1 Mission Statement2 Power Conversion Unit Options2.1 Turbomachinery Cycles for Nuclear Reactors2.1.1 Brayton Cycle2.1.2 Stirling cycle2.1.3 Rankine cycle2.2 Solid State Power Conversion2.2.1 Thermophotovoltaic Cells2.2.2 Thermoelectric Conversion2.2.3 Thermionic Power Conversion2.2.4 Magnetohydrodynamic Power Conversion2.3 Electrochemical Cells2.3.1 Voltaic cells2.3.2 Electrolytic Cells3 PCU Decision3.1 Litmus Test3.2 Extent-to-Which Test3.2.1 Small Mass and Size3.2.2 Launchable / Accident Safe3.2.3 Controllable3.2.4 High Reliability and Limited Maintenance3.3 Thermionics as the Power Conversion Unit4 Design and Analysis of Thermionic PCU System4.1 Introduction to Thermionic Technology4.2 System Description and Specifications4.2.1 Diode Type4.2.2 Emitter Material4.2.3 Emitter Temperature4.2.4 Collector Material4.2.5 Collector Temperature4.2.6 Electrode Spacing4.2.7 Estimate of Cesium Reservoir Size4.2.8 Cesium to Barium Conversion Rate4.3 Expected Performance Characteristics4.3.1 Efficiency4.3.2 Mass and Area4.3.3 Other4.4 Failure Modes and Redundancy4.5 Scalability4.6 Discussion4.7 Summary5 D-A Power Conversion & Transmission System5.1 DC-AC System5.1.1 Options5.1.2 D-A System Selection & Analysis5.2 Transformers5.3 Transmission Cable5.4 Discussion6 Heat Exchanger to Radiator6.1 Options6.2 Annular Heat Pipes – Concept6.3 Heat Pipe Design6.4 Thermal Analysis6.5 Coupling to Radiator6.6 Discussion7 Future Work8 ConclusionAppendix A - Calculated Efficiency for Thermionic SystemsAppendix B – Thermionics Mass CalculationsMSR – Power Conversion System1 Mission StatementThe goal of the power conversion unit (PCU) is to convert heat from the reactor into usable electricity. As for parameters specific to the Lunar Space Reactor (LSR), the goals of the PCU are:1) Remove heat from the core2) Produce at least 100kWe3) Send excess heat to the radiator for dissipation4) Convert the electricity produced to a voltage/current suitable for transmissionAfter heat is removed from the core via lithium heat pipes (see Core section), some thermal energy is converted by the PCU into electricity. The electricity then flows to the habitat, and excess heat is dissipated. This specifies three main components for the PCU:1) Power Conversion System2) Electricity Conversion/Transmission System3) Thermal Coupling to the Radiator1MSR – Power Conversion System2 Power Conversion Unit OptionsThis section outlines the possible power conversion options for the MSR, including abrief system description and the pros and cons for each option. Presented below are thepower conversion unit (PCU) options with emphasis on the parameters of the lunarsurface reactor parameters. Tables of operating parameters follow each system.2.1 Turbomachinery Cycles for Nuclear ReactorsOne of the biggest advantages of turbomachinery cycles as a power conversion unit isthat they have the capacity to run at high efficiencies, approaching 50%. In spaceapplications, however, it is important to resist the lure of a high efficiency system thatwould cause the radiator size to be prohibitively large. Given that radiator size scalesroughly as T4, the need for high efficiency systems was reevaluated. Threeturbomachinery cycles are described: Brayton, Stirling and Rankine cycles.2.1.1 Brayton CycleThe Brayton cycle uses a single-phase gaseous coolant to convert thermal energy toelectricity. In this cycle, energy enters at a constant pressure with a rise in temperature, asshown in Figure 2.1-1.Figure 2.1-1 - T-S Diagrams for Brayton Cycle [1]The Brayton cycle can operate in either open or closed mode. In open mode, a workingfluid is taken in from the environment (i.e. air in the atmosphere), circulated once throughthe reactor, used to power the turbines and then ejected from the system. In a closedBrayton cycle, a working fluid is recycled through the system continuously byrecompressing it. The only moving parts in a Brayton cycle are the shaft, the turbine andthe compressor as shown in Figure 2.1-2. 2MSR – Power Conversion SystemFigure 2.1-1– Closed and Open Brayton Cycles [1]Many factors determine the efficiency of a Brayton cycle. First, in order for a Braytoncycle to produce more power than it consumes, the turbine and the compressor must havevery high efficiencies – over 80%. Work is also lost in compressing the working fluid,reducing the overall efficiency. The Brayton efficiency depends mainly on the inlet andoutlet temperatures – higher inlet temperatures and lower outlet temperatures allow formore effective energy conversion [1]. The following equation for Brayton efficiencyassumes 100% efficient turbines and compressors:inoutoutneteTTQW 1(2.1-1)where ηe is the efficiency, Wnet is the work out, Qout is the total energy used in the cycle, and Tin & Tout are the inlet and outlet temperatures, respectively. Typical efficiencies for Brayton cycles routinely approach 70% Carnot efficiency.There are advantages to using a Brayton system, the most notable of which is the largeexperience base. In addition, the use of inert gaseous coolants such as CO2 or heliummakes them attractive from a materials standpoint, where corrosion is effectively a non-issue in choosing structural materials. Brayton cycles can also be built very compactly –one multi-megawatt system designed using dual Brayton cycles occupied the space of acylinder 1.8m in diameter and 1.2m high [2]. This cycle can also accommodate high inlettemperatures, leading to higher efficiencies, or higher outlet temperatures for the sameefficiency. This is especially useful when dealing with the hot working fluid in a fastreactor. Finally, using an open CO2 cycle, the Martian atmosphere can serve as a coolantif NASA’s Planetary Protection Policy allows for it.There are however many disadvantages to a Brayton system in the context of spacereactor design. The most notable disadvantage is the large mass required. While Braytonsystems can be very light and compact, a heat exchanger is necessary to remove heat3MSR – Power Conversion Systemfrom the primary coolant, because the system uses a gas and therefore must be physicallyisolated from the primary coolant system. This will result in a decreased efficiency and amassive heat exchanger. The reason for this is that the thermal conductivity of metals isapproximately 30 times greater than most gases, so a