skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Low Mass SCoRe-S Designs for Affordable Planetary Exploration

Abstract

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 radialmore » 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.« less

Authors:
;  [1];  [2]
  1. Institute for Space and Nuclear Power Studies, University of New Mexico, Albuquerque, NM 87131-0001 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
21054541
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 880; Journal Issue: 1; Conference: International forum-STAIF 2007: 11. conference on thermophysics applications in microgravity; 24. symposium on space nuclear power and propulsion; 5. conference on human/robotic technology and the vision for space exploration; 5. symposium on space colonization; 4. symposium on new frontiers and future concepts, Albuquerque, NM (United States), 11-15 Feb 2007; Other Information: DOI: 10.1063/1.2437462; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
22 GENERAL STUDIES OF NUCLEAR REACTORS; BERYLLIUM OXIDES; BORON CARBIDES; CONTROL; CRITICALITY; DESIGN; FAST NEUTRONS; FAST REACTORS; FISSION; FUEL ADDITIVES; POWER SYSTEMS; REACTIVITY; REACTOR VESSELS; SHUTDOWN; SPACE VEHICLES; THERMAL NEUTRONS; NESDPS Office of Nuclear Energy Space and Defense Power Systems

Citation Formats

Hatton, Steven A., El-Genk, Mohamed S., and Dept. Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131-0001. Low Mass SCoRe-S Designs for Affordable Planetary Exploration. United States: N. p., 2007. Web. doi:10.1063/1.2437462.
Hatton, Steven A., El-Genk, Mohamed S., & Dept. Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131-0001. Low Mass SCoRe-S Designs for Affordable Planetary Exploration. United States. doi:10.1063/1.2437462.
Hatton, Steven A., El-Genk, Mohamed S., and Dept. Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131-0001. Tue . "Low Mass SCoRe-S Designs for Affordable Planetary Exploration". United States. doi:10.1063/1.2437462.
@article{osti_21054541,
title = {Low Mass SCoRe-S Designs for Affordable Planetary Exploration},
author = {Hatton, Steven A. and El-Genk, Mohamed S. and Dept. Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131-0001},
abstractNote = {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.},
doi = {10.1063/1.2437462},
journal = {AIP Conference Proceedings},
number = 1,
volume = 880,
place = {United States},
year = {Tue Jan 30 00:00:00 EST 2007},
month = {Tue Jan 30 00:00:00 EST 2007}
}
  • The number of low-mass brown dwarfs and even free floating planetary-mass objects in young nearby star-forming (SF) regions and associations is continuously increasing, offering the possibility to study the low-mass end of the initial mass function in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus SF region one of which was recently discovered in parallel by Luhman et al. The underlying data we use is part of a new database from a deep near-infrared survey at the Calar Alto observatory. The survey is more than 4 mag deeper than the Twomore » Micron All Sky Survey and covers currently approx1.5 deg{sup 2}. Complementary optical photometry from Sloan Digital Sky Survey were available for roughly 1.0 deg{sup 2}. After selection of the candidates using different color indices, additional photometry from Spitzer/IRAC was included in the analysis. In greater detail, we focus on two very faint objects for which we obtained J-band spectra. Based on comparison with reference spectra, we derive a spectral type of L2 +- 0.5 for one object, making it the object with the latest spectral type in Taurus known today. From models, we find the effective temperature to be 2080 +- 140 K and the mass 5-15 Jupiter masses. For the second source, the J-band spectrum does not provide definite proof of the young, low-mass nature of the object, as the expected steep water vapor absorption at 1.33 mum is not present in the data. We discuss the probability that this object might be a background giant or carbon star. If it were a young Taurus member, however, a comparison to theoretical models suggests that it lies close to or even below the deuterium burning limit (<13 M{sub Jup}) as well. A first proper motion analysis for both objects shows that they are good candidates for being Taurus members.« less
  • Observations of accretion disks around young brown dwarfs (BDs) have led to the speculation that they may form planetary systems similar to normal stars. While there have been several detections of planetary-mass objects around BDs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively large mass ratios and projected separations, suggesting that they formed in a manner analogous to stellar binaries. We present the discovery of a planetary-mass object orbiting a field BD via gravitational microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass ratio (0.080 ± 0.001), relatively tightly separated (∼0.87 AU) binary composed of a planetary-mass object withmore » 1.9 ± 0.2 Jupiter masses orbiting a BD with a mass 0.022 M {sub ☉}. The relatively small mass ratio and separation suggest that the companion may have formed in a protoplanetary disk around the BD host in a manner analogous to planets.« less
  • Measurements of accretion rates onto planetary mass objects may distinguish between different planet formation mechanisms, which predict different accretion histories. In this Letter, we use Hubble Space Telescope (HST)/WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC 06214–00210 b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10{sup –9}-10{sup –11} M {sub ☉} yr{sup –1} for these three objects. Their accretion rates are an order of magnitude higher than expected from themore » correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large separation from the central star demonstrate the presence of massive disks around these objects. Models for the formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of Hα luminosity over accretion luminosity for objects with low accretion rates suggest that searches for Hα emission may be an efficient way to find accreting planets.« less
  • Theory and simulations suggest that it is possible to form low-mass hydrogen-burning stars, brown dwarfs (BDs), and planetary-mass objects (PMOs) via disk fragmentation. As disk fragmentation results in the formation of several bodies at comparable distances to the host star, their orbits are generally unstable. Here, we study the dynamical evolution of these objects. We set up the initial conditions based on the outcomes of the smoothed-particle hydrodynamics simulations of Stamatellos and Whitworth, and for comparison we also study the evolution of systems resulting from lower-mass fragmenting disks. We refer to these two sets of simulations as set 1 andmore » set 2, respectively. At 10 Myr, approximately half of the host stars have one companion left, and approximately 22% (set 1) to 9.8% (set 2) of the host stars are single. Systems with multiple secondaries in relatively stable configurations are common (about 30% and 44%, respectively). The majority of the companions are ejected within 1 Myr with velocities mostly below 5 km s{sup −1}, with some runaway escapers with velocities over 30 km s{sup −1}. Roughly 6% (set 1) and 2% (set 2) of the companions pair up into very low-mass binary systems, resulting in respective binary fractions of 3.2% and 1.2%. The majority of these pairs escape as very low-mass binaries, while others remain bound to the host star in hierarchical configurations (often with retrograde inner orbits). Physical collisions with the host star (0.43 and 0.18 events per host star for set 1 and set 2, respectively) and between companions (0.08 and 0.04 events per host star for set 1 and set 2, respectively) are relatively common and their frequency increases with increasing disk mass. Our study predicts observable properties of very low-mass binaries, low-mass hierarchical systems, the BD desert, and free-floating BDs and PMOs in and near young stellar groupings, which can be used to distinguish between different formation scenarios of very low-mass stars, BDs, and PMOs.« less
  • The evolution of a carbon-oxygen dwarf of mass Mroughly-equal0.6 Msun has been carried all the way from an initial nuclear burning stage, when it is the central star of a planetary nebula, to the stage of complete internal crystallization, after 10/sup 10/ yr of cooling. Shell hydrogen and helium burning, neutrino losses, and the effects of liquification and crystallization have been taken into account. We show how the luminosity-time relationship may be understood in terms of balances between competing physical processes and demonstrate that, after complete crystallization, the time scale for cooling to terrestrial-like temperatures, in our approximation, is simplymore » the optical depth of the outer, nonisothermal layer multiplied by a dimensional constant which, in years, is of the order of unity. A luminosity function based on the results covers the range -5< or approx. =log(L/Lsun)< or approx. =4 and agrees reasonably well with the observed luminosity function extending from the brighest planetary nebula nuclei to the dimmest observed white dwarfs, except perhaps for log(L/L/sub sun/)< or approx. =-4.5. Possible reasons for the apparent discrepancy at low luminosity, apart from the extreme obstacles against discovery, are discussed, one of the simplest is that the oldest dwarfs in the solar vicinity are distributed over a distance from the galactic plane that is approx.5 times larger than is the case for the youngest dwarfs; another possibility is that the opacity in the outer layers of the oldest dwarf models has been overestimated (or underestimatedexclamation) by a factor of 5 or more.« less