Paul Dixon

Bio: Paul Dixon is an academic researcher from Los Alamos National Laboratory. The author has contributed to research in topics: Secular equilibrium & Plutonium. The author has an hindex of 5, co-authored 13 publications receiving 120 citations.

Analysis of naturally produced technetium and plutonium in geologic materials.

Abstract: In uncontaminated natural materials, plutonium and technetium exist exclusively as products (daughters) of nuclear reactions in which uranium is the principal reactant (parent). Under conditions of chemical stability over geologic periods of time, the relative abundances of daughter and parent elements are fixed by the rates of nuclear reactions and the decay of the daughter radionuclide. The state of this nuclear secular equilibrium condition is the primary basis of the geochemical study of these elements in nature. Thus, it is critical that nuclear parent and daughter abundances are measured in the same sample. We have developed a quantitative procedure for measuring subpicogram quantities of plutonium and technetium in gram quantities of geologic matrices such as uranium ores. The procedure takes advantage of the aggressive properties of sodium peroxide/hydroxide fusion to ensure complete dissolution and homogenization of complex materials, the precision provided by isotope dilution techniques, and the...

Chlorine-36 investigations of groundwater infiltration in the Exploratory Studies Facility at Yucca Mountain, Nevada

Abstract: Chlorine-36, including the natural cosmogenic component and the component produced during atmospheric nuclear testing in the 1950`s and 1960`s (bomb pulse), is being used as an isotopic tracer for groundwater infiltration studies at Yucca Mountain, a potential nuclear waste repository. Rock samples have been collected systematically in the Exploratory Studies Facility (ESF), and samples were also collected from fractures, faults, and breccia zones. Isotopic ratios indicative of bomb-pulse components in the water ({sup 36}Cl/Cl values > 1,250 x 10{sup {minus}15}), signifying less than 40-yr travel times from the surface, have been detected at a few locations within the Topopah Spring Tuff, the candidate host rock for the repository. The specific features associated with the high {sup 36}Cl/Cl values are predominantly cooling joints and syngenetic breccias, but most of the sites are in the general vicinity of faults. The non-bomb pulse samples have {sup 36}Cl/Cl values interpreted to indicate groundwater travel times of at least a few thousand to possibly several hundred thousand years. Preliminary numerical solute-travel experiments using the FEHM (Finite Element Heat and Mass transfer) code demonstrate consistency between these interpreted ages and the observed {sup 36}Cl/Cl values but do not validate the interpretations.

Nature`s uncommon elements: Plutonium and technetium

Abstract: The authors have taken advantage of the extremely sensitive method of thermal ionization mass spectrometry to measure technetium and plutonium concentrations in sample masses that are smaller by as much as three orders of magnitude than those used in the early research efforts. The work reported in this paper extends the understanding of the geochemistry of plutonium and technetium by developing detailed descriptions of their associations in well characterized geologic samples, and by using modern neutron-transport modeling tools to better interpret the meaning of the results. Analyses were conducted on samples from three uranium ore deposits selected for their contrasting geochemical environments. The Cigar Lake deposit is an unweathered, unaltered primary ore in a reducing environment which is expected to closely approximate a system that is closed with respect to uranium and its products. The Koongarra deposit is a shallow system, both altered and weathered, subject to active ground water flow. Finally, a sample from the Beaverlodge deposit is included because it is a commercially-available uranium ore standard that allows demonstration of the precision of the analytical results.

Advanced simulation capability for environmental management (ascem): an overview of initial results

Abstract: ADVANCED SIMULATION CAPABILITY FOR ENVIRONMENTAL MANAGEMENT (ASCEM): AN OVERVIEW OF INITIAL RESULTS Mark Williamson,* Juan Meza,† David Moulton,‡ Ian Gorton,§ Mark Freshley,§ Paul Dixon,‡ Roger Seitz,¶ Carl Steefel,† Stefan Finsterle,† Susan Hubbard,† Ming Zhu,* Kurt Gerdes,* Russ Patterson,# and Yvette T. Collazo* *U.S. Department of Energy, Office of Environmental Management, Washington, DC, USA †Lawrence Berkeley National Laboratory, Berkeley, CA, USA ‡Los Alamos National Laboratory, Los Alamos, NM, USA §Pacific Northwest National Laboratory, Richland, WA, USA ¶Savannah River National Laboratory, Aiken, SC, USA #U.S. Department of Energy, Carlsbad, NM, USA The US Department Energy (DOE) Office of Environmental Management (EM) determined that uni- form application of advanced modeling in the subsurface could potentially help reduce the cost and risk associated with its environmental cleanup mission. In response to this determination, the EM Office of Technology Innovation and Development (OTID), Groundwater and Soil Remediation (GWS Simulation; Model; Groundwater; ASCEM BACKGROUND: INTRODUCTION TO EM NEEDS Fifty years of nuclear weapons production and government-sponsored nuclear energy research in the US during the Cold War generated large amounts of radioactive wastes, spent fuel, excess plutonium and uranium, thousands of contaminated facilities, and contaminated groundwater and soil. During most of that half century, the nation did not have the environmental regulatory structure or nu- clear waste remediation technologies that exist to- day. The result was a legacy of nuclear waste that was stored and disposed of in ways now considered unacceptable (11). At the end of US Government Fiscal Year 2010 (FY10), EM had 18 funded sites. Estimates report these sites to contain 40 million m 3 of contami- nated soil and 6.4 trillion L of contaminated groundwater (7). Current groundwater and soil re- mediation challenges that will continue to be ad- dressed in the next decade include cost-effective characterization, remediation, and monitoring of contaminants in the vadose zone and groundwater.

Water film flow along fracture surfaces of porous rock

Abstract: This study shows that hydraulic properties of individual fracture surfaces can be meaningfully defined and measured, and that water film flow is a mechanism contributing to fast, unsaturated flow in fractures. The hydraulic conductivity of an unconfined block of Bishop Tuff was measured over a range of near-zero matric potentials, where differences between hydraulic conductivities obtained without and with wax sealing of its lateral sides allowed isolation of film flow effects. Tensiometer and flux measurements showed that surface film flow in this system was significant for matric potentials greater (more positive) than about −250 Pa. In this range the average film thickness was shown to be potential dependent and proportional to the observed enhanced hydraulic conductivity. Measured average surface film thicknesses ranged from 2 to 70 μm, with average film velocities in the range of 2 to 40 m d−1 (about 103 times faster than that of the pore water under unit gradient saturated flow). Our experiments demonstrate that hydraulic properties of macroscopic surfaces of porous media are quantifiable, related to surface roughness, and potentially important in the flow of water in vadose environments. This study further shows that contrary to existing conceptual models, unsaturated flow in fractures cannot generally be predicted solely on the basis of aperture distribution information. The high velocities of these surface films suggest that film flow can be an important mechanism contributing to fast flow in unsaturated fractures and macropores, especially in media characterized by low-permeability matrix and along regions of convergent flow in partially saturated fractures.

An active fracture model for unsaturated flow and transport in fractured rocks

Abstract: The unsaturated zone at Yucca Mountain, a potential repository site of high-level nuclear waste, is a complex hydrologic system in which a variety of important flow and transport processes is involved. To quantify these processes as accurately as possible is a theoretically challenging and practically important issue. In this study, we propose a new formulation for modeling flow and transport in unsaturated fractured rocks. The formulation is mainly based on a hypothesis that only a portion of connected fractures are active in conducting water. Analysis of the relevant data with the new formulation suggests that about 18–27% of the connected fractures in the Topopah Spring welded (TSw) unit (the potential repository unit) of Yucca Mountain are active under ambient conditions. The relatively high percentage of active fractures is consistent with field observations from a variety of sources. Sensitivity analyses are performed to investigate effects of the “activity” of connected fractures on flow and transport behavior in unsaturated rocks.

Natural and anthropogenic 236U in environmental samples

Abstract: The interaction of thermal neutrons with 235U results in fission with a probability of ∼85% and in the formation of 236U (t1/2 = 2.3 × 107 yr) with a probability of ∼15%. While anthropogenic 236U is, therefore, present in spent nuclear fuel at levels of 236U/U up to 10−2, the expected natural ratios in the pre-anthropogenic environment range from 10−14 to 10−10. At VERA, systematic investigations suggest a detection limit below 236U/U = 5 × 10−12 for samples of 0.5 mg U, while chemistry blanks of ∼2 × 107 atoms 236U per sample limit the sensitivity for smaller samples. We have found natural isotopic ratios in uranium reagents separated before the onset of human nuclear activities, in uranium ores from various origins and in water from a subsurface well in Bad Gastein, Austria. Anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria, whereas river sediments from Garigliano river (Southern Italy) were close to the detection limit. Finally, our natural in-house standard Vienna-KkU was calibrated against a certified reference material (IRMM REIMEP-18 A).