Thermionics Quo Vadis? An Assessment of the DTRA's Advanced Thermionics Research and Development Program (2001) / Chapter Skim
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3 Overview of the Technology
3 Overview of the Technology
Pages 15-32
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From page 15... ...
For these reasons, thermionic converters have been considered potentially useful as power Collector @? ~1000 K Plasma "Cloud" 1 [a ~ ~e- ~ ~ External Load ........
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From page 16... ...
Efficient operation of a thermionic converter requires an emitter surface with a relatively low work function of 3 electron volts or less and an even lower collector work function of 1.5 electron volts or less. The converter must also have the right charge transport conditions in the interelectrode gap, the area between the emitter and collector surfaces, to allow electrons to flow from the emitter to the collector.
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From page 17... ...
First, a layer 1.00 o.so 0.80 0.70 o 17 of adsorbed cesium is formed on both the emitter and collector surfaces that reduces the surfaces' work functions and improves converter efficiency. Second, the cesium vapor is ionized and forms a plasma in the interelectrode gap.
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From page 18... ...
However, the outof-core system also requires a core-to-converter heat transport mechanism, such as a heat pipe assembly or pumped liquid metal recirculating system, to deliver the thermal energy to the thermionic converter since the converters are not in direct contact with the heat in the reactor core. In-core thermionic converters use thermionic fuel elements that contain both the thermionic converter (or converters)
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Spacecraft operate as close as low Earth orbit, only TABLE 3.1 Potential Missions for Solar and Nuclear Thermionic Power Systems Mission Space Near-Term, Far-term Transportation/ Manned Space-Based Space-Based Electric Radar Radar Propulsion Lunar, Planetary Space Surface Exploration Power Space-Based Electric Advanced powered Communications Weapons Advanced Deep Space Telescope Power 3 to 10s 100s 5 to 100s 100s to 100s to 10s to 100 1,000s 10s to 100 required 10 MW for 1,000s (kW) cargo support missions Location LEO High orbit Orbit transfer, deep space Application ~ ~ ~ Orbit transfer for solar and deep space thermionics Application for nuclear thermionics Mars to deep space ~{ Cargo and robotic missions Deep space ~{ Manned ~1 .
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and thermionic converters. Finally, nuclear reactor- and radioisotope-heated dynamic and direct converters are listed.
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However, there is a cost penalty on the front end to 21 10kW 100kW 1 MW 10MW Electrical Power Output develop thermionic systems for in-core thermionic fuel elements. The cost of testing in-core for issues such as fuel swelling and radiation effects on materials can be significant.
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From page 22... ...
With a thermionic nuclear reactor system, however, the high heat rejection temperature allows for a smaller heat rejection radiator to be used. Even when compared to most other nuclear power conversion candidate technologies that operate outside the nuclear core yet have higher efficiencies, a nuclear in-core thermionic system is still often more desirable in terms of specific mass.
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From page 23... ...
Selection of an adequate radiator heat pipe fluid for 500 to 600 K radiators is difficult, but is often ignored when considering low temperature systems. A sodium heat pipe has a very high transport capacity, which allows for a lighter weight and less complex radiator assembly.
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These flight demonstrated power conversion technologies are compared in Table 3.2. The power conversion systems that have flown on spacecraft are listed below: · Photovoltaic Systems are still in widespread use; · Radioisotope thermoelectric generator (RTG)
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Fuel cells (alkaline~f Fuel (Wh/kg) 5.30 188.68 0.3 133,000 1997 30 plus Pu238 51.00 19.61 7 825 1981 0.3 1550.00 0.65 plus fuel Nuclear thermionicg 4.17 239.81 5 5000 1987 1.0 plus fuel 25 Highly survivable against natural radiation and spacebased weapon attacks Heat source degrades 0.8%/yr from Pu238 half-life Converter degrades 0.8%/yr Cassini Requires fuel resupply Requires fuel storage, tank weight included Does not need sunlight Orbiter Requires maintenance between flights Highly survivable Cosmos 1818 and 1867, Topaz I ao.s6 watt-hours of energy storage required for every watt of load power needed in a low Earth orbit (LEO)
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Je! Propulsion Laboratory Solar Energy Technology (SET} Thermionic Program JPL initiated a solar thermionic converter life evaluation and generator test program in 1961.
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In 1970, General Atomics was selected to continue development of the thermionic fuel element and to construct a thermionic test reactor. The program included designing thermionic power systems for both space and remote terrestrial applications, as well as developing fabrication methods for single-cell and multicell thermionic fuel elements and a thermionic critical reactor experiment.
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From page 28... ...
1999. "Design, Fabrication, and Testing of a 10 kW-hr H2-O2 PEM Fuel Cell Power System for High Altitude Balloon Applications," 34th Intersociety Energy Conversion Engineering Conference Proceedings, Society of Automotive Engineers, Inc., August 1999.
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From page 29... ...
Fuel swelling is caused by the accumulation of fission gas in the bulk fuel, which distorts the emitter and eventually results in an electrical short between the emitter and collector surfaces of the fuel elements. Radiation damage on materials was also a concern.
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Major development was carried out in a number of areas, including the nucleonic theory of compact thermionic space reactor design, the control theory of nuclear thermionic reactors, high temperature materials, high temperature nuclear fuel, single-crystal refractory metals, radiation degradation studies, and thermionic converter physics. These programs are impressive in terms of their breadth, as well as their depth in demonstrating the feasibility of space nuclear thermionic systems.
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From page 31... ...
U.S.-Russian Cooperation: TOPAZ International Program In the mid 1960s, the former Soviet Union initiated a full scale program to develop and test in-core thermionic reactors using UO2 fueled thermionic fuel elements (Ponomarev-Stepnoi et al.
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From page 32... ...
The single cell design also demonstrated the technology of advanced lowmodulus nuclear fuel combined with creep resistant, single-crystal emitter claddings, which was intended to result in long lifetime for the nuclear reactor system. Ultimately, the multicell system was flown.
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