High-fidelity MCNP modeling of a D-T neutron generator for active interrogation of special nuclear material

Fast and robust methods for interrogation of special nuclear material (SNM) are of interest to many agencies and institutions in the United States. It is well known that passive interrogation methods are typically sufficient for plutonium identification because of a relatively high neutron production rate from ²⁴⁰Pu [1]. On the other hand, identification of shielded uranium requires active methods using neutron or photon sources [2]. Deuterium-deuterium (2.45 MeV) and deuterium-tritium (14.1 MeV) neutron-generator sources have been previously tested and proven to be relatively reliable instruments for active interrogation of nuclear materials [3] and [4]. In addition, the newest generators of this type are small enough for applications requiring portable interrogation systems.Active interrogation techniques using high-energy neutrons are being investigated as a method to detect hidden SNM in shielded containers [4] and [5]. Due to the thickness of some containers, penetrating radiation such as high-energy neutrons can provide a potential means of probing shielded SNM. In an effort to develop the capability to assess the signal seen from various forms of shielded nuclear materials, the University of Michigan Neutron Science Laboratory’s D-T neutron generator and its shielding were accurately modeled in MCNP. The generator, while operating at nominal power, produces approximately 1×10¹⁰ neutrons/s, a source intensity which requires a large amount of shielding to minimize the dose rates around the generator. For this reason, the existing shielding completely encompasses the generator and does not include beam ports. Therefore, several MCNP simulations were performed to estimate the yield of uncollided 14.1-MeV neutrons from the generator for active interrogation experiments. Beam port diameters of 5, 10, 15, 20, and 25 cm were modeled to assess the resulting neutron fluxes. The neutron flux outside the beam ports was estimated to be approximately 2×10⁴ n/cm² s.     

Computers and Composition 20/20: A Conversation Piece, or What Some Very Smart People Have to Say about the Future

At the 2011 Computers and Writing Conference, Town Hall speakers were asked to envision the future. This piece extends that conversation, with contributors presenting a range of ideas, often looking backward at our history before gazing into their crystal balls to envision what the future might bring. The pieces included here discuss writing, teaching writing, writing assessment, publishing, robotics, mobility, and other aspects of the field loosely termed computers and composition as it was, is, or may come to be in what we hope will be only the start of an ongoing conversation.     

Effects of Alkaline Treatment on Dissolved Carbohydrates in Suspensions of Norway Spruce Thermomechanical Pulp

The effects of alkaline treatment on dissolved carbohydrates in suspensions of Norway spruce thermomechanical pulp (TMP) were studied by agitating 1% consistency TMP suspensions at different pH values (5.4 to 12) for 3 h at 60°, followed by acidification and centrifugation to obtain water samples for analysis. It was found that at pH values above 8, dissolved galactoglucomannans were deacetylated, resulting in their substantial adsorption onto the TMP. Galacturonans began to dissolve from the TMP at pH values greater than 8. The amount of arabinogalactans apparently remained constant throughout the various alkaline treatments. At pH values above 10, arabino-(4-O-methylglucurono)xylans began to dissolve from the TMP.

The amount of dissolved and colloidal substances (DCS), measured by TOC, could be substantially decreased by alkaline treatment of filtrate from the 1% consistency suspension, followed by acidification and remixing the filtrate with its original pulp. At 10 m³/t specific water consumption, the TOC of laboratory-prepared circulation water dropped about 20% due to the alkaline treatment. The decrease in TOC was attributed mostly to the adsorption of deacetylated galactoglucomannans. Alkaline treatment of fiber-free mechanical pulp circulation water may be a useful technique to reduce the amount of organic substances circulating in process waters and leaving the pulp or paper mill in the effluent stream.     

Pressurized water reactor fuel crud and corrosion modeling

Pressurized water reactors circulate high-temperature water that slowly corrodes Inconel and stainless steel system surfaces, and the nickel/iron based corrosion products deposit in regions of the fuel where sub-cooled nucleate boiling occurs. The deposited corrosion products, called ‘crud’, can have an adverse impact on fuel performance. Boron can concentrate within the crud in the boiling regions of the fuel leading to a phenomenon known as axial offset anomaly (AOA). In rare cases, fuel clad integrity can be compromised because of crud-induced localized corrosion (CILC) of the zirconium-based alloy. Westinghouse and the Electric Power Research Institute have committed to understanding the crud transport process and develop a risk assessment software tool called boron-induced offset anomaly (BOA) to avoid AOA and CILC. This paper reviews the history of the BOA model development and new efforts to develop a micro-scale model called MAMBA for use in the Consortium for Advanced Light Water Reactor Simulation (CASL) program.     

Beyond Counterarguing: Simple Elaboration,Complex Integration,and Counterelaboration in Response to Variations in Narrative Focus and Sidedness

This study attempts to advance theorizing about narrative persuasion by explicating types of thoughts, beyond counterarguing, generated in response to a short narrative with persuasive intent. We examine responses to four types of narratives (focus: individual vs. community; by sidedness: one‐ vs. two‐sided) about causes and solutions for obesity in an attempt to increase support for policies to address the issue. Using a randomized experiment (n = 245), we show that narrative focus and sidedness interact to produce different patterns of thoughts, attributions, and policy support. Simple elaboration, counterelaboration, and counterarguing predicted causal attributions and policy support, but only simple elaboration mediated message effects on intended persuasive outcomes. We conclude by discussing the study's contributions to communication theory and practice.     

Function Trumps Form in Two Sugar Symporters,LacY and vSGLT

Active transport of sugars into bacteria occurs through symporters driven by ion gradients. LacY is the most well-studied proton sugar symporter, whereas vSGLT is the most characterized sodium sugar symporter. These are members of the major facilitator (MFS) and the amino acid-Polyamine organocation (APS) transporter superfamilies. While there is no structural homology between these transporters, they operate by a similar mechanism. They are nano-machines driven by their respective ion electrochemical potential gradients across the membrane. LacY has 12 transmembrane helices (TMs) organized in two 6-TM bundles, each containing two 3-helix TM repeats. vSGLT has a core structure of 10 TM helices organized in two inverted repeats (TM 1–5 and TM 6–10). In each case, a single sugar is bound in a central cavity and sugar selectivity is determined by hydrogen- and hydrophobic- bonding with side chains in the binding site. In vSGLT, the sodium-binding site is formed through coordination with carbonyl- and hydroxyl-oxygens from neighboring side chains, whereas in LacY the proton (H₃O⁺) site is thought to be a single glutamate residue (Glu325). The remaining challenge for both transporters is to determine how ion electrochemical potential gradients drive uphill sugar transport.     

Programs for kriging and sequential Gaussian simulation with locally varying anisotropy using non-Euclidean distances

Geological deposits display nonlinear features such as veins, channels or folds that result in complex spatial anisotropies that are difficult to model with currently available geostatistical techniques. The methodology presented in this paper for incorporating locally varying anisotropy in kriging or sequential Gaussian simulation is based on modifying how locations in space are related. Normally, the straight line path is used; however, when nonlinear features exist the appropriate path between locations follows along the features. The Dijkstra algorithm is used to determine the shortest path/distance between locations and a conventional covariance or variogram function is used. This nonlinear path is a non-Euclidean distance metric and positive definiteness of the resulting kriging system of equations is not guaranteed. Multidimensional scaling (landmark isometric mapping) is used to ensure positive definiteness. In addition to the variogram, the only parameters required for the implementation of kriging or sequential Gaussian simulation with locally varying anisotropy are (1) the local orientation and magnitude of anisotropy and (2) the number of dimensions required for multidimensional scaling. This paper presents a suite of programs that can be used to krige or simulate practically sized geostatistical models with locally varying anisotropy. The programs kt3d_LVA, SGS_LVA and gamv_LVA are provided.