Publications

RADTRAN is an internationally accepted program and code for calculating the risks of transporting radioactive materials. The first versions of the program, RADTRAN I and II, were developed for NUREG-0170 (USNRC, 1977), the first environmental statement on transportation of radioactive materials. RADTRAN and its associated software have undergone a number of improvements and advances consistent with improvements in both available data and computer technology. The version of RADTRAN currently bundled with RadCat is RADTRAN 6.0. This document provides a detailed discussion and a guide for the use of the RadCat 3.0 Graphical User Interface input file generator for the RADTRAN code. RadCat 3.0 integrates the newest analysis capabilities of RADTRAN 6.0 which includes an economic model, updated loss-of-lead shielding model, and unit conversion. As of this writing, the RADTRAN version in use is RADTRAN 6.0.

Abstract not provided.

A stabilized node-based uniform strain tetrahedral element is presented and analyzed for finite deformation elasticity. The element is based on linear interpolation of a classical displacement-based tetrahedral element formulation but applies nodal averaging of the deformation gradient to improve mechanical behavior, especially in the regime of near-incompressibility where classical linear tetrahedral elements perform very poorly. This uniform strain approach adopted here exhibits spurious modes as has been previously reported in the literature. We present a new type of stabilization exploiting the circumstance that the instability in the formulation is related to the isochoric strain energy contribution only and we therefore present a stabilization based on an isochoric-volumetric splitting of the stress tensor. We demonstrate that by stabilizing the isochoric energy contributions only, reintroduction of volumetric locking through the stabilization can be avoided. The isochoric-volumetric splitting can be applied for all types of materials with only minor restrictions and leads to a formulation that demonstrates impressive performance in examples provided. Copyright © 2008 John Wiley & Sons, Ltd.

Planar wire arrays are studied at 3-6 MA on the Saturn pulsed power generator as potential drivers of compact hohlraums for inertial confinement fusion studies . Comparison with zero-dimensional modeling suggests that there is significant trailing mass. The modeled energy coupled from the generator cannot generally explain the energy in the main x-ray pulse. Preliminary comparison at 1-6 MA indicates sub-quadratic scaling of x-ray power in a manner similar to compact cylindrical wire arrays. Time-resolved pinhole images are used to study the implosion dynamics. © 2009 American Institute of Physics.

Experiments with different stainless steel (SS) wire loads were performed on the 1 MA Zebra Z-pinch generator at University of Nevada, Reno. The wire array loads consisted of 7.6 (μm SS wires and had 10 wires for the planar wire array with an interwire gap of 1 mm and 8 wires for the cylindrical wire array of a 16 mm diameter. In addition, a single-wire experiment with a 25 (μm SS wire was carried out. The different wire loads were used to provide a broader spectrum of plasma conditions. Time-integrated and time-gated x-ray images, as well as time-integrated, spatially-resolved and spatially-integrated x-ray spectra, were collected and analyzed. Both K-shell and L-shell radiation were recorded using LiF and KAP crystal spectrometers, respectively. Non-LTE kinetic models of Fe and Ni are employed to derive plasma parameters. For axially resolved L-shell spectra, the resulting electron temperatures are between 230 and 300 eV (higher near the cathode) and electron densities vary from 10 19 to 10 20 cm -3 dependent on the load. The advantage of using Z-pinch plasmas for astrophysical applications is highlighted. © 2009 American Institute of Physics.

Many physical effects can produce unstable plasma behavior that affect K-shell emission from arrays. Such effects include: asymmetry in the initial density profile, asymmetry in power flow, thermal conduction at the boundaries, and non-uniform wire ablation. Here we consider how asymmetry in the radiation field also contributes to the generation of multidimensional plasma behavior that affects K-shell power and yield. To model this radiation asymmetry, we have incorporated into the MACH2 r-z MHD code a self-consistent calculation of the non-LTE population kinetics based on radiation transport using multi-dimensional ray tracing. Such methodology is necessary for modeling the enhanced radiative cooling that occurs at the anode and cathode ends of the pinch during the run-in phase of the implosion. This enhanced radiative cooling is due to reduced optical depth at these locations producing an asymmetric flow of radiative energy that leads to substantial disruption of large initial diameter (>5 cm) pinches and drives ID into 2D fluid (i.e., Rayleigh-Taylor like) flows. The impact of this 2D behavior on K-shell power and yield is investigated by comparing ID and 2D model results with data obtained from a series of single wire array stainless steel experiments performed on the Z generator. © 2009 American Institute of Physics.

A 1D Lagrangian magnetohydrodynamic z-pinch simulation code is extended to include wire ablation. The plasma transport coefficients are calibrated to reproduce the K-shell yields measured on the Z generator for three stainless steel arrays of diameter 55 mm and masses ranging from 1.8 to 2.7 mg. The resulting 1D scaling model is applied to a larger SS array (65 mm and 2.5 mg) on the refurbished Z machine. Simulation results predict a maximum K-shell yield of 77 kJ for an 82 kV charging voltage. This maximum drops to 42 kJ at 75 kV charging. Neglecting the ablation precursor leads to a ∼10% change in the calculated yield. © 2009 American Institute of Physics.

Highly collimated outflows or jets are produced by a number of astrophysical objects including protostars. The morphology and collimation of these jets is thought to be strongly influenced by the effects of radiative cooling, angular momentum and the interstellar medium surrounding the jet. Astrophysically relevant experiments are performed with conical wire array z-pinches investigating each of these effects. It is possible in each case to enter the appropriate parameter regime, leading the way towards future experiments where these different techniques can be more fully combined. © 2009 American Institute of Physics.

The major focus of this study was to examine the feasibility of applying diamond-like nanocomposite (DLN) coatings on the sidewalls of Ni alloy parts fabricated using lithographie, galvanoformung and abformung (LIGA: a German acronym that means lithography, electroforming, and molding) for friction and wear control. Planar test coupons were employed to understand the friction mechanisms in regimes relevant to LIGA microsytems. Friction tests were conducted on planar test coupons as well as between LIGA-fabricated test structures in planar-sidewall and sidewall-sidewall configurations. Measurements were made in dry nitrogen and air with 50% relative humidity by enclosing the friction tester in an environmental chamber. In contrast to bare metal-metal contacts, minimal wear was exhibited for the DLN-coated LIGA NiMn alloy parts and test coupons. The low friction behavior of DLN was attributed to its ability to transfer to the rubbing counterface providing low interfacial shear at the sliding contact. The coating coverage and chemistry on the sidewalls and the substrate-coating interface integrity were examined by transmission electron microscopy, Automated eXpert Spectral Image Analysis, and electron backscatter diffraction on cross sections prepared by focused ion beam microscopy. The role of novel characterization techniques to evaluate the surface coatings for LIGA microsystems technology is highlighted. © 2009 IEEE.

A future generation of high-performance low-power atomic systems is expected to require VCSEL linewidths below 10 MHz for compatibility with the natural atomic linewidth (5 MHz for cesium) that is realized with atomic beams, trapped atoms, and trapped ions. This paper describes initial efforts at Sandia to reduce VCSEL linewidth by increasing the effective cavity length of an 850-nm monolithic VCSEL. In particular, two aspects of VCSEL design will be discussed: the Q of the VCSEL cavity, and the linewidth enhancement factor of the active region material. We report a factor of two linewidth reduction, from 50 MHz for our standard oxide-aperture VCSEL to 23 MHz for an extended-cavity VCSEL. ©2009 SPIE.

Detailed exhaust speciation measurements were made on an HCCI engine fueled with iso-octane over a range of fueling rates, and over a range of fuel-stratification levels. Fully premixed fueling was used for the fueling sweep. This sweep extended from a fuel/air equivalence ratio (Φ{phonetic}) of 0.28, which is sufficiently high to achieve a combustion efficiency of 96%, down to a below-idle fueling rate of Φ{phonetic} = 0.08, with a combustion efficiency of only 55%. The stratification sweep was conducted at an idle fueling rate, using an 8-hole GDI injector to vary stratification from well-mixed conditions for an early start of injection (SOI) (40°CA) to highly stratified conditions for an SOI well up the compression stroke (325°CA, 35°bTDCcompression). The engine speed was 1200 rpm. At each operating condition, exhaust samples were collected and analyzed by GC-FID for the C1 and C2 hydrocarbon (HC) species and by GC-MS for all other species except formaldehyde and acetaldehyde. These two species were analyzed using high-performance liquid chromatography. In addition, standard emissions-bench exhaust analysis equipment was used to measure total HC, CO, CO2, O2, and NOX simultaneously with the sampling for the detailed-speciation analysis. Good overall agreement was found between the emissions-bench data and total HC from the detailed measurements. Unreacted fuel, iso-octane, was by far the most prevalent HC species at all operating conditions. Numerous other HC and oxygenated HC (OHC) species were found that could be identified as breakdown products of iso-octane. Several smaller HC and OHC species were also identified. At the highest Φ{phonetic}, emissions of all species were low, except iso-octane. As Φ{phonetic} was reduced, emissions of all species increased, but the rate of increase varied substantially for the different species. Analysis showed that these differences were related to the degree of breakdown from the parent fuel and the in-cylinder location where they formed. SOI-sweep results indicated that stratification improves combustion efficiency by reducing the fuel penetration to the crevice and cylinder-wall boundary-layer regions, as well as by creating a locally richer mixture that burns hotter and more completely.

This work explores the high-load limits of HCCI for naturally aspirated operation. This is done for three fuels with various autoignition reactivity: iso-octane, PRF80, and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liter displacement), mostly with a CR = 14 piston installed, but with some tests at CR = 18. Five load-limiting factors were identified: 1) NOx-induced combustion-phasing run-away, 2) wall-heating-induced run-away, 3) EGR-induced oxygen deprivation, 4) wandering unsteady combustion, and 5) excessive exhaust NOx. These experiments at 1200 rpm show that the actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the selected CA50, and in some cases, the tolerable level of NOx emissions. For iso-octane, which has the highest resistance to autoignition of the fuels tested, the NOx emissions become unacceptable at IMEPg = 473 kPa. This happens before wandering and unsteady combustion becomes an issue for IMEPg > 486 kPa. The NOx is caused by high peak-combustion temperatures resulting from the high intake temperature required for this low-reactivity fuel. Iso-octane operation with a CR = 18 piston reduces the intake-temperature requirement. Consequently, the exhaust NOx issue vanishes while the IMEPg can be increased to 520 kPa before wall-heating-induced run-away become an issue. For a very reactive fuel like PRF60, large amounts of EGR are required to control the combustion phasing. Therefore, the maximum IMEPg becomes limited at 643 kPa by the available oxygen as the EGR gases displace air. A fuel of intermediate reactivity, PRF80, exhibits the highest IMEPg for the conditions of this study - 651 kPa. For this fuel, the maximum IMEPg becomes limited by NOx-induced run-away. This happens because even small amounts of NOx recycled via residuals enhance the autoignition sufficiently to advance the ignition point. This leads to higher peak-combustion temperatures and more NOx formation, thus making a very rapid run-away situation inevitable.

We consider a Bayesian approach to nonlinear inverse problems in which the unknown quantity is a spatial or temporal field, endowed with a hierarchical Gaussian process prior. Computational challenges in this construction arise from the need for repeated evaluations of the forward model (e.g., in the context of Markov chain Monte Carlo) and are compounded by high dimensionality of the posterior. We address these challenges by introducing truncated Karhunen-Loève expansions, based on the prior distribution, to efficiently parameterize the unknown field and to specify a stochastic forward problem whose solution captures that of the deterministic forward model over the support of the prior. We seek a solution of this problem using Galerkin projection on a polynomial chaos basis, and use the solution to construct a reduced-dimensionality surrogate posterior density that is inexpensive to evaluate. We demonstrate the formulation on a transient diffusion equation with prescribed source terms, inferring the spatially-varying diffusivity of the medium from limited and noisy data. © 2008 Elsevier Inc.

For logistical reasons, the military requires that jet fuel (JP-8, F-34) be used in both jet engines and diesel engines. While JP-8-fueled diesel engines appear to operate successfully in many cases, negative impacts, including engine failures, are occasionally reported. As diesel combustion with JP-8 has not been explored in great detail, fundamental information about JP-8 fuel spray combustion is needed. In this study, we report measurements of liquid-phase penetration length, vapor penetration, and ignition delay made in an optically- accessible combustion vessel over a range of high- temperature, high-pressure operating conditions applicable to a diesel engine. Results show that the liquid-phase penetration of JP-8 is less than that of diesel, owing to the lower boiling point temperatures of JP-8. Despite the more rapid vaporization, the vapor penetration rate of JP-8 matches that of diesel and ignition does not advance. In fact, with no required cetane number specification for JP-8, ignition delay times are 25-50% longer for this 38-cetane-number JP-8 fuel sample compared to a 46-cetane-number #2 diesel sample. High-speed shadowgraph imaging shows that a cool flame precedes ignition for both diesel and JP-8 but the time of the cool flame heat release is delayed for JP- 8, consistent with the overall ignition delay trend.

A very general and robust approach to solving continuous-variable optimization problems involving uncertainty in the objective function is through the use of ordinal optimization. At each step in the optimization problem, improvement is based only on a relative ranking of the uncertainty effects on local design alternatives, rather than on precise quantification of the effect. One simply asks "Is that alternative better or worse than this one?"-not "HOW MUCH better or worse is that alternative to this one?" The answer to the latter question requires precise characterization of the uncertainty- with the corresponding sampling/integration expense for precise resolution. By looking at things from an ordinal ranking perspective instead, the trade-off between computational expense and vagueness in the uncertainty characterization can be managed to make cost-effective stepping decisions in the design space. This paper demonstrates correct advancement in a continuous-variable probabilistic optimization problem despite extreme vagueness in the statistical characterization of the design options. It is explained and shown how spatial correlation of uncertainty in such design problems can be exploited to dramatically increase the efficiency of ordinal approaches to optimization under uncertainty.

The Galerkin projection procedure for construction of reduced order models of compressible flow is examined as an alternative discretization of the governing differential equations. The numerical stability of Galerkin models is shown to depend on the choice of inner product for the projection. For the linearized Euler equations, a symmetry transformation leads to a stable formulation for the inner product. Boundary conditions for compressible flow that preserve stability of the reduced order model are constructed. Preservation of stability for the discrete implementation of the Galerkin projection is made possible using a piecewise-smooth finite element basis. Stability of the reduced order model using this approach is demonstrated on several model problems, where a suitable approximation basis is generated using proper orthogonal decomposition of a transient computational fluid dynamics simulation. © 2008 Elsevier Inc.

In an optically accessible single-cylinder engine fueled with hydrogen, acetone planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) are used to evaluate in-cylinder mixture formation. The experiments include measurements for engine operation with hydrogen injection in-cylinder either prior to or after intake valve closure (IVC). Pre-IVC injection is used to produce a near-homogeneous mixture for PLIF calibration experiments and to establish a baseline comparison for post-IVC injection. Calibration experiments and a temperature correction allow conversion of the acetone fluorescence signal to equivalence ratio. For post-IVC injection with start of injection (SOI) coincident with IVC, PLIF results are similar to pre-IVC injection. With retard of SOI from IVC, mixture inhomogeneities increase monotonically, with high hydrogen concentration spatially located near the injector and within a smaller volume. For injection late in the cycle, the turbulent fuel-rich area is sharply delineated from the more quiescent fuel-lean region. The PIV vector plots suggest that the observed spatial distribution of hydrogen for SOI retarded from IVC is a consequence of the in-cylinder flow field generated by the injection event. Specifically, in the measured r-θ plane of the cylinder and in the field of view imaged, the vector plots show a large-scale mean flow towards the injector. It is conjectured that the observed flow field results from jet-wall interactions that redirect the leading edge of some of the fuel jets back towards the injector, creating a counter-flow with respect to the other fuel jets, which inhibits further jet penetration. The net result is a high hydrogen concentration near the injector. This scenario confirms that the injector tip geometry, injector location, and injection timing are critical parameters with respect to in-cylinder mixing in direct-injection hydrogenfuelled engine.

A very general and robust approach to solving continuous-variable optimization problems involving uncertainty in the objective function is through the use of ordinal optimization. At each step in the optimization problem, improvement is based only on a relative ranking of the uncertainty effects on local design alternatives, rather than on precise quantification of the effect. One simply asks "Is that alternative better or worse than this one?"-not "HOW MUCH better or worse is that alternative to this one?" The answer to the latter question requires precise characterization of the uncertainty- with the corresponding sampling/integration expense for precise resolution. By looking at things from an ordinal ranking perspective instead, the trade-off between computational expense and vagueness in the uncertainty characterization can be managed to make cost-effective stepping decisions in the design space. This paper demonstrates correct advancement in a continuous-variable probabilistic optimization problem despite extreme vagueness in the statistical characterization of the design options. It is explained and shown how spatial correlation of uncertainty in such design problems can be exploited to dramatically increase the efficiency of ordinal approaches to optimization under uncertainty.

Low-temperature combustion of diesel fuel was studied in a heavy-duty, single-cylinder, optical engine employing a 15-hole, dual-row, narrow-included-angle nozzle (10 holes x 70° and 5 holes x 35°) with 103-μmdiameter orifices. This nozzle configuration provided the spray targeting necessary to contain the direct-injected diesel fuel within the piston bowl for injection timings as early as 70° before top dead center. Spray-visualization movies, acquired using a high-speed camera, show that impingement of liquid fuel on the piston surface can result when the in-cylinder temperature and density at the time of injection are sufficiently low. Seven single- and two-parameter sweeps around a 4.82-bar gross indicated mean effective pressure load point were performed to map the sensitivity of the combustion and emissions to variations in injection timing, injection pressure, equivalence ratio, simulated exhaust-gas recirculation, intake temperature, intake boost pressure, and load. High-speed movies of natural luminosity were acquired by viewing through a window in the cylinder wall and through a window in the piston to provide quasi-3D information about the combustion process. These movies revealed that advanced combustion phasing resulted in intense pool fires within the piston bowl, after the end of significant heat release. These pool fires are a result of fuel-films created when the injected fuel impinged on the piston surface. The emissions results showed a strong correlation with poolfire activity. Smoke and NOx emissions rose steadily as pool-fire intensity increased, whereas HC and CO showed a dramatic increase with near-zero pool-fire activity.

Particle image velocimetry (PIV) was used to measure velocity fields inside and around oscillating methane-air diffusion flames with a slot fuel orifice. PIV provided velocity and directional information of the flow field comprised of both the flame and air. From this, information on flow paths of entrained air into the flame were obtained and visualized. These show that at low fuel flow rates for which the oscillations were strongest, the responsible mechanism for the oscillating flow appeared to be the repetitive occurrence of flame quenching. PIV findings indicated that quenching appears to be associated primarily with air entrainment. Velocity was found to be considerably larger in regions where quenching occurred. The shedding of vortices in the shear layer occurs immediately outside the boundary of the flame envelope and was speculated to be the primary driving force for air entrainment. © 2008 Springer-Verlag.

The gas storage capacity of metal-organic frameworks (MOFs) is well-known and has been investigated using both experimental and computational methods. Previous Monte Carlo computer simulations of gas adsorption by MOFs have made several questionable approximations regarding framework-framework and framework-adsorbate interactions: potential parameters from general force fields have been used, and framework atoms were fixed at their crystallographic coordinates (rigid framework). We assess the validity of these approximations with grand canonical Monte Carlo simulations for a well-known Zn-based MOF (IRMOF-1), using potential parameters specifically derived for IRMOF-1. Our approach is validated by comparison with experimental results for hydrogen and xenon adsorption at room temperature. The effects of framework flexibility on the adsorption of noble gases and hydrogen are described, as well as the selectivity of IRMOF-1 for xenon versus other noble gases. At both low temperature (78 K) and room temperature, little difference in gas adsorption is seen between the rigid and flexible force fields. Experimental trends of noble gas inflation curves are also matched by the simulation results. Additionally, we show that IRMOF-1 selectively adsorbs Xe atoms in Xe/Kr and Xe/Ar mixtures, and this preference correlates with the trend in van der Waals parameters for the adsorbate atoms. © 2009 American Chemical Society.

We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Many experimental efforts to track fuel-air-residual mixture preparation in internal combustion engines have employed laser induced fluorescence (LIF) of tracers. Acetone and 3-pentanone are often chosen as tracers because of their relatively strong LIF signal, weak quenching, and reasonable match to thermo-chemical properties of common fuels such as iso-octane. However, the addition of these tracers to fuel-air mixtures could affect combustion behavior. In this work, we assess these effects to better understand limitations of tracer-based engine measurements. The effects of tracer seeding on combustion phasing, duration, and variation are studied in an HCCI engine using a recompression strategy to accommodate single- and multi-stage-ignition fuels. Using direct-injected (DI) fuels iso-octane and n-heptane, comparisons are made of combustion performance with and without seeding of the intake air (air seeding, as opposed to the more common fuel seeding, is a variation of LIF used to measure residual-gas concentration). Chemical and premixing effects of tracer addition are distinguished by substituting equivalent amounts of fuel for the tracer. Chemical kinetic simulations of iso-octane and n-heptane oxidation help explain the experimentally determined trends. Results show that the phasing of iso-octane combustion can be significantly impacted by premixing effects because of the sensitivity of ignition to charge temperature. For n-heptane, the chemical effects of tracer addition are shown to be more pronounced because of impact on low-temperature heat release. Acetone retards the combustion for both single- and two- stage-ignition fuels, whereas 3-pentanone advances iso- octane combustion while retarding n-heptane. Overall, we found that the impact of tracer addition is modest for the chosen operating conditions since varying the intake temperature can easily compensate for it.

A simplified model was developed and is presented in this report for simulating thermal transport coupled with chemical reactions that lead to the pyrotechnic ignition of TiH1.65/KClO4 powder. The model takes into account Joule heating via a bridgewire, thermal contact resistance at the wire/powder interface, convective heat loss to the surroundings, and heat released from the TiH1.65- and KClO4-decomposition and TiO2-oxidation reactions. Chemical kinetic sub-models were put forth to describe the chemical reaction rate(s) and quantify the resultant heat release. The simplified model predicts pyrotechnic ignition when heat from the pyrotechnic reactions is accounted for. Effects of six key parameters on ignition were examined. It was found that the two reaction-rate parameters and the thermal contact resistance significantly affect the dynamic ignition process whereas the convective heat transfer coefficient essentially has no effect on the ignition time. Effects of the initial/ambient temperature and electrical current load through the wire are as expected. Ignition time increases as the initial/ambient temperature is lowered or the wire current load is reduced. Lastly, critical needs such as experiments to determine reaction-rate and other model-input parameters and to measure temperature profiles, time to ignition and burn-rate data for model validation as well as efforts in incorporating reaction-rate dependency on pressure are pointed out.

Data and models of aerosol particle deposition in leak pathways are described. Pathways considered include capillaries, orifices, slots and cracks in concrete. The Morewitz-Vaughan criterion for aerosol plugging of leak pathways is shown to be applicable only to a limited range of particle settling velocities and Stokes numbers. More useful are sampling efficiency criteria defined by Davies and by Liu and Agarwal. Deposition of particles can be limited by bounce from surfaces defining leak pathways and by resuspension of particles deposited on these surfaces. A model of the probability of particle bounce is described. Resuspension of deposited particles can be triggered by changes in flow conditions, particle impact on deposits and by shock or vibration of the surfaces. This examination was performed as part of the review of the AP1000 Standard Combined License Technical Report, APP-GW-GLN-12, Revision 0, 'Offsite and Control Room Dose Changes' (TR-112) in support of the USNRC AP1000 Standard Combined License Pre-Application Review.