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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]

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]