pressure to meet mass targets, or worse, decrease mass during development—a contributor to the downfall of the SP-100 program. 

There are many possible reactor and power conversion technologies available for an FSP system.  Stirling power conversion is generally considered the best option for systems in the 20 to 50 kWe range, with Brayton power conversion systems generally preferred at higher power.  Also, it is a general consensus that a fast reactor using UO₂ fuel and stainless-steel cladding and structure provide the lowest cost and risk system.  Heat transport for this class of Stirling system is via heat pipes or pumped liquid metal.  For a higher-power Brayton system, a gas-cooled reactor should also be strongly considered. These technologies could all be developed affordably; each having specific strengths and weaknesses. The liquid metal systems have issues with pump technology and system reliability, while heat pipe systems have some fabrication and integration issues, and gas-cooled systems are generally heavy.  For more information see FSP_Reactor_Compare and FSP_Cooling_Pros_and_Cons references.

In 2008, NASA and DOE developed a 40 kWe Stirling-based FSP concept that utilized a pumped liquid metal (NaK-78) reactor with a stainless steel and UO₂ fuel system.   This concept was designed with all of the attributes required for an entry-level space fission system.  The power level and lifetime fall comfortably below the knees-in-the curve that could affect development (e.g. irradiation damage, reflector cooling, fuel burnup, passive safety).  For more information on the reactor module see FSP_Reference_Reactor and for the complete system see FSP_Reference_System.  NASA is expected to complete an electrically-heated Technology Demonstration Unit (TDU) in 2013. A successful demonstration will place the system at TRL 6 and ready for flight development. The TDU will also be the first step in a series of non-nuclear tests that, when combined with component irradiation testing, enable a zero-power critical test to be used to qualify the flight system without a nuclear-powered system test.  For more info see the FSP_TDU reference.

Surface power radiation shielding is quite different from a space power mission, where a shadow shield can be used. This is because of scattering from the regolith and spacecraft components, so the shield design usually needs to surround the entire reactor. As a result, surface reactor shields can be very heavy, and can dominate the system mass. Fortunately there are many potential options for using in-situ resources as shielding materials, which can provide substantial mass savings. Regolith could be used in several ways: berming, sandbagging, digging/burying, using natural topography (e.g. craters), or any combination of these. In-situ resources could also be used to fill permanent structures or cans as a fixed part of the reactor structure (e.g. sand could be scooped or vacuumed), and in the most optimistic scenarios in-situ water could fill shield tanks or perhaps concrete could be made. All shielding options that require in-situ resources will depend on significant robotic ability (or existing robotic/human infrastructure) to successfully complete the mission. This could add significantly to mission risk, but the mass savings potential is so large that in-situ options warrant serious consideration. Any option that can pre-deploy and verify in-situ shielding prior to human mission departure would have a significant advantage. A comparison of deployment and shielding architectures can be found in the FSP_Deployment_Architectures and FSP_Shielding_Options references.

The reference FSP concept was designed for use on the Moon, but with the idea of also being used on Mars. There are a few differences between the applications, which are discussed in the FSP_Moon_to_Mars reference. A comparison of the estimated mass of solar and fission lunar/Mars power systems is shown below.

 

Power System	Specific Power (W/kg)
40 kWe Solar + Storage on Lunar Surface (equator)	0.6 W/kg (with 100% power at night) 1.3 W/kg (with 50% power at night)
40 kWe Solar + Storage on Mars Surface (clean surface and atmosphere)	5 W/kg (with 100% power at night) 8 W/kg (with 50% power at night)
40 kWe Solar + Storage on Mars (dust storm, effective insolation 100 W/m2??)	1.8 W/kg (with 100% power at night) 2.8 W/kg (with 50% power at night)
40 kWe FSP system: SS/UO2, NaK-cooled, Stirling conversion.	8 W/kg (with shaped or regolith assisted shield)

These solar power system specific masses are based on 25% efficient, sun-tracking photovoltaic arrays with 50% round-trip efficient regenerative fuel cells.  Despite being closer to the sun, the Moon is in many ways more challenging for solar power than Mars because of the cold 14 days of darkness. The fission system is dramatically better than solar on the lunar equator, and also substantially better at the poles (not shown in the table above).  On Mars, the solar system that provides only 50% power at night is about the same mass, but this does not include the risk of major, prolonged dust storms.  Also, in both cases when higher power systems are utilized, the specific power of the fission system becomes much more attractive.  Ultimately, second generation technologies can provide fission systems with substantially higher specific power.

In summary, the FSP’s combination of mission need and simple development makes it an ideal entry-level space fission system.  As such, this system has seen the majority of work over recent decades. Please refer to the documents referenced within for more detail on the system and the development approach.