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DOE OSTI2021

Material Control & Accountancy for Molten Salt Reactors (FY2021 Report)

There is significant domestic and international interest, investment, and research and development momentum to pursue advanced nuclear reactor technologies. Molten salt reactor (MSR) concepts display the largest variability in fuel type and design features among the current advanced concepts. MSRs have been proposed with various core designs, sizes (power), and fuel cycles. Salt-fueled molten salt systems represent the only advanced reactor type with fuel that is not in a solid form during operation. These “liquid-fueled” MSRs are unique from perspectives of fuel fabrication, spent irradiated fuel and waste components, licensing, and material control and accountability (MC&A) including the potential of fissile material holdup. The liquid fuel salt is the defining distinction in comparison to other advanced reactors that propose TRI-structural ISOtropic particle fuel pebbles, various coolant options (e.g., molten salts or metals, high temperature gas), or small modular alternatives using solid fuel variants including both light water reactors and non-light water reactors. MSRs are appealing to the nuclear energy industry because of the diverse reactor characteristics they can support including various neutron energy spectra, fueling requirements, fuel cycles, and/or fuel utilization. However, because of the significant deviation and diversity of a salt-fueled system compared to traditional solid fuel light water-cooled reactors (LWRs), the history, regulatory licensing framework, modeling capabilities, and supporting engineering technology are either lacking or, in some cases, nonexistent. Therefore, the research community is actively supporting advanced MSR development on many of these fronts in particular to assist MSR vendors with licensing requirements. ORNL is leading the research and development of respective MC&A approaches for salt-fueled MSRs. This report summarizes the research performed at Oak Ridge National Laboratory (ORNL) under the US Department of Energy, Office of Nuclear Energy, Advanced Reactor Safeguards (ARS) program to investigate safeguards and security by design concepts, licensing and regulatory considerations, and dynamic system-level modeling to understand radioisotope concentrations for salt-fueled MSRs. The report builds upon the previous research and literature, identifies the MC&A challenges inherent to a salt-fueled MSR, reviews current regulatory frameworks for LWRs and their applicability towards salt-fueled MSRs, summarizes the status and progress of an MSR dynamic modeling tool, and discusses a prospective MC&A approach based on the Molten Salt Demonstration Reactor (MSDR) model.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2019

A New Model for Electron-capture Supernovae in Galactic Chemical Evolution

We examine the contribution of electron-capture supernovae (ECSNe), low-mass SNe from collapsing Fe cores (FeCCSNe), and rotating massive stars to the chemical composition of the Galaxy. Our model includes contributions to chemical evolution from both thermonuclear ECSNe (tECSNe) and gravitational collapse ECSNe (cECSNe). We show that if ECSNe are predominantly gravitational collapse SNe but about 15% are partial thermonuclear explosions, the model is able to reproduce the solar abundances of several important and problematic isotopes including ${}^{48}\mathrm{Ca}$, ${}^{50}\mathrm{Ti}$, and 54 Cr together with 58 Fe, 64 Ni, 82 Se, and 86 Kr and several of the Zn–Zr isotopes. A model in which no cECSNe occur, only tECSNe with low-mass FeCCSNe or rotating massive stars, proves also very successful at reproducing the solar abundances for these isotopes. Despite the small mass range for the progenitors of ECSNe and low-mass FeCCSNe, the large production factors suffice for the solar inventory of the above isotopes. Our model is compelling because it introduces no new tensions with the solar abundance distribution for a Milky Way model—only tending to improve the model predictions for several isotopes. Here, the proposed astrophysical production model thus provides a natural and elegant way to explain one of the last uncharted territories on the periodic table of astrophysical element production.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Numerical Analysis of Novel Plate Type Heat Exchanger with Oval-Twisted Channels

The novel heat exchanger (HX) designs implementing geometrical and surface modification combinations are expected to perform better than traditional HX technologies. Applied enhancement techniques seek to achieve (1) higher overall heat transfer performance, (2) increased compactness, and (3) simplified or comparable manufacturability. One innovative enhancement technique is to generate swirling flow vortices induced by channel or tube twisting. The turbulence generated by the twisted cross-section greatly enhances the heat transfer rate with minimal increases in pressure drop. Plate-type HXs and Printed Circuit Heat Exchangers (PCHXs) are compact designs that achieve high heat transfer rates per unit volume by utilizing several small channels, which maximizes the heat transfer surface area between the hot and cold fluids. Currently, advanced manufacturing technologies enable the design and fabrication of compact-type units with complicated channel geometries to achieve the highest performance and meet the compactness criteria of innovative HX technology. The proposed HX design concept combines the compactness of plate-type HXs and twisted channels, which provide additional turbulence and flow swirl enhancement. The plate-type oval-twisted HX (PTOTHX) is a crossflow configuration, with 16 short channels on one side (for hot fluid) and 8 long channels on the perpendicular side (for cold fluid). The inlet plenums have flow guide vanes to redirect flow and produce uniformity across the various flow paths. The compact size and purportedly improved heat transfer performance of the PTOTHX investigated herein prove its viability in various applications. Some notable potential nuclear applications of the PTOTHX include reactor core, spent fuel cooling, and residual heat dissipation. To establish a reference case, circular channels (PTCHX) are also considered in the present study for comparison with (PTOTHX). This paper aims to outline the numerical analysis procedures for determining the viability of the PTOTHX by comparing its heat transfer performance with the PTCHX units. The computational study used STAR-CCM+, a commercial computational fluid dynamics (CFD) code. Sensitivity analysis and model selection studies are conducted to determine the appropriate mesh density and turbulence model to provide the reported results. Numerical analyses comparing the Nusselt number (Nu) of the PTOTHX design with a comparable HX unit, including a circular cross-section and no twisting (PTCHX), show an overall heat transfer performance increase of 29-55% for balanced flow and 29-59% for imbalanced flow. Oval-cross-sectional twisted channels induce swirling flow vortices, enhancing the working fluid's convective heat transfer capabilities.

25 ENERGY STORAGE↗
DOE OSTI2017

Application of the Nonballistic Model to the Black Hole Candidate XTE J1752-223 and the Quasar NRAO 150

Optical and radio observations of the black hole candidate XTE J1752-223 have exhibited a slightly curved motion of the jet components, which is associated with its radio light curve. In addition, observations of the quasar NRAO 150 have revealed a core–jet structure wobbling with a high angular speed. In this paper, the phenomena displayed in these two different sources are interpreted as the precession of a bent jet. In such a scenario, hot spots reproduced at different separations from the core precess on the same precession cone, in which different components correspond to different propagation times to the observer. By fitting the kinematics of the components of XTE J1752-223 and its light curve with a curved pattern of precession period 314 days, we find that the propagation time can make an earlier event appear later, and the jet axis can oscillate during its precession. Simulating the quasar NRAO 150 with the same scenario reveals that the knots at larger separation from the core precess at a slower speed than those closer in. A possible mechanism relating to the cooling time of a component is proposed. These three new results are of importance in understanding the physics underlying the curved jet as well as the activity of the central engine of different black hole systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Experimental and Theoretical Study of the Electronic Structures of Lanthanide Indium Perovskites LnInO 3

Ternary lanthanide indium oxides LnInO 3 (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by Xray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO 3 materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO 6 octahedra and highly distorted LnO 12 polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO 3 compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4f n compounds PrInO 3 , NdInO 3 , and SmInO 3 , and the overall photoemission spectral profiles were shown to be a superposition of final-state 4f n-1 terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series Pr-Nd-Sm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO 3 is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO 3 and SmInO 3 , but a lower gap of 3.6 eV for PrInO 3 was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Machine Learning-Based Temperature Prediction for Runtime Thermal Management Across System Components

Elevated temperatures limit the peak performance of systems because of frequent interventions by thermal throttling. Non-uniform thermal states across system nodes also cause performance variation within seemingly equivalent nodes leading to significant degradation of overall performance. In this paper we present a framework for creating a lightweight thermal prediction system suitable for run-time management decisions. We pursue two avenues to explore optimized lightweight thermal predictors. First, we use feature selection algorithms to improve the performance of previously designed machine learning methods. Second, we develop alternative methods using neural network and linear regression-based methods to perform a comprehensive comparative study of prediction methods. We show that our optimized models achieve improved performance with better prediction accuracy and lower overhead as compared with the Gaussian process model proposed previously. Specifically we present a reduced version of the Gaussian process model, a neural network-based model, and a linear regression-based model. Using the optimization methods, we are able to reduce the average prediction errors in the Gaussian process from 4: 2 degrees C to 2: 9 degrees C. We also show that the newly developed models using neural network and Lasso linear regression have average prediction errors of 2: 9 degrees C and 3: 8 degrees C respectively. The prediction overheads are 0.22, 0.097, and 0.026 ms per prediction for reduced Gaussian process, neural network, and Lasso linear regression models, respectively, compared with 0.57 ms per prediction for the previous Gaussian process model. We have implemented our proposed thermal prediction models on a two-node system configuration to help identify the optimal task placement. The task placement identified by the models reduces the average system temperature by up to 11: 9 degrees C without any performance degradation. Furthermore, these models respectively achieve 75, 82.5, and 74.17 percent success rates in correctly pointing to those task placements with better thermal response, compared with 72.5 percent success for the original model in achieving the same objective. Lastly, we extended our analysis to a 16-node system and we were able to train models and execute them in real time to guide task migration and achieve on average 17 percent reduction in the overall system cooling power.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2023

Building an EPA Class VI Permit Application

Summary To accelerate the commercialization of carbon capture and storage (CCS), the US Department of Energy (US DOE) is building on decades of characterization efforts and pilot-scale projects through their CarbonSAFE program. Administered through their National Energy Technology Laboratory, this program seeks to bring fully integrated projects to the sector that can store more than 50 million tonnes of CO2 over a 30-year period. The program, which was enacted before the enhancement of Internal Revenue Code Section 45Q, is in the capture assessment, characterization, and permitting phase. The objectives of this paper are to discuss (a) the injection permitting requirements of the CarbonSAFE projects; (b) information gathering in support of the permit; (c) the timelines of field development and permit-related activities; (d) the major technical components of the field development plan; and (e) early feedback from the regulators toward acceptance of the permit. In Mississippi, more than 30,000 acres have been characterized by six deep characterization wells, a deep groundwater well, and 92 line miles of 2D seismic as part of the CarbonSAFE Project ECO2S. During the acquisition of seismic data, all receiver lines were live, which resulted in the generation of a pseudo-3D seismic design. The incorporation of a 3D seismic survey was not included as part of this project due to logistical difficulties presented by the undulating, wooded surface terrain. A suite of openhole geophysical logs was taken from each well, allowing for a detailed interpretation of prospective storage reservoirs and confining intervals to complement the analysis carried out on the 290 ft of a whole core that was cut through the prospective confining zone and storage reservoir. The detailed geologic and reservoir data were assembled and entered into a 3D model to assess the injection capacity and the area of review (AoR). This information fed into the detailed corrective action, monitoring, testing, and postinjection site care (PISC) modeling. The results have been exceptional. The geologic assessment has revealed three primary storage targets, ranging in depth from 3,500 ft to 6,000 ft. These storage reservoirs net 1,300 ft of sandstone, with mean porosity and permeability of 29% and 3.6 darcies, respectively. Together, these reservoirs have storage capacities that may exceed 20 million tonnes per square mile, making this a gigatonne prospect. Forward modeling of the project resulted in an AoR of 16 sq miles, injecting about 8000 t/d, for 30 years, via two deep injection wells. The excellent confining characteristics of the caprock, relatively simple geologic structure, and lack of historical well drilling activity in this area provide excellent containment of the injected CO2. Based on this work, the project has proposed 20 years of PISC. To date, only two US CO2 injection permits have been granted. These projects relied on a singular capture point feeding a singular sequestration point (source to sink), and considerations have not been made to garner CO2 emissions from other industrial sources. The Kemper County Storage Complex is a first-of-its-kind storage hub concept that looks to develop an area capable of storing significant quantities of CO2 from the region. Also, this work will show how characterization efforts, geological and numerical modeling efforts, and plan development were constructed in support of permit and incentives acceptance.

Energy & Fuels↗
DOE OSTI2025

Solid State Transformer Architecture and Control Compensation for Common Mode Currents

A high-altitude electromagnetic pulse (HEMP) or similar geomagnetic disturbance (GMD) has the potential to impact the operation of large-scale electric power grids. By introducing low-frequency common-mode (CM) currents, these events can degrade the performance of critical system components, such as large power transformers by introducing CM currents which can lead to magnetic saturation of the transformer core. In this work, a solid-state transformer (SST) is developed to replace susceptible equipment and improve grid resiliency by safely absorbing these CM disturbances. This device will be referred to as a common-mode solid-state transformer (CM-SST). An SST architecture based on a four-legged AC/DC converter is developed. This architecture enables active control of CM signals without disturbing the AC voltages or the real and reactive power delivery capabilities. A system-level model of this architecture is created, and time-domain simulations are performed to evaluate the SST’s performance in response to simulated CM disturbances. A control strategy for mitigating CM current is also investigated. Hamiltonian surface shaping and power flow control (HSSPFC) is used to design a nonlinear controller for the SST’s output inverter. The objectives of the controller are to suppress CM-induced AC current offsets and regulate AC currents to desired setpoints. Nonlinear system analysis is applied to design and validate the controller. Two cases are tested: (a) the proposed four-leg inverter and (b) a standard three-leg inverter. The results show that the proposed controller rapidly mitigates CM disturbances while maintaining high-quality AC current waveforms in the four-leg configuration. Finally, the hardware performance of an SST prototype is evaluated. In particular, the ability of the SST to safely redirect and absorb CM currents is demonstrated, showing how it can protect neighboring conventional transformers in the system. The study confirms that appropriate control laws allow the SST to protect both itself and adjacent transformers during a HEMP or GMD event.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2021

Hierarchical Fractional Advection-Dispersion Equation (FADE) to Quantify Anomalous Transport in River Corridor over a Broad Spectrum of Scales: Theory and Applications

Fractional calculus-based differential equations were found by previous studies to be promising tools in simulating local-scale anomalous diffusion for pollutants transport in natural geological media (geomedia), but efficient models are still needed for simulating anomalous transport over a broad spectrum of scales. This study proposed a hierarchical framework of fractional advection-dispersion equations (FADEs) for modeling pollutants moving in the river corridor at a full spectrum of scales. Applications showed that the fixed-index FADE could model bed sediment and manganese transport in streams at the geomorphologic unit scale, whereas the variable-index FADE well fitted bedload snapshots at the reach scale with spatially varying indices. Further analyses revealed that the selection of the FADEs depended on the scale, type of the geomedium (i.e., riverbed, aquifer, or soil), and the type of available observation dataset (i.e., the tracer snapshot or breakthrough curve (BTC)). When the pollutant BTC was used, a single-index FADE with scale-dependent parameters could fit the data by upscaling anomalous transport without mapping the sub-grid, intermediate multi-index anomalous diffusion. Pollutant transport in geomedia, therefore, may exhibit complex anomalous scaling in space (and/or time), and the identification of the FADE’s index for the reach-scale anomalous transport, which links the geomorphologic unit and watershed scales, is the core for reliable applications of fractional calculus in hydrology.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2016

STELLAR ORIGINS OF EXTREMELY {sup 13}C- AND {sup 15}N-ENRICHED PRESOLAR SIC GRAINS: NOVAE OR SUPERNOVAE?

Extreme excesses of {sup 13}C ({sup 12}C/{sup 13}C < 10) and {sup 15}N ({sup 14}N/{sup 15}N < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron- to micron-sized {sup 13}C- and {sup 15}N-enriched presolar SiC grains ({sup 12}C/{sup 13}C < 16 and {sup 14}N/{sup 15}N < ∼100) from Murchison, and their correlated Mg–Al, S, and Ca–Ti isotope data when available. These grains are enriched in {sup 13}C and {sup 15}N, but with quite diverse Si isotopic signatures. Four grains with {sup 29,30}Si excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton- and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with {sup 30}Si excesses and {sup 29}Si depletions show lower-than-solar {sup 34}S/{sup 32}S ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also {sup 13}C enriched, but have a range of higher {sup 14}N/{sup 15}N. We found that {sup 15}N-enriched AB grains (∼50 < {sup 14}N/{sup 15}N < ∼100) have distinctive isotopic signatures compared to putative nova grains, such as higher {sup 14}N/{sup 15}N, lower {sup 26}Al/{sup 27}Al, and lack of {sup 30}Si excess, indicating weaker proton-capture nucleosynthetic environments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

A partial mechanistic understanding of the North American monsoon

Abstract An understanding of the major governing processes of North American monsoon (NAM) is necessary to guide improvement in global and regional climate modeling of the NAM, as well as NAM's impacts on the summer circulation, precipitation, and drought over North America. A mechanistic understanding of the NAM is suggested by incorporating local‐ and synoptic‐scale processes. The local‐scale mechanism describes the effect of the temperature inversion over the Gulf of California (GC) on controlling low‐level moisture during the 2004 NAM. The strong low‐level inversion inhibits the exchange between the moist air in the marine boundary layer (MBL) and the overlying dry air. This inversion weakens with increasing sea surface temperatures (SSTs) in GC and generally disappears once SSTs exceed 29.5°C, allowing the moist air, trapped in the MBL, to mix with free tropospheric air. This leads to a deep, moist layer that can be transported by across‐gulf (along‐gulf) flow toward the NAM core region (southwestern U.S.) to form thunderstorms. On the synoptic scale, climatologies from 1983 to 2010 exhibit a temporal correspondence between coastal warm tropical surface water, NAM deep convection, NAM anticyclone center, and NAM‐induced strong descent. A hypothesis is proposed to explain this correspondence, based on limited soundings at the GC entrance (suggesting this local mechanism may also be active in that region), the climatologies, and the relevant literature. The warmest SSTs moving up the coast may initiate NAM convection and atmospheric heating, advancing the position of the anticyclone and the region of descent northward.

Erfani, Ehsan↗
NASA NTRS2024

Development of a Data Fusion Methodology for Lineload Aerodynamic Databases for a Launch Vehicle during Liftoff and Transition

The need for databases for the distributed loading on launch vehicles during the early portion of flight necessitates the use of expensive computational flows in regimes where wake effects dominate. While also being expensive, this is a regime that computational tools tend to historically have problems simulating accurately. To help tackle this problem, a method of data fusion to combine computational results to wind tunnel derived force and moment data is developed. Using this method, significant reduction in computational costs and increases in confidence of the final product is possible and has been used to generate several databases for the Space Launch System (SLS) at NASA. While the full details of database generation are not part of this work, the crucial method at its core is developed here. Two SLS geometries are used throughout the work to demonstrate the techniques. These are two of the larger geometries and represent both planned crewed missions to the Moon as well as potential cargo missions to deep space. The method uses principal component analysis (PCA) to generate a reduced ordered model (ROM) to help fill in the full parameter space. Other similar techniques are explored, but were not found to have a significant result on the predictions of the ROM. Because the full number of components are kept to generate the model, this lack of difference is expected. This method is then extended to ensure that predicted surfaces match trusted force and moment data derived from wind tunnel testing. This extension is done by setting up a constrained optimization problem in order to minimize the deviation from the surface resolved computational data while still integrating to the desired values. When generating the constrained optimization problem, a weighting factor to balance these competing needs is introduced. The work compares previously introduced weighting terms from similar work to the proposed terms and shows that the previously used terms do not have as desirable behavior in this flow regime. This method is then expanded by developing a technique to incorporate uncertainty quantification into the developed data fusion methodology. This expansion takes a two pronged approach. One examines transferring the uncertainties in the force and moment database and characterizes how those adjustments change the predicted lineloads. The second looks at model form error and looks how rebuilding the model using slightly different data changes the predictions. These two terms are then combined in order to create an uncertainty model that takes both effects into account. The limitations of the proposed methods is then discussed as well as possible techniques to address these shortcomings.

Launch Vehicles↗
DOE OSTI2013

TYPE Ia SINGLE DEGENERATE SURVIVORS MUST BE OVERLUMINOUS

In the single-degenerate (SD) channel of a Type Ia supernovae (SNe Ia) explosion, a main-sequence (MS) donor star survives the explosion but it is stripped of mass and shock heated. An essentially unavoidable consequence of mass loss during the explosion is that the companion must have an overextended envelope after the explosion. While this has been noted previously, it has not been strongly emphasized as an inevitable consequence. We calculate the future evolution of the companion by injecting 2-6 Multiplication-Sign 10{sup 47} erg into the stellar evolution model of a 1 M{sub Sun} donor star based on the post-explosion progenitors seen in simulations. We find that, due to the Kelvin-Helmholtz collapse of the envelope, the companion must become significantly more luminous (10-10{sup 3} L{sub Sun }) for a long period of time (10{sup 3}-10{sup 4} yr). The lack of such a luminous ''leftover'' star in the LMC supernova remnant SNR 0609-67.5 provides another piece of evidence against the SD scenario. We also show that none of the stars proposed as the survivors of the Tycho supernova, including Tycho G, could plausibly be the donor star. Additionally, luminous donors closer than {approx}10 Mpc should be observable with the Hubble Space Telescope starting {approx}2 yr post-peak. Such systems include SN 1937C, SN 1972E, SN 1986G, and SN 2011fe. Thus, the SD channel is already ruled out for at least two nearby SNe Ia and can easily be tested for a number of additional ones. We also discuss similar implications for the companions of core-collapse SNe.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1992

A quasi-Hertzian stress field from an internal source: A possible working model for the Vredefort structure

The Vredefort structure is a large domal feature about 110 km southeast of Johannesburg, South Africa, situated within and almost central to the large intracratonic Witwatersrand Basin. This structure consists of an Archean core of ca. 45 km in diameter, consisting largely of granitic gneiss, surrounded by a collar of metasedimentary and metavolcanic supracrustal rocks of the Dominian Group, Witwatersrand and Ventersdorp Supergroups, and Transvaal Sequence. The interpretation of images of the gravity and magnetic fields over Vredefort has permitted the delineation of several important features of the structure and of its environment. The outline of the collar strata is a prominent feature of both the gravity and the magnetic fields. The Vredefort structure shares this distinctive geometry with other structures (e.g., Manicouagan, Decaturville, Sierra Madera) of debated impact origin. In all these, successively older strata with steep outward dips are encountered while traversing inward to the center of the structure. A further attribute of these structures is the shortening of the outcrop of a particular stratigraphic unit compared to the original perimeter of that unit. To account for the geometric attributes of the Vredefort structure a mechanical scheme is required where there is radial movement of horizontal strata toward, with uplift in, the center of the Vredefort structure. Two models can be proposed: (1) one in which there is a rapid rise and violent disruption of cover rocks in response to expansion of a fluid accumulation; and (2) one in which there is, in contrast, a nonexplosive, quasi-Hertzian stress field resulting from a diapiric process. Both models can accommodate the geometry and structural components of Vredefort.

Antoine, L. A. G.↗
DOE OSTI2014

An optical-near-IR study of a triplet of super star clusters in the starburst core of M82

We present HST/STIS optical and Gemini/NIFS near-IR IFU spectroscopy and archival Hubble Space Telescope (HST) imaging of the triplet of super star clusters (A1, A2, and A3) in the core of the M82 starburst. Using model fits to the Space Telescope Imaging Spectrograph (STIS) spectra and the weakness of red supergiant CO absorption features (appearing at ∼6 Myr) in the NIFS H-band spectra, the ages of A2 and A3 are 4.5 ± 1.0 Myr. A1 has strong CO bands, consistent with our previously determined age of 6.4 ± 0.5 Myr. The photometric masses of the three clusters are 4-7 × 10{sup 5} M{sub ☉}, and their sizes are R{sub eff} = 159, 104, 59 mas (∼2.8, 1.8, 1.0 pc) for A1, A2, and A3. The STIS spectra yielded radial velocities of 320 ± 2, 330 ± 6, and 336 ± 5 km s{sup –1} for A1, A2, and A3, placing them at the eastern end of the x{sub 2} orbits of M82's bar. Clusters A2 and A3 are in high-density (800-1000 cm{sup –3}) environments, and like A1, are surrounded by compact H II regions. We suggest the winds from A2 and A3 have stalled, as in A1, due to the high ISM ambient pressure. We propose that the three clusters were formed in situ on the outer x{sub 2} orbits in regions of dense molecular gas subsequently ionized by the rapidly evolving starburst. The similar radial velocities of the three clusters and their small projected separation of ∼25 pc suggest that they may merge in the near future unless this is prevented by velocity shearing.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

PINGU: A vision for neutrino and particle physics at the South Pole

The Precision IceCube Next Generation Upgrade (PINGU) is a proposed low-energy in-fill extension to the IceCube Neutrino Observatory. With detection technology modeled closely on the successful IceCube example, PINGU will provide a 6 Mton effective mass for neutrino detection with an energy threshold of a few GeV. Also, with an unprecedented sample of over 60 000 atmospheric neutrinos per year in this energy range, PINGU will make highly competitive measurements of neutrino oscillation parameters in an energy range over an order of magnitude higher than long-baseline neutrino beam experiments. PINGU will measure the mixing parameters Θ 23 and Δm $2\atop{32}$ , including the octant of Θ 23 for a wide range of values, and determine the neutrino mass ordering at 3σ median significance within five years of operation. PINGU's high precision measurement of the rate of v T appearance will provide essential tests of the unitarity of the 3 ×3 PMNS neutrino mixing matrix. PINGU will also improve the sensitivity of searches for low mass dark matter in the Sun, use neutrino tomography to directly probe the composition of the Earth's core, and improve IceCube's sensitivity to neutrinos from Galactic supernovae. Finally, reoptimization of the PINGU design has permitted substantial reduction in both cost and logistical requirements while delivering performance nearly identical to configurations previously studied.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1997

Particle Acceleration in Active Galactic Nuclei

The high efficiency of energy generation inferred from radio observations of quasars and X-ray observations of Seyfert active galactic nuclei (AGNs) is apparently achieved only by the gravitational conversion of the rest mass energy of accreting matter onto supermassive black holes. Evidence for the acceleration of particles to high energies by a central engine is also inferred from observations of apparent superluminal motion in flat spectrum, core-dominated radio sources. This phenomenon is widely attributed to the ejection of relativistic bulk plasma from the nuclei of active galaxies, and accounts for the existence of large scale radio jets and lobes at large distances from the central regions of radio galaxies. Reports of radio jets and superluminal motion from galactic black hole candidate X-ray sources indicate that similar processes are operating in these sources. Observations of luminous, rapidly variable high-energy radiation from active galactic nuclei (AGNs) with the Compton Gamma Ray Observatory show directly that particles are accelerated to high energies in a compact environment. The mechanisms which transform the gravitational potential energy of the infalling matter into nonthermal particle energy in galactic black hole candidates and AGNs are not conclusively identified, although several have been proposed. These include direct acceleration by static electric fields (resulting from, for example, magnetic reconnection), shock acceleration, and energy extraction from the rotational energy of Kerr black holes. The dominant acceleration mechanism(s) operating in the black hole environment can only be determined, of course, by a comparison of model predictions with observations. The purpose of the work proposed for this grant was to investigate stochastic particle acceleration through resonant interactions with plasma waves that populate the magnetosphere surrounding an accreting black hole. Stochastic acceleration has been successfully applied to the problem of ion and electron energization in solar flares, and is capable of accounting for a wide range of both neutral and charged particle emissions. It is also a component in diffusive shock acceleration, since pitch-angle scattering (which is necessary for multiple shock crossings) is accompanied by diffusion in momentum space, which in turn yields a net systematic energy gain; however, stochastic energization will dominate the first-order shock process only in certain parameter regimes. Although stochastic acceleration has been applied to particle energization in the lobes of radio galaxies, its application to the central regions of AGNs has only recently been considered, but not in detail. We proposed to systematically investigate the plasma processes responsible for stochastic particle acceleration in black hole magnetospheres along with the energy-loss processes which impede particle energization. To this end we calculated acceleration rates and escape time scales for protons and electrons resonating with Alfven waves, and for electrons resonating with whistlers. Assuming either a Kolmogorov or Kraichnan wave spectrum, accretion at the Eddington limit, magnetic field strengths near equipartition, and turbulence energy densities approx. 10% of the total magnetic field energy density, we find that Alfven waves accelerate protons to Lorentz factors approx, equals 10(exp 4) - 10(exp 6) before they escape from the system. Acceleration of electrons by fast mode and whistler waves can produce a nonthermal population of relativistic electrons whose maximum energy is determined by a competition with radiation losses.

Miller, James A.↗
DOE OSTI2026

Production of Heavy α -Elements and 44 Ti in Cas A: Comparison to Abundances from 1D Core-collapse Supernova Models and Evidence for Carbon–Oxygen Shell Mergers

The merger between the carbon (C) and oxygen (O) shells hours to days before the collapse of a massive star significantly changes its nucleosynthesis, which is reflected in the elemental ratios observed in supernova remnants (SNRs). We present a nucleosynthesis study of 44 Ti production in core-collapse supernovae (CCSNe), highlighting large silicon (Si), sulfur (S), calcium (Ca), and, most importantly, argon (Ar) to neon (Ne) ratios as diagnostics for carbon–oxygen (C–O) shell mergers. We compare yields from eight different sets of CCSNe models to observations of Cassiopeia A (Cas A), and show that C–O shell mergers are consistently the models that best match X-ray and infrared observations. These models produce high Ar/Ne ratios (≳0.1), due to 20 Ne depletion and production of 36 Ar and 38 Ar, while lower ratios are obtained from nonmerger cases. Based on the Ar/Ne diagnostic, we compare the range of expected 44 Ti produced by C–O shell mergers, which is up to ∼20%–30% of the overall 44 Ti, but expected to be located outside the reverse shock. Based on the sets of models considered, the photon flux expected from the 44 Ti synthesized in the C–O shell merger in Cas A is below the NuSTAR and COSI detection limits, compatible with current limits locating most of the 44 Ti interior to the reverse shock, but might be detectable from proposed missions like ASCENT. Finally, for the SNR of 1987A, a dominant C–O merger origin of the observed 44 Ti is unlikely based on the observed redshift in its 44 Ti line.

79 ASTRONOMY AND ASTROPHYSICS↗