MSR Power Conversion System 1 Mission Statement The 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 2 3 4 Remove heat from the core Produce at least 100kWe Send excess heat to the radiator for dissipation Convert the electricity produced to a voltage current suitable for transmission After 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 2 3 Power Conversion System Electricity Conversion Transmission System Thermal Coupling to the Radiator 1 MSR Power Conversion System 2 Power Conversion Unit Options This section outlines the possible power conversion options for the MSR including a brief system description and the pros and cons for each option Presented below are the power conversion unit PCU options with emphasis on the parameters of the lunar surface reactor parameters Tables of operating parameters follow each system 2 1 Turbomachinery Cycles for Nuclear Reactors One of the biggest advantages of turbomachinery cycles as a power conversion unit is that they have the capacity to run at high efficiencies approaching 50 In space applications however it is important to resist the lure of a high efficiency system that would cause the radiator size to be prohibitively large Given that radiator size scales roughly as T4 the need for high efficiency systems was reevaluated Three turbomachinery cycles are described Brayton Stirling and Rankine cycles 2 1 1 Brayton Cycle The Brayton cycle uses a single phase gaseous coolant to convert thermal energy to electricity In this cycle energy enters at a constant pressure with a rise in temperature as shown 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 working fluid is taken in from the environment i e air in the atmosphere circulated once through the reactor used to power the turbines and then ejected from the system In a closed Brayton cycle a working fluid is recycled through the system continuously by recompressing it The only moving parts in a Brayton cycle are the shaft the turbine and the compressor as shown in Figure 2 1 2 2 MSR Power Conversion System Figure 2 1 1 Closed and Open Brayton Cycles 1 Many factors determine the efficiency of a Brayton cycle First in order for a Brayton cycle to produce more power than it consumes the turbine and the compressor must have very 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 and outlet temperatures higher inlet temperatures and lower outlet temperatures allow for more effective energy conversion 1 The following equation for Brayton efficiency assumes 100 efficient turbines and compressors e Wnet T 1 out Qout Tin 2 1 1 where e 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 large experience base In addition the use of inert gaseous coolants such as CO 2 or helium makes them attractive from a materials standpoint where corrosion is effectively a nonissue 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 a cylinder 1 8m in diameter and 1 2m high 2 This cycle can also accommodate high inlet temperatures leading to higher efficiencies or higher outlet temperatures for the same efficiency This is especially useful when dealing with the hot working fluid in a fast reactor Finally using an open CO2 cycle the Martian atmosphere can serve as a coolant if NASA s Planetary Protection Policy allows for it There are however many disadvantages to a Brayton system in the context of space reactor design The most notable disadvantage is the large mass required While Brayton systems can be very light and compact a heat exchanger is necessary to remove heat 3 MSR Power Conversion System from the primary coolant because the system uses a gas and therefore must be physically isolated from the primary coolant system This will result in a decreased efficiency and a massive heat exchanger The reason for this is that the thermal conductivity of metals is approximately 30 times greater than most gases so a very large surface area is required for an effective heat exchanger from liquid metal to gas Another disadvantage as with any turbomachinery is fast moving parts For the turbine to produce enough electricity it must spin about 40 000rpm These very high speeds introduce mechanical stresses to turbine parts increasing the possibility for turbine failure Such a failure is difficult to fix as it requires shutting down the reactor for maintenance Finally in order to achieve even modest efficiencies the Brayton cycle demands a very high inlet temperature further stressing moving materials already at high temperatures The combination of rapidly moving parts and high temperatures both producing physical stresses presents quite a difficult problem to the engineer One Brayton cycle that seems promising in the context of a lunar or Martian reactor is the supercritical CO2 cycle Using CO2 instead of helium allows for much lower inlet temperatures 830K for CO2 compared to 1170K for He at the tradeoff of a much higher pressure of 10 30MPa Such a high pressure in a near vacuum atmosphere presents a challenge to structural materials once again The main advantages of this system are its efficiency and its size cycles with inlet temperatures of 830K have shown efficiencies of up to 50 and as an example a 300MWe turbine was designed with a diameter of only one meter This could potentially decrease in size much more to accommodate our 100kWe system An example CO 2 cycle complete with heat exchanger recuperator and turbine designed by Dostal resulted in a cylindrical Power Conversion Unit PCU 18m high and 7 6m in diameter all components inclusive The PCU had a net efficiency of up to 49 produced 246Mwe and was 54 the size of an equivalent