Showing papers by "Atomic Energy of Canada Limited published in 1994"

Fragmentation of necklike structures.

Abstract: Intermediate mass fragment (IMF: 3[le][ital Z][le]20) emission from necklike structures joining projectilelike and targetlike residues has been observed for peripheral [sup 129]Xe+[sup nat]Cu collisions at [ital E]/[ital A]=50 MeV. These fragments are emitted primarily at velocities between those of the projectilelike and targetlike residues. Relative to the charge distribution of fragments evaporated from projectilelike residues, the distribution for neck'' emission shows an enhanced emission for fragments with 4[le][ital Z][sub IMF][le]12. This feature is consistent with expectations for the fragmentation of a noncompact cylindrical configuration.

Contaminant Enrichment and Properties of Soil Adhering to Skin

Abstract: The adhesion of contaminated soil to skin has potentially important health implications, because the contaminants may ultimately be ingested or absorbed through the skin. Previous studies indicated that the adhering soil is enriched in contaminant concentration relative In the original soil because of the selective adhesion of finer particles. This study investigated this enrichment using 11 markedly different soils. Two sandy soils consistently gave very high contaminant enrichment ratios, with a mean enrichment of 10-fold. The other soils all had enrichment ratios above unity. Scanning electron microscopy illustrated the potential for strong adhesion of very fine clay particles. The contaminant enrichment ratios were positively correlated to enrichments in specific surface area, organic matter content, and extractable Fe content. Correlations to soil textural properties and detailed particle-size analysis of the adhering soil indicated that 50 to 100 {mu}m may be a critical particle size: larger grains and aggregates do not adhere readily to skin. Because of this, enrichment ratios will vary positively with the proportion of particles in the whole soil that am greater than 50 {mu}m. A simple model is provided to predict enrichments using information from routine soil particle-size analysis. 44 refs., 4 figs., 4 tabs.

The Hydrogeochemistry Of Fractured Plutonic Rocks In The Canadian Shield

Abstract: The composition of groundwater has been determined in selected granitic, gabbroic and gneissic plutons in the Canadian Shield as part of the Canadian Nuclear Fuel Waste Management Program. A similar geochemical evolution of the groundwater is seen with increasing depth or with flow along the hydraulic gradient, irrespective of rock type. This pattern is comparable to that seen in mine groundwaters from throughout the Canadian Shield. Near-surface groundwaters are typically dilute, slightly alkaline and Ca-(Na)-HCO3 in composition. They evolve to higher pH, Na-(Ca)-HCO3 compositions along the flow path because of interaction with plagioclase, calcite precipitation and ion exchange on clay minerals. At greater depths (>≈200 m), groundwater salinity increases as a result of the dissolution of Cl-rich salts in the rock matrix and, in certain areas, mixing with Na-Cl brines from seawater or basinal formation water. Stable and radioactive isotopic data for the groundwaters help to interpret these characteristics and indicate high residence times for the deeper, saline groundwater.

Collective expansion in central Au+Au collisions.

Abstract: Energy spectra for intermediate mass fragments produced in central Au+Au collisions at [ital E]/[ital A]=100MeV indicate a collective expansion at breakup. For the first time, values for this collective expansion energy per nucleon are extracted independently for each charge. Typically, these values are one-third to one-half of the incident kinetic energy per nucleon in the c.m. system, but they decrease with [ital Z][sub [ital f]], suggesting that all fragments do not participate equally in the collective expansion.

Self-consistent modelling of nonlinear visco-elastic polycrystals: an approximate scheme

Abstract: An approximate self-consistent scheme is presented for calculating the stress evolution in the individual grains of a polycrystal subjected to constant load conditions (creep), from initial loading to steady state. This formulation couples elastic, creep and growth strains, and provides a very flexible framework for treating the time-dependent response of anisotropic nonlinear visco-elastic polycrystals. It explicitly includes the anisotropy in the mechanical and thermal properties of the individual grains and of the polycrystal, and it accounts for intergranular interactions. The formulation is applied to the problem of irradiation creep (linear) and of thermal creep (nonlinear) of a reactor pressure tube, subjected to internal pressure. The effect on the overall response of the aggregate, of an irradiation-induced stress-independent deformation rate (growth) at the grain level is also considered. In a separate application, the coupling between the creep and the growth mechanisms in a non-textur...

A Note on Irrotational Curvilinear Flow Past a Weir

Abstract: Curvilinear flows are commonly encountered in hydraulic engineering practice. Detailed velocity surveys of the curvilinear flow field over a circular-crested two dimensional weir were obtained using laser doppler velocimetry (LDV). Using these test data, the basic assumptions related to the streamline geometry and irrotational of flow over weirs made in all existing theoretical weir modes are verified. The test date yielded the pattern, slope and curvature of the streamlines

Rate constant and activation energy determination for reaction of e−(aq) and •OH with 2-butanone and propanal

Abstract: The techniques of pulse radiolysis and absorption spectroscopy have been used to determine kinetic parameters for e−(aq) and •OH reaction with 2-butanone and propanal. The temperature dependent rat...

Modelling of time-dependent hydride growth at crack tips in zirconium alloys

Abstract: The steady-state velocity model of delayed hydride cracking (DHC) in hydride-forming metals developed by Dutton and Puls has been extended to non-steady-state conditions. The development of a non-steady-state DHC model is important since it makes it possible to incorporate, for the first time, a realistic hydride fracture criterion as well as to determine time-dependent hydride growth at the crack tip. To accomplish this, a computer program, PDECHEB, has been used to simulate a moving-boundary problem at the tip of a growing hydride. Mathematical models that define the moving-boundary problem have been developed and tested. Preliminary results, assuming a constant critical hydride length, show that this program gives reasonable predictions on the hydride growth behaviour and parameters such as incubation time, maximum hydride length, and DHC velocity, as a function of stress intensity factor, yield stress, hydrogen concentration in solid solution, and temperature. Since, for simplicity, the theoretically expected dependence of the critical hydride length on the above parameters was not included in this study, very close agreement with experimental results is not expected. Nevertheless, comparison with limited experimental data shows qualitatively good agreement.

Eliashberg treatment of the microwave conductivity of niobium

Abstract: High-precision microwave measurements have recently been carried out on superconducting Nb samples. A BCS-like coherence peak has been observed below ${\mathit{T}}_{\mathit{c}}$. We find that a strong-coupling calculation of the microwave conductivity, using electron-phonon spectral functions obtained from tunneling measurements, is unable to account for the height of the peak. We also explain how, for microwave measurements at lower frequency, the height of the coherence peak can be used to provide information about the low-frequency part of the electron-phonon spectral function and the associated inelastic scattering rate.

Mitigation of Harmful Effects of Welds in Zirconium Alloy Components

Abstract: Welding produces local residual tensile stresses and changes in texture in components made from zirconium alloys. In the heat-affected zone in tubes or plates, the basal plane normals are rotated into the plane of the component and perpendicular to the direction of the weld. Thin-walled Zircaloy-2 tubes containing an axial weld do not reach their full strength because they always fail prematurely in the weld when pressurized to failure in a fixed-end burst test. Reinforcing the weld by increasing its thickness by 25% moves the failure to the parent metal and improves the biaxial strength of the tube by 20 to 25% and increases the total elongation by 200 to 450%. In components made from Zr-2.5Nb, the texture in the heat-affected zone promotes delayed hydride cracking (DHC) driven by tensile residual stress. Although the texture is not much affected by heat-treatments below 630 C and large grain interaction stresses remain as a result of mixed textures, macro-residual tensile stresses can be relieved by heat treatment to the point where the probability of cracking is very low.

Coherent motions in windbreak flow

Abstract: Experimental data on windbreak flows have been analysed, using several statistical methods. Our analysis demonstrates that coherent structures exist in the mixing region of a porous windbreak (50% porosity) flow, and are largely responsible for the momentum transport that re-accelerates the leeward flow. A comparison between windbreak flow and laboratory turbulent mixing layers suggests that the dominant structures in these two flows are similar. Some previous numerical and experimental results are interpreted in light of the coherent structures and the self-similar property in the mixing region of a windbreak flow.

Seismic reflectivity of the Sudbury Structure North Range from borehole logs

Abstract: Results from borehole geophysical logs, full waveform sonic logs, VSP and laboratory core sample measurements indicate that lithologic variations within the Sudbury Igneous Complex (SIC) and footwall rocks are the primary cause of reflections observed regionally on the multi-channel seismic reflection profiles. The effects of macroscopic fracturing and low-grade alteration on the seismic response are only important to a depth of ∼ 320 m. The major lithologic units of the SIC and footwall rocks in the North Range of the Sudbury structure have contrasting physical properties: Felsic norite and quartz-gabbro have higher Vp (∼ 6300 m/s) than granophyre (∼ 6000 m/s) due to their higher pyroxene content and the presence of quartz rather than calcic plagioclase in the granophyre. Velocities are higher (Vp ∼ 6500 m/s) within the brecciated footwall rocks due to an overall increase in mafic mineral content. The contrasting velocities and densities of these units imply that the granophyre/quartz-gabbro contact and the SIC/footwall transition can be mapped regionally using seismic reflection methods. Subunits within these units are also highly reflective, consistent with the nature of the seismic data, but are likely discontinuous laterally.

Magneto-optics of type-II superconductors.

Abstract: The magneto-optical activity of superconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ observed by Karra\"{\i} et al. is not present in many commonly employed models of vortex dynamics. Here we propose a simple, unifying picture for the frequency-dependent magneto-optic response of type-II superconductors at low temperatures. We bring together Kohn's theorem, vortex core excitations, and vortex pinning and damping into a simple expression for the conductivity tensor. The theory describes magneto-optical activity observed in infrared transmission measurements of thin films of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$.

Electron radiation dose tailoring by variable beam pulse generation

Abstract: A method and apparatus for controlling the delivered dose across a target area from an electron beam produced by a high energy, high power pulsed rf linear accelerator. Pulse width modulation as a function of scan position is applied to the rf drive pulses of the accelerator operated in a long pulse mode and fast feedback control is used to maintain uniform rf field intensity on an intra-pulse basis. In an alternative embodiment, rf drive pulses are inhibited as a function of scan position.

Growth and Characterization of Oxide Films on Zirconium-Niobium Alloys

Abstract: Pressure tubes for CANDU reactors are made from extruded and cold-drawn Zr-2.5Nb alloy. Their microstructure consists of elongated {alpha}-Zr grains containing about 1 atom % Nb, surrounded by a thin network of metastable {beta}-Zr phase, containing about 20 atom % Nb. Alloys of Zr-1Nb an Zr-20Nb were prepared, heat treated, and oxidized in 573 K water to produce bulk microstructures and oxides that would simulate those normally found on a much finer scale in pressure tubes. Oxidation of Zr-20Nb ({beta}-Zr phase) was more rapid than that for the Zr-1Nb (predominantly {alpha}-Zr phase) but, despite this, the hydrogen absorption was considerably lower. During corrosion testing, the metastable {beta}-Zr undergoes partial decomposition to omega phase. The oxides show contrasting morphologies in terms of crystallite size (20 to 60 nm for oxides on {alpha}-Zr versus about 15 nm for oxides on {beta}-Zr). In addition to monoclinic ZrO{sub 2}, there is evidence for either tetragonal ZrO{sub 2} or the mixed oxide, 6ZrO{sub 2}Nb{sub 2}O{sub 5} in the {beta}-Zr oxide. Scanning transmission electron microscopy (STEM) imaging shows niobium associated with the oxide formed over the {beta}-Zr phase in oxidized pressure tube material. Hydrogen depth profiling by {sup 15}N nuclear reaction analyses has been used tomore » investigate the diffusion of hydrogen in these oxides. The oxide films were implanted with hydrogen and the progressive dispersion of the implanted hydrogen, as a result of annealing, was used to investigate hydrogen diffusion as a function of temperature. The nondispersive nature of the implanted hydrogen peaks in the Zr-1Nb oxide after annealing was suggestive of the presence of interconnected porosity in those oxides. The broadened peaks in the Zr-20Nb oxide after annealing are indicative of a normal diffusion process in a nonporous medium. The implications of these observations will be discussed in terms of corrosion and hydrogen uptake in Zr-2.5Nb pressure tubes.« less

Formation and stability of Fe-rich precipitates in dilute Zr(Fe) single-crystal alloys

Abstract: The formation and stability of Fe-rich precipitates in two α-Zr(Fe) single-crystal alloys with nominal compositions I, 50 parts per million by atom (ppma) Fe, and II, 650 ppma Fe, have been investigated. Optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to examine the characteristics of Fe-rich precipitates. The SEM and TEM micrographs showed that in as-grown alloy II, Zr2Fe precipitates were located at “stringers. ”Precipitates were not observed in as-grown alloy I. Annealing treatments below 700 °C, for alloy I, and 820 °C, for alloy II, resulted in the diffusion of excess Fe (above the α-phase solution limit) to the free surface with the subsequent formation of Zr3Fe precipitates in both alloys. Dissolution of Zr3Fe surface precipitates of alloy I (annealing above the solvus) left precipitate-like features on the surfaces. Zr2Fe precipitates in as-grown alloy II were readily dissolved by β-phase annealing.

Modelling Procedures for Predicting the Lifetimes of Nuclear Waste Containers

Abstract: Approaches to modelling the corrosion of nuclear waste containers are reviewed. The required containment of many thousands of years makes this a daunting task. The process has been simplified for a disposal vault in which redox conditions evolve from initially oxidizing to eventually non-oxidizing. The corrosion behaviour can be divided into two periods: an early hot, oxidizing period when localized corrosion damage is to be expected; and a later cool, non-oxidizing period when localized processes would be stifled, or repassivated, and general corrosion will predominate. At present, deterministic models to predict localized corrosion damage during the early period are unavailable or, at best, preliminary. Generally, the approach to predicting localized penetration of the container has been stochastic in nature and extreme value statistical analyses have been used to predict the expected penetration of carbon steel or copper containers by pitting. Experiments to determine the rate of crevice propagation in titanium are discussed and a model developed to predict failure of titanium waste containers by either crevice corrosion or hydrogen-induced cracking described. General corrosion occurring in the second, less oxidizing, period is more amenable to modelling by deterministic methods. Models based on electrochemical descriptions of the interfacial kinetics are described for carbon steel and copper containers, two materials expected to corrode actively under waste vault conditions. To date, no adequate model exists to predict the slow general corrosion of passivated materials.