NASA and the Department of Energy (DOE) are conducting concept studies to define a possible Fission Surface Power (FSP) system for Mars missions. Design options and development strategies were examined based on affordability and risk. The key system requirements were 40 kWe net power output, 8 year design life, and launch within eight years of beginning of system development. The resulting system uses a low temperature, uranium dioxide-fueled, liquid metal-cooled fission reactor coupled to free-piston Stirling convertors. The system is considered a low development risk based on the use of terrestrial-derived reactor technology, high efficiency power conversion, and conventional materials. The low-risk approach was selected over other options that could offer higher performance and/or lower mass.
The FSP system block diagram, shown on right, is defined by four major subsystems:
- Power Conversion
- Heat Rejection
- Power Conditioning and Distribution (PCAD)
Heat is transferred from the Reactor to the Power Conversion and from the Power Conversion to the Heat Rejection. Electrical power generated by the Power Conversion is processed through the PCAD to the User Loads. The PCAD provides power for Power Conversion startup and for auxiliary loads associated with the Reactor and Heat Rejection. The PCAD also provides the primary communications link for command, telemetry, and health monitoring of the FSP system.
Prior to selecting the current concept, a matrix of options was generated by selecting reactor fuel, primary coolant, power conversion type, and radiator coolant. The affordability goal for this study led to a decision by the NASA/DOE team to limit reactor fuel-clad temperature to 900 K to minimize fuel and material development costs and maximize the use of existing technology. Fuel options were UO2, UN, U10Zr (all fast-spectrum), and UZrH (moderated). The coolant options included pumped liquid metal (either sodium-potassium (NaK) or sodium (Na)), potassium heat pipes, and inert gas (HeXe). The power conversion options included Free-Piston Stirling (FPS), Closed Brayton Cycle (CBC), PbTe/TAGS Thermoelectrics (TE), and Organic Rankine Cycle (ORC). All of the options, with the exception of the TE concept, use a water-based radiator cooling system. The higher rejection temperature for TE required the use of NaK as the radiator coolant.
There were two system configurations studied by the NASA/DOE team: landed and emplaced. The landed configuration, shown on left, assumes the FSP system remains on a dedicated lander with integral, Earth-delivered shielding. The primary advantage of this configuration is ease of deployment because no regolith moving equipment is needed for the installation and it is possible that it could be deployed without crew assistance. The system is located approximately 1 km from the outpost in order to reduce shield mass. The integral, shaped 4-pi shield reduces radiation levels to less than 5 rem/yr at 1 km in the direction of the outpost and less than 50 rem/yr elsewhere. The PCAD subsystem converts the 400 V power conversion output to 2000 V for transmission and 120 Vdc for distribution to the loads. The installation of the power transmission cable would require crew assistance or a tele-operated rover.
The emplaced configuration, shown on right, assumes the FSP system is off-loaded from a cargo lander and installed with the assistance of crew and construction equipment. The line-of-sight reactor shielding to the habitat is provided by regolith, either by burying the reactor or moving regolith to form a berm. Additional Earth-delivered shielding would be provided to protect power conversion and heat rejection equipment mounted above the reactor. The FSP system is located approximately 100 m from the outpost. The combination of regolith and integral shielding reduces radiation levels to less than 5 rem/yr at 100 m in all directions (360°). The improved shielding allows the potential for simple, short-term maintenance tasks such as electrical part replacements or radiator surface cleaning during temporary zero-power shutdowns. The PCAD subsystem directly transmits the 400 V power conversion output to the load distribution node where it is converted to 120 Vdc. The shielding geometry assumptions for the two configurations are shown below.
The landed configuration is considered an excellent choice if the first system must be delivered before suitable construction infrastructure is available for the emplaced configuration. However, the FSP system mass for the landed configuration was estimated to be approximately twice that of the emplaced configuration. The landed configuration would also impose some potential constraints on crew operations and outpost expansion. Based on the lower mass and reduced operational constraints, the NASA/DOE team selected the emplaced configuration as the reference approach. However, the self-shielded, landed configuration with high voltage transmission was retained as a potential design option.
The figure on the right presents a notional layout for the concept. The deployed span is approximately 34 m tip-to-tip and 5 m above grade. The bottom edge of the radiators is approximately 1 m above the surface to minimize the potential for dust on the radiator surfaces. The reactor is located at the bottom of a 2 m excavation with an upper plug shield protecting the equipment above from direct radiation.