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This paper compares the effectiveness of two control mechanisms for the S∧4 reactor, namely: (a) rotating BeO drums with 120° thin segments of enriched B4C in the radial reflector; and (b) sliding windows in the radial reflector. Investigated are the effects of using these control mechanisms on the differential reactor control worth, power generation profiles, and spatial neutrons flux distributions is the S∧4 reactor. For both control mechanism, the radial reflector has the same dimensions and volume. Results show that the difference in reactor performance with the two control mechanisms is small. The sliding reflector configuration features slightly lower mass and power peaking, and relatively more even fission power profiles in the core. The differential control worth for the sliding reflector segments is almost constant compared to that using rotating control drums, potentially simplifying the reactor control operation. The presence of a strong neutron absorber in the rotating drums slightly decreases the amount of excess reactivity at BOL compared with the sliding reflector configuration. However, the higher rate of reactivity depletion in the S∧4 reactor with the latter may negate this advantage. [ABSTRACT FROM AUTHOR]

Beyond Earth atmosphere, natural space radiation from Galactic Cosmic Rays and Solar Energetic Protons (SEPs) represents a significant hazard to both manned and robotic missions. For lunar settlements, protecting astronauts from SEPs is a key safety issue that needs to be addressed by identifying appropriate shielding materials. This paper investigates the interaction of SEPs with the lunar regolith, and quantifies the effectiveness of the regolith as a biological shield for a human habitat, compared to aluminum, presently the standard shielding material. Also calculated is the shielding thickness to reduce the dose in the habitat to those recommended by International Radiation Protection Committee and by NASA for operation on the international space station. The present calculations are for the most energetic solar event of February 1956, which included high energy protons up to 1000 MeV. Results show that the lunar regolith is as effective as aluminum for shielding lunar outposts. A large thickness of the regolith (∼30 g/cm2) would be needed to reduce the dose in the habitat from high energy protons below the 30 days flight crew limit of 25 Rem (or 250 mSv) and significantly more shielding would be needed (∼150 g/cm2) to reduce the dose down to the limit for radiation workers of 5 Rem (or 50 mSv). [ABSTRACT FROM AUTHOR]

This paper developed a design and investigated the performance of 38 kWe turbo-machine unit for space nuclear reactor power systems with Closed Brayton Cycle (CBC) energy conversion. The compressor and turbine of this unit are scaled versions of the NASA's BRU developed in the sixties and seventies. The performance results of turbo-machine unit are calculated for rotational speed up to 45 krpm, variable reactor thermal power and system pressure, and fixed turbine and compressor inlet temperatures of 1144 K and 400 K. The analyses used a detailed turbo-machine model developed at the University of New Mexico that accounts for the various energy losses in the compressor and turbine and the effect of compressibility of the He-Xe (40 mole/g) working fluid with increased flow rate. The model also accounts for the changes in the physical and transport properties of the working fluid with temperature and pressure. Results show that a unit efficiency of 24.5% is achievable at rotation speed of 45 krpm and system pressure of 0.75 MPa, assuming shaft and electrical generator efficiencies of 86.7% and 90%. The corresponding net electric power output of the unit is 38.5 kWe, the flow rate of the working fluid is 1.667 kg/s, the pressure ratio and polytropic efficiency for the compressor are 1.60 and 83.1%, and 1.51 and 88.3% for the turbine. [ABSTRACT FROM AUTHOR]

In order to reduce the size and mass of the single-shaft turbo-machines, with little impact on the size of the heat transfer components in the CBC loop, He-Xe binary mixture with a molecular weight of 40 g/mole has been the working fluid of choice in space nuclear reactor power systems with Close Brayton Cycle (CBC) for energy conversion. This working fluid is also a suitable coolant for the fission reactors heat source designed with fast neutron energy spectra. For space nuclear reactors with thermal neutron energy spectra, however, the high capture neutron cross-section of Xe will reduce the beginning-of-life excess reactivity of the reactor, decreasing its effective operation lifetime. In addition, the neutron activation of Xe in the reactor will introduce a radioactivity source term in the CBC loop. Alternative working fluids with no activation concerns and comparable performance are N2 and the binary mixtures of He-N2. This paper calculates the transport properties of these working fluids and compares their values to those of noble gas binary mixtures at the temperatures and pressures expected in CBC space reactor power system applications. Also investigated is the impact of using these working fluids on the pressure losses, heat transfer coefficient, and the aerodynamic loading of the blades in the CBC turbo-machines. [ABSTRACT FROM AUTHOR]

The 2004 NASA Vision for Space Exploration calls for the return of mankind to the moon by no later than 2020, in preparation for an adventure to Mars and beyond. An envisioned lunar outpost will provide living quarters for initially 5– 10 astronauts for up to 2 weeks, and latter for science experiments, and recovery of mineral and indigenous resources for the day-to-day operation and production of propellant. These activities would require electrical and thermal powers in the order of 10’s – 100’s of kilowatts 24/7. Potential power options include photovoltaic, requiring massive batteries or fuel cells for energy storage during the long nights on the moon, and nuclear reactor power systems, which are much more compact and operate independent of the sun. This paper examines the benefit of using the lunar regolith as a supplemental neutron reflector on decreasing the launch mass of the Sectored Compact Reactor (SCoRe-S), developed at the Institute for Space and Nuclear Power Studies. In addition to providing at least $2.00 of hot-clean excess reactivity at the beginning of life, various SCoRe-S concepts investigated in this paper are at least $1.00 sub-critical when shutdown, and when the bare reactor cores are submerged in wet sand and flooded with seawater, following a launch abort accident. Design calculations performed using MCNP5 confirmed that using lunar regolith as supplementary reflector reduces the launch mass of the SCoRe-S cores by ∼ 34% – 35%, or 150 – 200 kg, while satisfying the above reactivity requirements. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Fast neutron spectrum space reactors are an appropriate choice for high thermal powers, but for low powers, they need to be large enough to satisfy the excess reactivity requirement while remaining sub-critical when immersed in wet sand and flooded with seawater following a launch abort accident. All Sectored, Compact space Reactors (SCoRe) are fast spectrum reactors. They have a 0.1 mm thick 157Gd2O3 coating on the outer surface of the reactor vessel to reduce the effect of wet sand as a reflector, and 157GdN fuel additive to reduce the effect of thermal neutron fission on the criticality of the flooded reactor following a launch abort event. This paper identifies the SCoRe-S cores with the smallest BeO radial reflector thicknesses which satisfy the reactivity design requirements. These are a cold-clean and hot-clean excess reactivity > $4.00 and $2.00, respectively, at least $1.00 subcritical at shutdown, and when submerged in wet sand and flooded with seawater following a launch abort event. Reducing the radial reflector thickness increases the 157GdN fuel additive but decreases the diameter, and hence the shutdown margin, of the B4C/BeO control drums. The smallest reactor to satisfy the reactivity design requirements is SCoRe-S7, which has a thick BeO radial reflector (15.3 cm), and its mass (425.3 kg) is 21 kg heavier than the next larger SCoRe-S8 core with a thinner BeO reflector of 13.3 cm. The SCoRe-S7 is also 1.5 kg heavier than the SCoRe-S11, with the largest core, but the thinnest radial reflector thickness of 10.7 cm. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Detailed thermal-hydraulic analyses of the S∧4 reactor are performed to reduce the maximum fuel temperature of the Submersion-Subcritical Safe Space (S∧4) reactor to below 1300 K. The fuel pellet diameter is reduced from 1.315 cm to 1.25 cm, decreasing the thermal resistance of the pellets and each of the 1.54 cm diameter coolant channels in the reactor core are replaced with several 0.3 cm ID channels to increase the effective heat transfer area and to encourage mixing of the flowing helium-28% xenon coolant. The calculated maximum fuel temperature decreased from more than 1900 K to 1302 K and the relative pressure drop across the reactor core increased from 1.98% to 2.57% of the inlet pressure. Moving the concentric inlet and outlet pipes 1 cm towards the center of the reactor core encouraged more flow through the center region, further reducing the maximum fuel temperature by 14 degrees to 1288 K, with a negligible effect on the core pressure losses. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Space fission reactors can provide reliable, high power levels for periods of more than 10 years to support human outposts and for space travel to the farthest planets in the solar system. The operation lifetimes of these reactors depend on many factors chief among which are the hot-clean excess reactivity and the fuel burnup rate (or operation power) and the accumulation and decay of fission products. Other important parameters are the fuel average temperature and fissile inventory and the Doppler reactivity effect. Determining the operation lifetime for space reactors is a critical input to mission planning, requiring the use of sophisticated fuel burnup and criticality computational tools and benchmarking the results against actual data, if readily available. This paper performs parametric and comparative studies using widely used codes and a simplified approach for determining the operation lifetimes of two space reactors: the Sectored, Compact Reactor (SCoRe) that is liquid metal cooled, and the Submersion-Subcritical, Safe Space (S∧4) reactor that is cooled by a He-Xe binary gas mixture. The codes investigated against experimental data from a LWR are: (a) Monteburns 2.0, coupling MCNP5 1.30 to Origen2.2, (b) MCNPX 2.6b’s internal burn package incorporating CINDER90, and (c) TRITON a code in the SCALE5 package using NEWT and Origen-S. From the results Monteburns and MCNPX performed the best, and are selected to compare their predictions of the lifetimes of the two space reactors with those of a simplified method. This method couples MCNP5 to a burnup analysis model in Simulink® considering only the 10 most probable low Z and 10 most probable high Z elements of the fission yield peaks plus 149Sm. Results show that the predicted operational lifetimes using the simplified method are within -6.6 to 12.8% of those calculated using the widely used Monteburns 2.0 and MCNPX 2.6bc1 codes. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Closed Brayton Cycles with centrifugal flow, single-shaft turbo-machines are being considered, with gas cooled nuclear reactors, to provide 10’s to 100’s of electrical power to support future space exploration missions and Lunar and Mars outposts. Such power system analysis is typically based on the cycle thermodynamics, for given operating pressures and temperatures and assumed polytropic efficiencies of the compressor and turbine of the Brayton energy conversion units. Thus the analysis results not suitable for modeling operation transients such as startup and changes in the electric load. To simulate these transients, accurate models of the turbine and compressor in the Brayton rotating unit, which calculate the changes in the compressor and turbine efficiencies with system operation are needed. This paper presents flow models that account for the design and dimensions of the compressor impeller and diffuser, and the turbine stator and rotor blades. These models calculate the various enthalpy losses and the polytropic efficiencies along with the pressure ratios of the turbine and compressor. The predictions of these models compare well with reported performance data of actual hardware. In addition, the results of a parametric analysis to map the operations of the compressor and turbine, as functions of the rotating shaft speed and inlet Mach number of the gas working fluid, are presented and discussed. The analysis used a binary mixture of He-Xe with a molecular weight of 40 g/mole as the working fluid. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The solid-core, gas-cooled, Submersion-Subcritical Safe Space (S∧4) reactor is developed for future space power applications and avoidance of single point failures. The Mo-14%Re reactor core is loaded with uranium nitride fuel in enclosed cavities, cooled by He-30%Xe, and sized to provide 550 kWth for seven years of equivalent full power operation. The beryllium oxide reflector disassembles upon impact on water or soil. In addition to decreasing the reactor and shadow shield mass, Spectral Shift Absorber (SSA) materials added to the reactor core ensure that it remains subcritical in the worst-case submersion accident. With a 0.1 mm thick boron carbide coating on the outside surface of the core block and 0.25 mm thick iridium sleeves around the fuel stacks, the reflector outer diameter is 43.5 cm and the combined reactor and shadow shield mass is 935.1 kg. With 12.5 atom% gadolinium-155 added to the fuel, 2.0 mm diameter gadolinium-155 sesquioxide intersititial pins, and a 0.1 mm thick gadolinium-155 sesquioxide coating, the S∧4 reactor has a slightly smaller reflector outer diameter of 43.0 cm, and a total reactor and shield mass of 901.7 kg. With 8.0 atom% europium-151 added to the fuel, 2.0 mm diameter europium-151 sesquioxide interstitial pins, and a 0.1 mm thick europium-151 sesquioxide coating, the reflector’s outer diameter and the total reactor and shield mass are further reduced to 41.5 cm and 869.2 kg, respectively. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The S∧4 reactor has a sectored, Mo-14%Re solid core for avoidance of single point failures in reactor cooling and Closed Brayton Cycle (CBC) energy conversion. The reactor is loaded with UN fuel, cooled with a He-Xe gas mixture at ∼1200 K and operates at steady thermal power of 550 kW. Following a launch abort accident, the axial and radial BeO reflectors easily disassemble upon impact so that the bare reactor is subcriticial when submerged in wet sand or seawater and the core voids are filled with seawater. Spectral Shift Absorber (SSA) additives have been shown to increase the UN fuel enrichment and significantly reduce the total mass of the reactor. This paper investigates the effects of SSA additions on the temperature and burnup reactivity coefficients and the operational lifetime of the S∧4 reactor. SSAs slightly decrease the temperature reactivity feedback coefficient, but significantly increase the operating lifetime by decreasing the burnup reactivity coefficient. With no SSAs, fuel enrichment is only 58.5 wt% and the estimated operating lifetime is the shortest (7.6 years) with the highest temperature and burnup reactivity feedback coefficients (-0.2709 ¢/K and -1.3470 $/atom%). With europium-151 and gadolinium-155 additions, the enrichment (91.5 and 94 wt%) and operating lifetime (9.9 and 9.8 years) of the S∧4 reactor are the highest while the temperature and burnup reactivity coefficients (-0.2382 and -0.2447 ¢/K; -0.9073 and 0.8502 $/atom%) are the lowest. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Fast neutron spectrum space reactors are an appropriate choice for high thermal powers, but for low powers, they may not satisfy the excess reactivity requirement while remaining sub-critical when immersed in wet sand and flooded with seawater following a launch abort accident. This paper identifies the smallest size fast spectrum, Sectored, Compact Reactor loaded with Single UN fuel pins (SCoRe-S7), which satisfy the requirements of cold clean excess reactivity > $4.00 and remains at least $1.00 subcritical at shutdown and in submersion conditions. Results indicate that increasing the diameter of the SCoRe-S core reduces its active height and the UN fuel enrichment, but increases the Spectrum-Shift Absorber (SSA) of 157GdN additive to the fuel. All SCoRe-S cores also have a 0.1 mm thick 157Gd2O3 SSA coating on the outer surface of the reactor vessel to reduce the effect of the wet sand reflector, while the SSA fuel additive reduces the effect on the criticality of the flooded reactor caused by thermal neutron fission. The active core height decreases from 42.4 cm for the smallest SCoRe-S7 to as much as to 37.4 cm for the largest core of SCoRe-S11. For a 1.8 MWth reactor thermal power the UN fuel specific power decreases from 17.0 in the SCoRe-S7 to 11.5 Wth/kg in the -S11. The corresponding reactor total mass, including the BeO reflector, increases from 440 kg to 512 kg. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper presents the results of a 3-D performance analysis of Skutterudite-Based Segmented Unicouple (SKU) with nine μm thick Molybdenum (Mo) coatings on n- and p-legs near the hot junction. The analysis performed at hot and cold junction temperatures of 973 K and 300 K investigated the effect of the coating on lateral distributions of the electric current, voltage, and temperatures in the legs and the conversion efficiency and electric power generated by the unicouple. The p- and n-legs are 20 mm long and have hot and cold segments of CeFe3.5Co0.5Sb12 and Bi0.4Sb1.6Te3, and of CoSb3 and Bi2Te2.95Se0.05, respectively. The cross-section area of the p-leg is 11 mm2. The cross sectional area of the n-leg and the lengths of the Mo coatings and of various segments of the legs are determined for maximum conversion efficiency using a 1-D optimization model. The 3-D analysis performed with ANSYS finite element commercial software used the dimension determined by the 1-D model. Results show that the electric currents near the outer surface of the Mo coatings flow in the opposite direction to those in the n- and p-legs. In very thin parts of legs near the interface with the coatings, electric currents flow in the same direction as in the coatings, but decrease rapidly with inward distance from the interface, eventually becoming zero, then flowing in the same direction as the electric current in the legs. Such current circulation increases joules losses in the coated parts of the hot segments of the legs, decreasing the conversion efficiency. The increased heat flow from the hot shoe through the metallic coatings contributes to decreasing the conversion efficiency, but increasing the electric power generated by SKU. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

In many space nuclear power systems, the primary and/or secondary loops use liquid metal working fluids, and require accumulators to accommodate the change in the liquid metal volume and maintain sufficient subcooling to avoid boiling. This paper developed redundant and light-weight bellows-type accumulators with and without a mechanical spring, and compared the operating condition and mass of the accumulators for different types of liquid metal working fluids and operating temperatures: potassium, NaK-78, sodium and lithium loops of a total capacity of 50 liters and nominal operating temperatures of 840 K, 860 K, 950 K and 1340 K, respectively. The effects of using a mechanical spring and different structural materials on the design, operation and mass of the accumulators are also investigated. The structure materials considered include SS-316, Hastelloy-X, C-103 and Mo-14Re. The accumulator without a mechanical spring weighs 23 kg and 40 kg for a coolant subcooling of 50 K and 100 K, respectively, following a loss of the fill gas. The addition of a mechanical spring comes with a mass penalty, in favor of higher redundancy and maintaining a higher liquid metal subcooling. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A high temperature water heat pipes radiator design is developed for a space power system with a sectored gas-cooled reactor and three Closed Brayton Cycle (CBC) engines, for avoidance of single point failures in reactor cooling and energy conversion and rejection. The CBC engines operate at turbine inlet and exit temperatures of 1144 K and 952 K. They have a net efficiency of 19.4% and each provides 30.5 kWe of net electrical power to the load. A He-Xe gas mixture serves as the turbine working fluid and cools the reactor core, entering at 904 K and exiting at 1149 K. Each CBC loop is coupled to a reactor sector, which is neutronically and thermally coupled, but hydraulically decoupled to the other two sectors, and to a NaK-78 secondary loop with two water heat pipes radiator panels. The segmented panels each consist of a forward fixed segment and two rear deployable segments, operating hydraulically in parallel. The deployed radiator has an effective surface area of 203 m2, and when the rear segments are folded, the stowed power system fits in the launch bay of the DELTA-IV Heavy launch vehicle. For enhanced reliability, the water heat pipes operate below 50% of their wicking limit; the sonic limit is not a concern because of the water, high vapor pressure at the temperatures of interest (384 – 491 K). The rejected power by the radiator peaks when the ratio of the lengths of evaporator sections of the longest and shortest heat pipes is the same as that of the major and minor widths of the segments. The shortest and hottest heat pipes in the rear segments operate at 491 K and 2.24 MPa, and each rejects 154 W. The longest heat pipes operate cooler (427 K and 0.52 MPa) and because they are 69% longer, reject more power (200 W each). The longest and hottest heat pipes in the forward segments reject the largest power (320 W each) while operating at ∼ 46% of capillary limit. The vapor temperature and pressure in these heat pipes are 485 K and 1.97 MPa. By contrast, the shortest water heat pipes in the forward segments operate much cooler (427 K and 0.52 MPa), and reject a much lower power of 45 W each. The radiator with six fixed and 12 rear deployable segments rejects a total of 324 kWth, weights 994 kg and has an average specific power of 326 Wth/kg and a specific mass of 5.88 kg/m2. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The space nuclear reactors being considered to support the Jupiter Icy Moons Orbiter (JIMO-1) mission-1 sometime in the next decade are compact and fast spectrum with void fractions ranging from 20–40%. In order to secure launch approval, it has to be demonstrated that these reactors will remain sufficiently subcritical when submerged in water or wet sand and subsequently flooded with water, following a launch abort accident. The resulting shift in the neutron spectrum towards thermal increases reactivity, potentially making the reactors supercritical. Incorporating “Spectral Shift Absorbers” (or SSAs), elements such as boron, europium, gadolinium or rhenium, which have significantly higher absorption cross-sections for thermal versus fast neutrons, can offset the reactivity increase. It has always been the assumption that the worst-case submersion accident is with a fully flooded reactor; however, this work shows that, depending on the type and amount of SSA in the reactor, a submerged but unflooded reactor core could be more reactive, raising a safety concern. This condition, referred to as “reactivity inversion”, is investigated for the following SSA elements: boron, boron-10, samarium-149, europium, europium-151, gadolinium, gadolinium-155, gadolinium-157 and rhenium, by varying the ratio of SSA to fissile atoms in the reactor. The effect of placing a coating of SSA material on the outside of the reactor vessel is also investigated. Gadolinium-157 is the most effective SSA material investigated as a core additive, yielding the greatest increase in the safety worth of a submerged space reactor with a comparatively small decrease in excess reactivity. Samarium-149 yields a similar increase in safety worth, but with a much larger decrease in excess reactivity. Natural europium and gadolinium are also promising: however, the decrease in the excess reactivity is greater with europium than with gadolinium. Boron and boron-10 are not particularly attractive as core additives, but boron-10 yields the greatest benefit as a coating. Rhenium is useful only if the desired safety worth is small (< $3) and at a significant mass penalty. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Gadolinium-155 and europium-151 are examined as alternative spectral shift absorbers to rhenium in the Scalable AMTEC Integrated Reactor space power System (SAIRS) heat-pipe reactor. Spectral shift absorbers counteract the reactivity increase when a compact, highly-enriched space nuclear reactor is submerged in seawater or wet sand and flooded following a launch abort accident. After all excess rhenium is removed from the reactor core, gadolinium-155 or europium-151 is added to the core in the form of a 0.1 mm oxide coating on the inside of the reactor vessel and/or as a nitride additive to the UN fuel. To compensate for increased parasitic neutron absorption, the UN fuel enrichment in the SAIRS reactor is increased to from 83.5% to a maximum of 94%. With 12 atom% 155GdN added to the reactor fuel, the outer diameter of the axial reflector decreased by 2 cm, and with a 155Gd2O3 coating on the inside of the reactor vessel, the reactor has $2.47 of excess reactivity at Beginning of Mission (compared to $2.08 for the rhenium base-case) and a worst case submersion and flooding accident reactivity of -$1.12 (compared to -$0.93 for the base-case). The resulting reactor and shield weigh 951.20 kg, for a savings of 100.94 kg over the base-case. When 9 atom% 151EuN is used in the fuel, the outer diameter of the axial reflector is reduced by 4 cm, and the reactor has $2.53 excess reactivity and -$1.13 of reactivity in the worst-case submersion and flooding accident scenario. The europium-case represents a mass savings of 143.16 kg over the base-case for a total reactor and shield mass of 908.98 kg. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The performance and specific electrical powers of Radioisotope Power Systems (RPSs) with Skutterudite-based Thermoelectric Arrays (SKTAs) of 2×7 and 4×7 unicouples connected electrically in series are calculated as functions of load voltage from 24 to 38 VDC. The p-legs of the unicouples are made of CeFe3.5Co0.5Sb12 and the n-legs are made of CoSb3, and the optimized cross-section areas of these legs for maximum efficiency operation are calculated as functions of the RPS terminal voltage. The RPS employs five General Purpose Heat Source (GPHS) bricks, generating 1250 Wth at BOM, and 40 SKTAs, which are connected in series in two or four parallel strings. The peak electrical power and peak conversion efficiency of the SKTAs (20 mm long) occurs at the same electrical current because the unicouples operate at constant hot junction temperature (973 K) and constant input thermal power. The cold junction temperature of the unicouples and the radiator base temperature are determined from coupling the unicouples performance optimization model with a detailed radiation thermal analysis for sizing the aluminum fins for rejecting waste heat into space. The calculated peak specific powers and the corresponding electrical powers of the RPSs at Beginning-Of Mission (BOM) are compared with those calculated at the peak electric power at different load voltages. Results indicated that the value of the load voltage strongly affects the performance of the RPS. Both the RPS peak specific powers and those corresponding to the peak conversion efficiency / peak electrical power increase with increasing the load voltage up to certain values then drop with further increasing that voltage. The highest BOM specific powers of the RPS is ∼ 6.859 –6.973 We/kg, ∼ 43.3% and 45.7% higher than that of the RTG (4.786 We/kg) with 7 GPHS brick and SiGe unicouples. Such specific electrical powers of the RPS with SKTAs are attainable with 28.6% less 238PuO2 fuel and 2.85% to 11.62% more BOM electrical power (122 We) than the RTG (105 We). © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Skutterudite-based thermoelectric unicouples are currently being considered for use in next generation Radioisotope Power Systems (RPSs) to reduce the amount of 238PuO2 fuel and the mission’s cost and increase the system specific power. In these unicouples, typically operateing at a hot junction temperature of 973K, the loss of the volatile antimony from the n- and p-legs near the hot junction, if not suppressed, could gradually degrade their performance. Recently, it has been shown that such a loss of antimony could be effectively suppressed by applying a thin metallic coating on the legs near the hot junction. However, to minimize the impact on the conversion efficiency, the thickness and the length of the coating need to be optimized. An analytical model for coated skutterudite-based segmented unicouples is developed and used to optimize the length and thickness of the coating and quantity its effect on the conversion efficiency and load electrical power. Several coating materials are investigated: Tantalum (Ta), Titanium (Ti), Molybdenum (Mo), and Vanadium (V). The required coating length for maintaining the antimony vapor pressure off the exposed portions of the legs ≤ 10-3 Pa depends on its thickness, which varied from 1.0 to 9 μm. Results indicated that increasing the coating thickness decreases the conversion efficiency, but increases the load electrical power. The smallest decrease in the conversion efficiency occurs when using Ti or V coating, while using Ta or Mo coating results in a large decrease in the conversion efficiency. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

A performance test of a Skutterudite-based unicouple (MAY-04) with a metallic coating to suppress the sublimation of antimony from the legs near the hot junction is performed in vacuum (∼ 9 ×10-7 torr) for ∼ 2,000 hours at hot and cold junction temperatures of 892.1 ± 11.9 K and 316.1 ± 5.5 K, respectively. The p-leg is made of CeFe3.5Co0.5Sb12 and the n-leg is made of CoSb3. Presented are the measured voltage-current characteristics, electrical power, open-circuit voltage, and the Seebeck coefficients of the legs as functions of cumulative test time. Also presented is the estimate of the conversion efficiency, ∼ 96 hrs after the start of test. To demonstrate the effectiveness of the metallic coating, the measurements for MAY-04 are compared with those of two uncoated unicouples of the same leg materials (MAR-03 and JUN-03), which had been tested earlier. The cross-sectional areas of the legs in MAY-04 are larger than those in MAR-03 and JUN-03, tested in argon cover gas at ∼ 0.051–0.068 MPa for 450 and 1200 hours, respectively. The open circuit voltage, Voc (204 mV) of MAY-04 at Beginning-Of-Test (BOT) is almost the same as that of MAR-03, but higher than that of JUN-03 (∼180 mV). Although the argon gas effectively decreased antimony loss from legs of MAR-03 and JUN-03, marked degradations in performance occurred with time in these tests. Conversely, the metallic coating in MAY-04 effectively reduced the performance degradation with cumulative test time. The estimated peak efficiency of MAY-04, shortly after BOT (10.65%) is only ∼ 0.37 percentage point lower than the theoretical value, assuming zero side heat losses and zero contact resistance per leg. The peak power of MAY-04 decreased by only ∼12%, from its BOT value of ∼1.6 We to ∼1.4 We after ∼2,000 of cumulative testing. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Conceptual designs of Advanced Radioisotope Power System (ARPS) with Cascaded Thermoelectric Converters (CTCs) are developed and optimized for maximum efficiency operation for End-Of Mission (EOM) electrical power of at least 100 We. These power systems each employs four General Purpose Heat Source (GPHS) bricks generating 1000 Wth at Beginning-of-Life (BOL) and 32 Cascaded Thermoelectric Modules (CTMs). Each CTM consists of a top and a bottom array of thermoelectric unicouples, which are thermally, but not electrically, coupled. The top and bottom arrays of the CTMs are connected electrically in series in two parallel strings with the same nominal voltage of > 28 VDC. The SiGe unicouples in the top array of the CTMs are optimized for nominal hot shoe temperature of 1273 K and constant cold shoe temperature of either 780 K or 980 K, depending on the thermoelectric materials of the unicouples in the bottom array. For a SiGe cold junction temperature of 780 K, the unicouples in the bottom array have p-legs of TAGS-85 and n-legs of 2N-PbTe and operate at constant hot junction temperature of 765 K and nominal cold junction temperature of 476.4 K. When the SiGe cold junction temperature is 980 K, the unicouples in the bottom arrays of CTMs have p-legs of CeFe3.5Co0.5Sb12 or CeFe3.5Co0.5Sb12 and Zn4Sb3, segments and n-legs of CoSb3 and operate at constant hot junction temperature of 965 K and nominal cold junction temperatures of 446.5 K or 493.5 K, respectively. The CTC-ARPSs have a nominal efficiency of 10.82% – 10.85% and generate BOL power of 108 We. This system efficiency is ∼ 80% higher than that of State-of-the-Art (SOA) Radioisotope Thermoelectric Generators (RTGs), requiring 7 GHPS bricks and generating 105 We at BOL. The CTC-ARPSs have specific powers of 8.2 We/kg to 8.8 We/kg, which are 71% to 83% higher, respectively, than that of the SOA-RTGs, and use ∼ 43% less 238PuO2 fuel. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A methodology for the neutronics design of space power reactors is presented. This methodology involves balancing the competing requirements of having sufficient excess reactivity for the desired lifetime, keeping the reactor subcritical at launch and during submersion accidents, and providing sufficient control over the lifetime of the reactor. These requirements are addressed by three reactivity values for a given reactor design: the excess reactivity at beginning of mission, the negative reactivity at shutdown, and the negative reactivity margin in submersion accidents. These reactivity values define the control worth and the safety worth in submersion accidents, used for evaluating the merit of a proposed reactor type and design. The Heat Pipe-Segmented Thermoelectric Module Converters space reactor core design is evaluated and modified based on the proposed methodology. The final reactor core design has sufficient excess reactivity for 10 years of nominal operation at 1.82 MW of fission power and is subcritical at launch and in all water submersion accidents. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper presents results of three performance tests of Skutterudites and Segmented Thermoelectric (STE) unicouples performed at average hot and cold shoe temperatures of ∼ 973 K and 300 K, respectively, to verify theoretical predictions. The first two tests (MAR-03 and JUN-03) involved non-segmented Skutterudites unicouples of slightly different dimension but same materials for the n- (CoSb3) and p- (CeFe3.5Co0.5Sb12) legs. The test duration is 450 hours for MAR-03 and 1200 hours for JUN-03. The third test (JUL-03) is for a Skutterudites/Segmented (STE) unicouple, in which the p-leg has two segments of CeFe3.5Co0.5Sb12 and Bi0.4Sb1.6Te3 and the n-leg has two segments of CoSb3 and Bi2Te2.95Se0.05. The segments in the n- and p-legs have different lengths and cross-sectional areas. The JUL-03 test duration is 645 hours. All tested unicouples are fabricated at JPL and assembled and tested in the vacuum facility at the University of New Mexico in argon at ∼ 0.051 to 0.068 MPa to suppress the sublimation of antimony from the legs near the hot shoe. Detailed measurements of the open circuit voltage, voltage across the n- and p-legs, the voltage-current (V-I) characteristics, and the hot and cold shoe temperatures are performed in all tests. In JUL-03, additional measurements of the interfacial temperatures and the voltage across the segments in the n- and p-legs are obtained as functions of test duration. Estimates of beginning-of-life (BOL) conversion efficiencies of 10.7% for Shutterudites and 13.5% for STE unicouples are within 10% of theoretical predictions assuming zero side heat losses and zero contact resistances. Estimates of these losses in the tests are 2.3 W in MAR-03 to 9.3 W in JUL-03, thus actual efficiencies in the tests are ∼ 40–50% lower. Because cross sectional areas of the legs of JUL-03 are much larger than of both MAR-03 and JUN-03, the measured BOL peak electrical power per unicouple is 1.295 We versus 0.671 We for the latter. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Three Cascaded Thermoelectric Converters (CTCs) are optimized for potential use in Multi-Mission Advanced Radioisotope Power Systems (MM-ARPS) for electrical powers up to 1 kWe, or even higher, in support of 7–10 year missions. The peak efficiencies of these CTCs of 9.43% to 14.32% are 40% to 110% higher than that of SiGe in State-of-the-Art (SOA) Radioisotope Thermoelectric Generators (RTGs). Such high efficiencies would significantly reduce the amount of 238PuO2 fuel and the total system mass for a lower mission cost. Each CTC is comprised of a SiGe top unicouple that is thermally, but not electrically, coupled to a bottom unicouple with one of the following three choices of thermoelectric materials: (a) p-leg of TAGS-85 and n-leg of 2N-PbTe; (b) p-leg of CeFe3.5Co0.5Sb12 and n-leg of CoSb3; and (c) segmented p-leg of CeFe3.5Co0.5Sb12 and Zn4Sb3 and n-leg of CoSb3. The length of the top and bottom unicouples is 10 mm, but the cross-sectional areas of the n- and p-legs of the unicouples are optimized for maximum efficiency operation. They vary with the thermal power inputs of 1, 2, and 3 Wth per SiGe unicouple, and the heat rejection temperature of 375 K, 475 K, and 575 K, from the bottom unicouple. Such geometrical optimization is at nominal hot shoe temperature of 1273 K for the SiGe unicouple and cold shoe temperature of either 780 K or 980 K, depending on the materials of the bottom unicouples. The hot shoe temperature of the bottom unicouples is 20 K lower than the cold shoe of the top SiGe unicouple, but the rate of heat input is the same as the rate of heat rejection from the top unicouple. The present results are conservative as they assume a contact resistance of 150 μΩ-cm2 per leg for the top and the bottom unicouples in the CTCs; however, decreasing this resistance to 50 μΩ-cm2 per leg could increase the current efficiency estimates by an additional 1 – 2 percentage points. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A conceptual design of a Heat Pipe-Segmented Thermoelectric Module Converters (HP-STMCs) space reactor power system (SRPS) for a net power of 110 kWe is developed. The parametric analysis changed the number of radiator’s potassium heat pipes from 224 to 336 and calculated the effects on the operation parameters and total mass of the system. The reactor has a hexagonal core comprised of 126 heat pipe modules, each consists of three UN, 1.5 cm OD fuel pins brazed to a central lithium heat pipe of identical diameter. The Re cladding of the fuel pins is brazed along the active core length to the lithium heat pipe using 6 Re tri-cusps. The reactor control is accomplished using 12 B4C/BeO control drums, a large diameter one on each side of the hexagonal core and a small diameter one at each corner. The control drums are placed within the radial BeO reflector (7.1–9.1 cm thick). The fuel pin peak-to-average power ratio in the reactor core is 1.12–1.19. Despite its very high density and fabrication challenge, using rhenium structure in the reactor core is necessary for three main reasons: (a) the high reactor temperature (>= 1500 K); (b) excellent compatibility with the UN fuel and lithium; (c) to cause a spectrum shift that ensures having sufficient negative reactivity margin during a water submersion accident. The reference HP-STMC system with 324, 2.42–3.03 cm OD potassium heat pipes in the radiator is 9.60 m long and has a cone angle of 30°. The nominal operation of the reactor’s lithium heat pipes and of the radiator’s potassium heat pipes is at or below ∼ 45% of the prevailing wicking and sonic limit, respectively. The masses of the reactor and radiation shadow shield are 753.7 kg and 999.5 kg, respectively; the average heat pipes temperature in the reactor is 1513 K; the mass of the reactor’s lithium heat pipes with a C-C finned condenser that is 1.5 m long is 516.1 kg; the mass of the radiator is 557.5 kg, with an outer surface area of 87 m2 (6.41 kg/m2) and effective temperatures of 752 K and 734 K for the front and rear radiator sections, respectively. These estimates are for a constant collector temperature for the STMCs of 1300 K and STMCs’ thermal and electrical losses of 5% and 8%, respectively. The estimates of the total mass and specific power of the reference HP-STMCs SRPS, pending future detailed design and analysis, are 4261 kg and 25.8 We/kg, respectively. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A detailed design and performance results of C-C finned, and armored potassium heat pipes radiator for a 110 kWe Heat Pipes-Segmented Thermoelectric Module Converters (HP-STMCs) Space Reactor Power system (SRPS) are presented. The radiator consists of two sections; each serves an equal number of STMCs and has 162 longitudinal potassium heat pipes with 0.508 mm thick C-C fins. The width of the C-C fins at the minor diameter of the radiator is almost zero, but increases with distance along the radiator to reach 3.7 cm at the radiator’s major diameter. The radiator’s heat pipes (OD = 2.42 cm in front and 3.03 cm in rear) have thin titanium (0.0762 mm thick) liners and wicks (0.20 mm thick with an effective pore radius of 12–16 μm) and a 1.016 mm thick C-C wall. The wick is separated from the titanium liner by a 0.4 mm annulus filled with liquid potassium to increase the capillary limit. The outer surfaces of the heat pipes in the front and rear sections of the radiator are protected with a C-C armor that is 2.17 mm and 1.70 mm thick, respectively. The inside surface of the heat pipes in the front radiator is thermally insulated while the C-C finned condensers of the rear heat pipes are exposed, radiating into space through the rear opening of the radiator cavity. The heat pipes in both the front and the rear radiators have a 1.5 m long evaporator section and each dissipates 4.47 kW while operating at 43.6% of the prevailing sonic limit. The front and rear radiator sections are 5.29 m and 2.61 m long with outer surface area and mass of 47.1 m2 and 314.3 kg, and 39.9 m2 and 243.2 kg, respectively. The total radiator is 7.63 m long and has minor and major diameters of 1.48 m and 5.57 m, respectively, and a total surface area of 87 m2; however, the effective radiator area, after accounting for heat rejection through the rear of the radiator cavity, is 98.8 m2. The radiator’s total mass including the C-C armor is 557.5 kg and the specific area and specific mass are 6.41 kg/m2 and 5.07 kg/kWe, respectively. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The mechanical and thermo-physical properties of Mechanically Alloyed (MA)-Oxide Dispersion Strengthened (ODS) steels are reviewed and their potential for use in space nuclear reactor power systems is examined. The three MA-ODS alloys examined are Inconel MA-ODS754 (77.55Ni, 20Cr, 1Fe, 0.5Ti, 0.3Al, 0.05C, and 0.6Y2O3), Incoloy MA-ODS956 (74.45Fe, 20Cr, 4.5Al, 0.5Ti, 0.05C, 0.5Y2O3), and Incoloy MA-ODS957 (84.55Fe, 14Cr, 0.3Mo, 0.9Ti, 0.25Y2O3). The major advantages of these alloys are: (a) their strength at high temperatures (>1000 K) is relatively higher and decreases slower with temperature than niobium (Nb) and molybdenum (Mo) refractory alloys; (b) they are relatively lightweight and less expensive; (c) they have been shown to experience low swelling and embrittlement with exposure to high-energy neutrons (> 0.1 MeV) up to a fluence of 1023 n/cm2; and (d) their high resistance to oxidation and nitration at high temperatures, which simplifies handling and assembly. These MS-ODS alloys are also lighter and much stronger than 316-stainless steel and super-alloys such as Inconel 601, Haynes 25, and Hastalloy-X at moderately high temperatures (688–1000 K). The little data available on the compatibility of the MA-ODS alloys with alkali liquid metals up to 1100 K are encouraging, however, additional tests at typical operation temperatures (1000–1400 K) in liquid metal cooled and alkali metal heat pipe-cooled space nuclear reactors are needed. The anisotropy of the MA-ODS alloys when cold worked, and in particularly when rolled into tubes, should not hinder their use in space nuclear power systems, in which the operation pressure is either near atmospheric or as high as 2 MPa. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Design and performance analysis of the nuclear reactor’s lithium heat pipes for a 110-kWe Heat Pipes-Segmented Thermoelectric Module Converters (HP-STMCs) Space Reactor Power system (SRPS) are presented. The evaporator length of the heat pipes is the same as the active core height (0.45 m) and the C-C finned condenser is of the same length as the STMC panels (1.5 m). The C-C finned condenser section is radiatively coupled to the collector shoes of the STMCs placed on both sides. The lengths of the adiabatic section, the values of the power throughput and the evaporator wall temperature depend on the radial location of the heat pipe in the reactor core and the number and dimensions of the potassium heat pipes in the heat rejection radiator. The reactor heat pipes have a total length that varies from 7.57 to 7.73 m, and a 0.2 mm thick Mo-14%Re wick with an average pore radius of 12 μm. The wick is separated from the Mo-14%Re wall by a 0.5 mm annulus filled with liquid lithium, to raise the prevailing capillary limit. The nominal evaporator (or reactor) temperature varies from 1513 to 1591 K and the thermal power of the reactor is 1.6 MW, which averages 12.7 kW for each of the 126 reactor heat pipes. The power throughput per heat pipe increase to a nominal 15.24 kW at the location of the peak power in the core and to 20.31 kW when an adjacent heat pipe fails. The prevailing capillary limit of the reactor heat pipes is 28.3 kW, providing a design margin >= 28%. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Coated Particles Fuel Compacts (CPFC) have recently been shown to offer performance advantage for use in Radioisotope Heater Units (RHUs) and design flexibility for integrating at high thermal efficiency with Stirling Engine converters, currently being considered for 100 We. Advanced Radioisotope Power Systems (ARPS). The particles in the compact consist of 238PuO2 fuel kernels with 5-μm thick PyC inner coating and a strong ZrC outer coating, whose thickness depends on the maximum fuel temperature during reentry, the fuel kernel diameter, and the fraction of helium gas released from the kernels and fully contained by the ZrC coating. In addition to containing the helium generated by radioactive decay of 238Pu for up to 10 years before launch and 10–15 years mission lifetime, the kernels are intentionally sized (>= 300 μm in diameter) to prevent any adverse radiological effects on reentry. This paper investigates the advantage of replacing the four iridium-clad 238PuO2 fuel pellets, the two floating graphite membranes, and the two graphite impact shells in current State-Of-The-Art (SOA) General Purpose Heat Source (GPHS) with CPFC. The total mass, thermal power, and specific power of the CPFC-GPHS are calculated as functions of the helium release fraction from the fuel kernels and maximum fuel temperature during reentry from 1500 K to 2400 K. For the same total mass and volume as SOA GPHS, the generated thermal power by single-size particles CPFC-GPHS is 260 W at Beginning-Of-Mission (BOM), versus 231 W for the GPHS. For an additional 10% increase in total mass, the CPFC-GPHS could generate 340 W BOM; 48% higher than SOA GPHS. The corresponding specific thermal power is 214 W/kg, versus 160 W/kg for SOA GPHS; a 34% increase. Therefore, for the same thermal power, the CPFC-GPHS is lighter than SOA GPHS, while it uses the same amount of 238PuO2 fuel and same aeroshell. For the same helium release fraction and fuel temperature, binary-size... [ABSTRACT FROM AUTHOR]

Static and dynamic energy conversion technologies for Advanced Radioisotope Power Systems (ARPSs) are reviewed and their impact on the system’s total mass and specific electrical power and the amount of 238PuO2 fuel needed for the heat source are assessed and compared. Conversion technologies considered are Segmented and cascaded Thermoelectric, Alkali-Metal Thermal-to-Electric Conversion, and Free Piston Stirling Engines (FPSEs) and, for comparison, SiGe thermoelectric. Estimates for a 100 We ARPS indicate that when using SiGe thermoelectric, operating between 1273 K and 573 K, 8 General Purpose Heat Source (GPHS) modules would be required and the system’s specific power is ∼ 4.6 We/kg. Using STE converters, operating between 973 K and 373 K, 5 GPHS modules are required and the ARPS’s specific power is ∼ 7.28 We/kg. The next generation STE converters that could operate between 1273 K and 573 K, for a projected system efficiency of 13.8%, decrease the number of GPHS modules needed to 4 and increase the system’s specific power to ∼ 9.9 We/kg. With cascaded SiGe-STE converters, operating between 1273 K and 373 K, the system’s efficiency could be as much as 16%, requiring only 3 GPHS modules, for an estimated specific power of 10.7 We/kg. This specific power is more than twice that for SOA RTG. With the current version 1.0 of FPSEs, the 100 We ARPS needs only two GPHS modules, but its specific power (4.1 we/kg) is slightly lower than that of SOA RTG (4.6 We/kg). Future introduction of versions 1.1 and 2.0 engines, with slightly higher conversion efficiency and significantly lower mass, could increase the system’s specific power to ∼ 7.5 We/kg, using the same number of GPHS modules as version 1.0 engines. With Na-AMTEC and K-AMTEC, the 100 We ARPS needs 3 and 4 GPHS modules, respectively, for an estimated specific power of 5.3 and 5.8 We/kg, respectively. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Alkali Metal Thermal-to-Electric Conversion (AMTEC), although currently at a Technology Readiness Level-3 (TRL-3), has an excellent potential for use in Space Nuclear Reactor Power (SNRP) systems for NASA’s deep-space exploration missions. In addition to operating at a conversion efficiency > 20%, representing the highest fraction (> 60%) of Carnot efficiency of all other static and dynamic conversion technology options, the relatively high heat rejection radiator temperature (650–700 K) reduces the size and mass of the radiator and of the SNRP system. A high-power AMTEC unit design has been developed and optimized for operating at reactor exit temperatures ≤ 1180 K and radiator temperature ≤ 680 K. Depending on the reactor exit temperature, the nominal electrical power of the AMTEC unit, measuring 594 mm × 410 mm × 115 mm and weighting 44.3 kg, could be as high as 5.6 kWe, with a margin of >= 5% for an additional load-following increase. A conceptual design of a 100 kWe SNRP system with these high-power AMTEC units is developed and presented in this paper. The total mass of major subsystems, including the converters, nuclear reactor, shadow radiation shield, and radiator, is calculated and compared with that for the SP-100. Despite the large specific mass of the AMTEC units compared to the SiGe thermoelectrics in the SP-100 system, the lower masses of the reactor, radiation shield, and radiator make the present AMTEC-SNRP system > 26% lighter, for the same electrical power. An optimized AMTEC-SNRP system could potentially operate at a specific power > 30 We/kg (or specific mass < 33 kg/kWe), use non-refractory structures of super-steel alloys with well-know properties, relatively low density, low Ductile-To-Brittle (DTB) transition temperatures, and good compatibility with space and planetary environments containing CO2 and oxygen. The radiator area for the baseline 100 kWe AMTEC SNRP system is < 27 m2, which, together with operating... [ABSTRACT FROM AUTHOR]

A high electrical power, Alkali Metal Thermal-To-Electric Conversion (AMTEC) unit design is developed, and performance estimates as functions of the beta″-alumina solid electrolyte (BASE) temperature (or anode vapor pressure), condenser temperature, and the type of the working fluid (sodium or potassium) are calculated and discussed. The Na and K-AMTEC units, measuring 410 mm × 594 mm × 115 mm in outside dimensions, are identical except for the type of the BASE and working fluid. The peak efficiency of the Na-AMTEC at a BASE temperature of 1123 K is 29.2%, decreasing to 26.8% at a BASE temperature of 1073 K. The corresponding specific powers of the Na-AMTEC unit are 76 and 54 We/kg, respectively. For nominal operation at 85% of peak electrical power at BASE temperatures of 1123 K and 1073 K, the conversion efficiency of the Na-AMTEC decreases to 26.7% and 24.5%, respectively, but the corresponding specific powers increase significantly to 125 and 91 We/kg, respectively. The Na-AMTEC nominally generates 4.0 and 5.6 kWe when operating at BASE temperatures of 1073 K and 1123 K, respectively. When operating at the same anode vapor pressure of 76.8 kPa, the BASE temperature in the K-AMTEC is only 1002 K, versus 1123 K for the Na-AMTEC, generating only 2.0 kWe at a specific electrical power of 45 We/kg. In addition to the lower specific power, the specific radiator area for the K-AMTEC is significantly larger than for the Na-AMTEC because of the lower conversion efficiency (∼ 22.5%). The Na-AMTEC operates at higher conversion efficiency and higher electrical power than the K-AMTEC for condenser temperatures >= 620 K. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

A high power, liquid-anode, Alkali Metal Thermal-To-Electric Conversion (AMTEC) unit design is developed and optimized. Optimized parameters include: number of BASE elements; spacing between BASE elements and their arrangement in rows and columns; electrode’s surface area per BASE element; and number and diameter of the orifices in the internal radiation heat shield. The effects of these parameters on the size, mass and performance of the AMTEC unit when operating at BASE and condenser temperatures of 1123 K and 670 K, respectively, are discussed. The specific power of the Na AMTEC unit is maximum at a WRh1.5 electrode’s surface area of 140.8 cm2 per BASE element. A total number of 128 BASE elements, electrically connected in series, deliver the nominal power of 5.6 kWe which is 84% of the peak power (6.7 kWe). The BASE elements are arranged in 4 rows of 32 elements each, resulting in a nearly square footprint, minimum mass and parasitic heat losses, and maximum specific power and efficiency. At the nominal operating current of ∼100 A, the radiation and conduction parasitic heat losses amount to only 7% of the total heat input. The optimized AMTEC unit design measures 594 mm × 410 mm × 115 mm, weighs 44.3 kg, and has an efficiency of 27.6% at the nominal power of 5.6 kWe (a specific power of 125 We/kg). For a 100-kWe power system with a nuclear reactor core exit temperature of ∼1180 K, 18 of these units would provide excellent redundancy and reliability in energy conversion. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Experiments are conducted to compare the performance of the smooth electrodes, grooved emitter, and grooved collector converters at the same emitter and collector temperatures. The focus of the experiments is to investigate the effects of collector temperature on the performance of grooved electrode converters. The converters with various electrode configurations and a 0.5 mm gap are tested at emitter temperatures, TE = 1473 – 1673 K, collector temperatures, TC = 773 – 1023 K, and cesium pressures, PCs = 10 – 500 Pa. All electrodes are planar and fabricated with polycrystalline Molybdenum. The grooved electrodes have concentric macro-grooves 0.5 mm wide, 0.5 mm deep, and 1.0 mm apart. The output electrical power density and the conversion efficiency are calculated from the measured I-V curves and the maximum power density and conversion efficiency are determined from the calculated PD-V and η-V envelopes. At TE = 1473 K, the electrical power density for the smooth electrodes converter at the optimum collector temperature (873 K) is 2.25 W/cm2 at η = 13.8 %, decreasing to 1.46 W/cm2 at η = 12.4 % as TC increases by only 75 K to 948 K. However, the corresponding decrease in the electrical power density for the grooved collector converter is only 0.04 W/cm2 (from 1.24 W/cm2 at η = 11.4 % and TC = 873 K to 1.20 W/cm2 at η =10.7 % and TC = 948 K) and that for the grooved emitter converter is 0.12 W/cm2 (from 0.82 W/cm2 at η = 8.1 % and TC = 873 K to 0.70 W/cm2 at η = 6.8 % and TC = 948 K). At the higher emitter temperatures of 1573 K and 1673 K, similar results are obtained. In all experimental conditions, however, the grooved emitter converter shows the lowest power density and conversion efficiency, followed by the grooved collector converter, and the smooth electrodes converter, except for TF = 1673 K and TC = 1023 K at which the converter with a grooved collector operates at the highest power density (2.38... [ABSTRACT FROM AUTHOR]

For the past 25 years, unicouples made of Silicon Germanium alloys (Si[sub 0.8]Ge[sub 0.2] and Si[sub 0.63]Ge[sub 0.38]) have been used in the majority of the spacecraft powered by Radioisotope Thermoelectric Generators (RTGs) to explore various planets of the solar system as well as the sun. These unicouples operate typically at hot and cold side temperatures of 1175-1273 K and 573 K, respectively, converting the heat produced in the General Purpose Heat Source (GPHS) modules to electricity at efficiencies < 7%. Recently, Segmented Thermoelectric Unicouples (STUs) made of n-type Bi[sub 2]Te[sub 3] and CoSb[sub 3]-based alloys and p-type Bi2Te[sub 3] and CeFe[sub 4]Sb[sub 12]-based alloys have been developed and tested successfully with a demonstrated efficiency to date of ∼ 10% and the potential for achieving 14 -15% when operated at hot and cold side temperatures of 973 K and 300 K, respectively. This paper compares the performance of SiGe unicouples and skutterudite STUs, at same hot side temperature of 973 K and cold side temperatures of 300, 573, and 673 K, and the same total length and cross sectional dimensions of the p-leg. The area of the n-leg and the lengths of the segments of various materials in the STUs are determined based on maximizing either the electrical power density or the conversion efficiency. Results showed that replacing SiGe with skutterudite STUs in future radioisotope power systems (or even in conjunction with a space nuclear reactor and space probes), could increase the thermal to electric conversion efficiency at a heat source temperature of 973 K by ∼ 40% to 198% for T[sub c] = 673 K and 300K, respectively. This would reduce by more than half the mass of the [sup 238]PuO[sub 2] fuel needed and the radiator's surface area for rejecting the waste heat. [ABSTRACT FROM AUTHOR]

The effects of simulated reduced gravity on nucleate boiling and Critical Heat Flux (CHF) of the hydrofluoroether, HFE-7100 dielectric liquid from a 1.0-cm² smooth copper surface are investigated experimentally. Reduced gravity is simulated by varying the inclination angle of the surface, θ, from 0° (upward-facing, 1-g) to 180° (downward facing, zero-g) in 10°-30° increments. Results show that nucleate boiling heat fluxes at surface superheats, ΔT[sub sat] > 13 K are highest at 0° and decrease with increased θ to 180°. At lower superheats, however, the nucleate boiling heat flux data are inconclusive, with the lowest and highest values occurring at 120° and 180°, respectively. The developed correlations for saturation nucleate boiling, as a function of &theta, is within ± 10% of the data. Correlations for the normalized CHF to that at 0° (or CHFR), and of the surface superheat at CHF (≤ 31.7 K) are also developed as functions of &theta. The CHFR decreases slowly as &theta increases from 0° to 120°, then faster for θ > 120°, consistent with earlier work for dielectric and non-dielectric liquids. The surface superheat at CHF also decreases as θ increases reaching its lowest value at 180°. Photographs of saturation nucleate boiling at θ = 0°, 30°, 120°. and 180°, support the nucleate boiling heat flux measurements. This work demonstrated that dissipation heat fluxes of 24.45 and 4.30 W/cm² could be removed from a high performance computer chip by saturation nucleate boiling of HFE-7100 at 0° (or l g) and 180° (or zero-g), at surface superheats of 31.7 and 19.9K, respectively. [ABSTRACT FROM AUTHOR]