Prospects for a low specific mass multi-megawatt nuclear space power plant were examined assuming closed cycle coupling of a high-temperature fission reactor with magnetohydrodynamic (MHD) energy conversion and utilization of a nonequilibrium helium/xenon frozen inert plasma (FIP). Critical evaluation of performance attributes and specific mass characteristics was based on a comprehensive systems analysis assuming a reactor operating temperature of 1800 K for a range of subsystem mass properties. Total plant efficiency was expected to be 55.2% including plasma pre-ionization power, and the effects of compressor stage number, regenerator efficiency and radiation cooler temperature on plant efficiency were assessed. Optimal specific mass characteristics were found to be dependent on overall power plant scale with 3 kg/kWe being potentially achievable at a net electrical power output of 1-MWe. This figure drops to less than 2 kg/kWe when power output exceeds 3 MWe. Key technical issues include identification of effective methods for non-equilibrium pre-ionization and achievement of frozen inert plasma conditions within the MHD generator channel. A three-phase research and development strategy is proposed encompassing Phase-I Proof of Principle Experiments, a Phase-II Subscale Power Generation Experiment, and a Phase-III Closed-Loop Prototypical Laboratory Demonstration Test.
The principle technical obstacle to deep-space utilization of high-power nuclear electric propulsion (NEP) is the need for end-to-end system specific mass approaching 1 kg/kW. One of the few potentially feasible approaches for achieving the requisite power-to-weight performance are closed-cycle nuclear space power plants utilizing a High Temperature Gas Reactor (HTGR) coupled with a magnetohydrodynamic (MHD) generator, a concept readily scalable for application to multi-MW-class NEP systems. In contrast to turbo-generator systems, MHD generators extract energy at temperatures beyond solid material limits and provide high-temperature heat rejection. This advantage translates into weight savings via reduction in space radiator size and weight. If these potential specific power gains can be realized, it would open up entirely new vistas for rapid deep-space transport.
For many science-based robotic missions and surface power applications, power plant outputs of 100-120 kWe are considered readily achievable with currently available nuclear technologies, and various Advanced Radioisotope (AR) or Nuclear Fission Reactor (NFR) electric power generation systems could be engineered and developed with minimal R&D needs. For missions with larger electric power requirements, however, innovative low specific mass NFR systems with efficient electric power conversion are clearly needed. Here, we propose consideration of nuclear closed-cycle MHD (CCMHD) without bottoming cycle owing to the intrinsic high efficiency and compactness.
Thus far, thermoelectric converters (El-Genk and Saber, 2004; El-Genk and Tournier, 2004), stirling engines (Thieme and Schreiber, 2004), and turbo-brayton cycles (Zagarola et al., 2004; Godfroy et al., 2004) have been considered and studied as power conversion systems with AR power source for relatively smaller missions and with NFR for larger ones. For larger missions over 1 MWe, gas-cooled NFR and vapor core reactors with CCMHD energy conversion have also been considered and studied (Litchford, et al., 2001; Knight and Anghaie, 2004).
Baseline thermodynamic cycle analysis results for the proposed nuclear Brayton CCMHD space power generation system are summarized in Table 1 and Fig. 2(a) assuming 5-MW thermal reactor power. In this case, we deduce a re-circulating regenerative power of about 8 MW implying 12.9 MW of thermal input power at the entrance to the MHD generator under steady state operating conditions. The total MHD electric power output is 4.51 MW of which 1.67 MW is consumed by the compressor bank and 0.08 MW is consumed by the pre-ionization system, yielding a net electric power output of 2.76 MW and an overall plant efficiency of 55.2 %. Achievable plant efficiency is primarily determined by the difference between the available gas temperature at the exit of the reactor and the heat rejection temperature in the radiators.
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