A high performance NEP reactor offers more potential benefit to long-term human space exploration than any other space reactor application. The reactor for a high-performance NEP systems would power a multi-megawatt (MMW) system (>>1 MWe) with a very low specific mass (<< 10 kg/kWe). Whereas NTRs offer a factor of 2 performance benefit over chemical rocket technology, a higher performance NEP system can offer orders of magnitude better performance (ISP) – a truly enabling and paradigm shifting technology. Advances are continuously being made in all of the non-nuclear technologies needed for an NEP system, most importantly power conversion, heat rejection, and thrusters. Significant advances are being made in ion thrusters (HiPEP) and plasma thrusters (e.g. VASIMR), such that the deployment of high power EP systems appear possible within a decade or two. Conversely, a reactor for this kind of NEP system is essentially no closer to deployment than it was 50 years ago; which is another testament that non-nuclear system development is vastly easier than nuclear system development. The difficulty in this reactor is not achieving high power, but delivering power with a very low mass system, which implies very high temperature operation.
A high-performance MMW reactor has some of the same issues as an NTR, most importantly the fuel. An MMW system will need a very high temperature fuel with a high uranium density (to allow low mass). Additionally, this fuel will have to go to high nuclear burnup, because every additional U-235 atom burned increases the energy density achieved by the system. A high temperature structural material that can withstand significant irradiation damage must be found, which is also compatible with the fuel form. Once these two major reactor issues are resolved, a high temperature, high efficiency power conversion system must be developed that can integrate well with the reactor. On the plus side, the balance of the system can be developed in a non-nuclear fashion, to some extent utilizing the lessons learned and infrastructure from the earlier development of entry-level and second generation systems. Even then, the high power density of the core might make resistance-heated system testing impractical, so that a nuclear-powered ground test will be needed; unless several smaller reactors had been previously qualified and operated, and the data from those systems could be used to qualify the MMW system.
There are many potential technologies that have been proposed for low-mass, high temperature MMW space reactors. Refractory-metal gas-cooled reactors have been seriously considered in previous programs, with high temperature fuel in a particle/pellet configuration. This reactor could then be coupled to a refractory Brayton power conversion system. High-temperature liquid-metal cooled reactors (e.g. Li) have also been considered; coupled to potential liquid-metal Rankine systems or perhaps a refractory Brayton system. Both these options might offer hot-end temperatures between 1300 K and 1600 K, which is high enough to enable a high efficiency thermodynamic cycle while utilizing a high temperature radiator. For an MMW reactor, radiator size is one of the most important issues, to keep mass low and minimize issues such as deployment, shielding (scatter), and maneuverability. Thermionic systems are a possible candidate, in that they offer very high temperatures with reasonable efficiencies. Molten salt reactors have many attractive attributes (potential for low-mass, long-lived reactors) but a salt would need to be found that can reliably operate at temperatures high enough to make an attractive system. Finally, on paper, MHD power conversion systems could provide very attractive ultra-high temperature performance. Ultimately, this space fission system class (MMW NEP) has several sub-classes within it, and hopefully would evolve to higher and higher performance systems as they evolve.
It is debatable which propulsion system would be easier and more affordable to develop: NTP or MMW NEP. Starting from where we are today, an NTP system might be easier to develop; although both systems are well beyond the bounds of what could be an affordable system. If an evolutionary approach is undertaken for space fission power reactors, then eventually the MMW NEP system would be much easier to develop. Solar electric propulsion (SEP) systems could also serve as precursor NEP systems. What is not debatable is that the MMW NEP system increases our ability to explore and expand into space much more than an NTP system.
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