The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) at Lewis Field is planning a Technology Demonstration Unit (TDU) as an early step in the potential development of Fission Surface Power (FSP) systems. The goal of the TDU is to assemble the major components of a power system (heat source, power conversion, heat rejection, power conditioning and distribution) and conduct non-nuclear, integrated system testing in thermal-vacuum to evaluate overall performance. The TDU activity is a key element of the FSP technology development project which has a major objective to mature power conversion and other FSP components to a technology readiness level (TRL) 6 by the end of FY 2014. FSP systems are a potential option for future human exploration missions to Mars.
The conceptual layout for the TDU test system is shown in the figure to the right. The heat source, power conversion, and heat rejection would be contained inside the thermal-vacuum facility. The PCAD would be located outside of the vacuum facility. An external Data Acquisition and Control system would collect data and provide a user interface for commands and controls.
A 12 kWe Power Conversion Unit (PCU) is being developed by NASA and Sunpower, Inc., for use in the non-nuclear technology demonstration of a fission reactor power system. The PCU, shown below, features a unique sodium-potassium (NaK) heat exchanger that allows it to be integrated with a pumped NaK heat transfer loop. The PCU is comprised of two, 6 kWe free-piston Stirling engines. Fabrication of the engines has been completed, and testing is underway. The engines have demonstrated full power capability when operated individually with electrical heating, and will be tested in a dual-opposed configuration by Sunpower prior to integration with a NaK reactor simulator and water heat rejection system in late 2014 at NASA Glenn Research Center.
The PCU would interface with a liquid-metal heat source and a water Heat Rejection Subsystem (HRS) shown below. The heat source would supply thermal input via a pumped sodium-potassium (NaK) fluid loop at approximately 850 K. The heat rejection subsystem would provide cooling via a pumped water (H2O) fluid loop at approximately 375K. The initial HRS configuration may be an external facility chiller. The final HRS would consist of a water heat transport loop coupled to a series of two-sided radiator panels in vacuum. In either case, the PCU interface would be the same. The vacuum facility, liquid-metal heat source, and HRS are Installation- Provided Government Property.
The required PCU output power is 12 kWe. The heat source and HRS will be sized accordingly to accommodate the 12 kWe PCU output. The 12 kWe PCU power output is derived from a notional FSP concept developed by the government. A block diagram of the FSP system consists of a liquid-metal heat source that supplies thermal input to four, parallel-string convertors coupled to two radiator assemblies. Each convertor has a dedicated, pumped heat rejection loop and a dedicated electrical controller. Each radiator assembly is shared by two convertors. The use of two, separate radiator assemblies would allow them to be deployed on the Mars surface as symmetric wings from a central, vertical truss structure. The four electrical controllers feed a common DC bus that provides the power interface for user loads, system parasitic loads (pumps, motors, etc.), and auxiliary power sources. The FSP system produces a net power of 40 kWe available for user loads. The gross power provided by the four convertors would be 48 kWe, or 12 kWe per convertor. This would provide sufficient power margin for electrical losses (approximately 3 kWe) and system parasitic loads (approximately 5 kWe). This notional FSP system has a design life of 8 years.
The TDU test system includes the heat source, a single convertor and controller, a single heat rejection loop coupled to a partial radiator assembly, and the DC bus and user load interface. The heat source will be simulated with electrical resistance heaters and scaled based on the required heat input of one convertor. The user loads will also be simulated. The PCU portion of the block diagram consists of the convertor and controller. In this architecture, the controller is assumed to include the following functions: alternator control, DC rectification, voltage regulation, bus disconnect, startup power circuitry, data acquisition, and health monitoring.
The objectives of the TDU system test are as follows:
- Reduce Development Risk
- Verify System-Level Performance in Realistic Environment
- Characterize Component Performance in a System Context
- Obtain Comprehensive Temperature, Pressure, and Flow Data under Steady-State and Transient Operations
- Develop Safe and Reliable Control Methods
- Validate Analytical Codes
- Gain System Operations Experience
- Invigorate NASA/DOE Core Competencies in Nuclear Systems
- Stimulate Industrial Infrastructure for Component Design and Fabrication
- Demonstrate Manufacturing Methods
- Obtain As-Built Mass and Cost Data
- Provide Tangible and Measurable Technology Milestone