Showing papers by "Atomic Energy of Canada Limited published in 1991"

Magnetic excitations in the heavy-fermion superconductor URu2Si2.

Abstract: Antiferromagnetic order and fluctuations in the heavy-fermion superconductor ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ have been studied by magnetic neutron scattering. Below ${\mathit{T}}_{\mathit{N}}$=17.5 K, ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ is a type-I antiferromagnet with an anomalously small ordered moment of (0.04\ifmmode\pm\else\textpm\fi{}0.01)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ polarized along the tetragonal c axis. Dispersive resonant excitations exist in the ordered state with a zone-center gap of 0.43 THz. The excitations are polarized along the ordered moment and have a large dipolar matrix element, which suggests that they are coupled transitions between singlet crystal-field-like states. For energy transfer above 3 THz, peaks have not been identified in the magnetic excitation spectra, but instead a continuous spectrum of scattering peaked around the ordering wave vector indicates the presence of overdamped antiferromagnetically correlated spin fluctuations. Upon heating above ${\mathit{T}}_{\mathit{N}}$, the resonant excitations abruptly become heavily damped but the magnetic scattering at higher energies does not change at ${\mathit{T}}_{\mathit{N}}$. Instead, the disappearance of the antiferromagnetic modulation of the higher-energy scattering coincides with the maximum in the resistivity of ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$.

Oxidation of tryptophan and N-methylindole by N3.cntdot., Br2.-, and (SCN)2.- radicals in light- and heavy-water solutions: a pulse radiolysis study

Abstract: Reactions of N{sub 3}{sup {sm bullet}}, Br{sub 2}{sup {sm bullet}{minus}}, and (SCN){sub 2}{sup {sm bullet}{minus}} with tryptophan (TRPH) and N-methylindole (NMI) have been investigated in H{sub 2}O and D{sub 2}O solutions. The main transients produced were the TRP{sup {sm bullet}} radical and the TRPH{sup {sm bullet}+} radical cation from TRPH and the NMI{sup {sm bullet}+} radical cation from NMI. Their extinction coefficients ({epsilon}) as well as the rate constants for their formation and decay were determined in the pH range 3-10. The pK of the TRPH{sup {sm bullet}+} radical cation was confirmed to be 4.2 {plus minus} 0.1. These rate constants were within experimental error, independent of pH, and the differences between the rate constants in H{sub 2}O and D{sub 2}O were small. On the basis of the results, it was concluded that of the two possible reactions - electron or hydrogen atom transfer - the data are more consistent with electron transfer in all cases. However, secondary acid-base reactions have an important effect on the overall redox equilibria, especially with N{sub 3}{sup {sm bullet}}. Experimental data on these effects have also been obtained.

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: II. Tumor development and immune response

Abstract: This paper describes preliminary findings on the influence of 60-Hz (2-mT) magnetic fields on tumor promotion and co-promotion in the skins of mice. The effect of magnetic fields on natural killer (NK) cell activity in spleen and blood was also examined. Groups of 32 juvenile female mice were exposed to the magnetic field as described in part I. The dorsal skin of all animals was treated with a subthreshold dose of the carcinogen 7,12-dimethyl-benz(a)anthracene (DMBA). One week after the treatment, two groups were sham exposed (group A) or field exposed at 2 mT (group B) 6 h/day for 21 weeks, to test whether the field would act as a tumor promoter. No tumors developed in these two groups of mice. To test whether the magnetic field would modify tumor development by directly affecting tumor growth or by suppressing immune surveillance, two additional groups of mice were treated weekly with the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) and then either sham exposed (group C) or field exposed (group D). The time to appearance of tumors was shorter (but not statistically so) in the group exposed to magnetic fields and TPA. Some differences in NK cell activity and spleen size were observed between the sham- and field-exposed groups.

DNA lesions that signal the induction of radioresistance and DNA repair in yeast.

Abstract: DNA recombinational repair, and an increase in its capacity induced by DNA damage, is believed to be the major mechanism that confers resistance to killing by ionizing radiation in yeast. We have examined the nature of the DNA lesions generated by ionizing radiation that induce this mechanism, using two different end points: resistance to cell killing and ability of the error-free recombinational repair system to compete for other DNA lesions and thereby suppress chemical mutation. Under the various conditions examined in this study, the "maximum" inducible radiation resistance was increased approximately 1.5- to 3-fold and suppression of mutation about 10-fold. DNA lesions produced by low-LET γ rays at doses greater than about 20 Gy given in oxygen were shown to be more efficient, per unit dose, at inducing radioresistance to killing than were lesions produced by neutrons (high-LET radiation). This suggests that DNA single-strand breaks are more important lesions in the induction of radioresistance than ...

Groundwater 87Sr/86Sr values in the Eye-Dashwa Lakes pluton, Canada: Evidence for plagioclase-water reaction

Abstract: The Eye-Dashwa Lakes pluton is a Sr-rich felsic intrusive of the sanukitoid suite located in NW Ontario in the Superior Province of the Canadian Shield. The 87Sr/86Sr values of the CaNaCl-type saline, deeper groundwaters fall in the narrow range of 0.7057–0.7070, similar to the 87Sr/86Sr value in plagioclase but not to that in K-feldspar, biotite, or the whole rock. This suggests that the water isotopic composition is the result of plagioclase-water interaction. The fracture minerals gypsum and calcite have similar 87Sr/86Sr ratios, indicating that they have crystallized from waters similar to the present deep groundwaters. The shallow, CaHCO3-type, dilute groundwaters show a wider spread in the Sr ratio of 0.7065–0.7278, reflecting their derivation from minerals in the overlying soil. The results for the saline groundwaters are consistent with thermodynamic and kinetic models that demonstrate the relatively rapid dissolution of plagioclase with respect to other rock-forming silicates, and should have application to most brines located in silicate host rocks.

Color and Oxidative Stability of Nitrite-Free Cured Meat after Gamma Irradiation

Abstract: The effects of 5 and 10 kGy irradiation on the color and oxidative stability of meats treated with nitrite or a nitrite-free curing system were investigated. The nitrite-free curing system consisted of the preformed cooked cured-meat pigment, sodium ascorbate and sodium tripolyphosphate with or without sodium acid pyrophosphate. Irradiation had no detrimental effects on the color or flavor of either cured samples. Polyphosphates had a beneficial effect on oxidative stability but had a slight detrimental effect on color stability of irradiated samples.

A Four-Year Mobility Study of Selected Trace Elements and Heavy Metals

Abstract: The environmental impact of accidental or chronic releases of contaminants to the unsaturated soil zone, either through surface spills or contaminated groundwater, has become a major concern. This study was designed to provide data after 1 and 4 yr to compare the leaching and upward transport of Tc, I, Np, Cs, U, Th, Cr, and Mo in cores of the same soil in outdoor lysimeters. Over the 4-yr study a broad range of climatic, soil moisture, and temperature conditions existed, which were representative of long-term means as well as some extremes. The amount of rainfall leached through the cores decreased with time. The leaching resulted in a downward movement of fine soil particles, especially in the lower horizons. Element mobility in the leaching cores for the 4 yr decreased in the order I > Tc ≫ Cr > Np > Mo > Cs ≥ U > Th. This differs slightly from the ranking predicted after 1 yr. After 1 yr, Mo appeared to be more mobile than Cr or Np. After 4 yr, I, Tc, Np, and Cs migrated to the soil surface from depth. Analysis of the soil solid/liquid partition coefficient, Kd, values indicates the retention of these elements is primarily related to soil organic matter content and dependent on porewater concentration. The Kd model is appropriate for I in all soil horizons and for Tc, Np, and Cs in the Ae horizon of the groundwater cores. The Freundlich isotherm or a kinetic expression best describes the sorption for all other elements and horizons.

Magnetic structures and excitations of CsMnI3: A one-dimensional Heisenberg antiferromagnet with easy-axis anisotropy.

Abstract: We have performed measurements on the easy-axis Heisenberg antiferromagnet ${\mathrm{CsMnI}}_{3}$ to determine the magnetic structure and excitations. The structural anisotropy of the hexagonal perovskite structure (space group P${6}_{3}$/mmc) leads to quasi-one-dimensional behavior such that the ratio of the intra- to interchain exchange ${\mathit{J}}_{1}$/${\mathit{J}}_{2}$\ensuremath{\approxeq}200. Transitions to three-dimensional ordered magnetic states occur at ${\mathit{T}}_{\mathit{N}1}$=11.41\ifmmode\pm\else\textpm\fi{}0.01 K and ${\mathit{T}}_{\mathit{N}2}$=8.21\ifmmode\pm\else\textpm\fi{}0.02 K. Superconducting quantum interference device (SQUID) magnetometer measurements were performed with magnetic fields of up to 5.2 T applied parallel to the crystal c axis. No other phase transitions were detected at temperatures down to 1.8 K. Neutron-diffraction measurements confirmed that the low-temperature phase has antiferromagnetic order within the chains and a canted spin structure similar to that of the low-temperature phase of ${\mathrm{CsNiCl}}_{3}$ with a canting angle of 51\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}. In the higher-temperature phase the moments were found to lie almost parallel to the c axis, and to have a very small ferrimagnetic component along the c axis in each a-b plane. Inelastic neutron scattering was used to measure the dispersion of magnetic excitations in both ordered magnetic phases. The form of the dispersion was very similar in both cases and was adequately fitted to linear spin-wave theory, giving exchange parameters ${\mathit{J}}_{1}$=-0.198\ifmmode\pm\else\textpm\fi{}0.002 THz, ${\mathit{J}}_{2}$=-0.0010\ifmmode\pm\else\textpm\fi{}0.0001 THz, and single-ion anisotropy D=-0.000 50\ifmmode\pm\else\textpm\fi{}0.000 02 THz. There were, however, several deviations that suggest that linear spin-wave theory does not give a complete description of the magnetic excitations in this class of magnet.

Measured stopping powers of 12C and 14N ions in thin elemental foils

Abstract: Absolute stopping values for 12 C and 14 N ions in Be, C, Al, Si, Ni, Ag and Au were measured over the energy region 200 to 2000 keV. Surface barrier particle detectors were used to measure the energy loss of heavy ions passing through thin, self-supporting films. The effective charge ratio of heavy ions to hydrogen was examined and found to be a complex function of ion energy and the stopping medium.

Supported high silica zeolites

Abstract: A process for applying a highly crystalline form of silica, for example Silicalite, to a supporting structure, with a small thermal mass, such as a screen or a metal surface is described. A slurry of finely-divided high silica zeolite and a silicone or polytetrafluoroethylene binder is coated onto the support structure. Optionally, a noble catalyst is applied to the high silica zeolite either before or after the high silica zeolite is coated onto the support structure. The support structures coated according to this process can be used to catalyze flameless combustion of hydrogen or hydrocarbon gases with suitable amounts of air or oxygen if a noble metal catalyst is applied, or as an adsorbent structure in the absence of the metal catalyst.

Eddy current probe, incorporating multi-bracelets of different pancake coil diameters, for detecting internal defects in ferromagnetic tubes

Abstract: Eddy current probes for detecting internal defects in a ferromagnetic tube are disclosed. The probe uses a plurality of eddy current measuring means, each being operated at a different operating point on the impedance diagram. By operating more than one such eddy current measuring means simutaneously, noises by, for example, permeability variation of a ferromagnetic material and internal magnetic deposit, can be made less influential.

Steam Generator Deposits—A Detailed Analysis and Some Inferences

Abstract: Detailed examinations of deposits removed from the Crystal River Unit 3 steam generators have been carried out in an attempt to determine the mechanisms responsible for the adherence of such deposits to tubes and tube support plates. The information was intended to assist in finding ways to remove deposits and, possibly, to prevent their formation. These analyses have not led to a mechanism for deposit adhesion, but do offer insights into the deposition process. Deposition onto tube surfaces is a two-step process, and spalling occurs at the interface between the passive film and the corrosion product film. The deposition surface can strongly affect deposit adherence and removal. The deposits are quite porous and show evidence for wick boiling. Cu appears to concentrate in regions where wick boiling occurs and is preferentially dissolved by the EPRI—SGOG steam-generator cleaning solvent. Possibly an initial Cu-dissolution step provides a more porous network within the deposit for subsequent dissol...

Relationships between the partitioning of lead in sediments and its accumulation in the marine mussel, Mytilus edulis near a lead smelter

Abstract: Surficial sediment samples and specimens of the filter-feeding marine mussel, Mytilus edulis, were collected at thirteen sites near a Pb/Zn ore stockpile and smelting complex (Canada). Aside from measuring the total S content in the sediment samples, each of these samples were subjected to a sequential extraction procedure designed to determine the partitioning of Fe and Pb among various geochemical phases and compared with the Pb levels measured in the mussel tissues. About one third of the Fe and more than 90% of the Pb were extracted from the sediments. Sediments collected within Belledune Harbour generally yielded the greatest amount of Pb whereas the highest levels of S were measured in Dalhousie Harbour sediments. Regression analysis indicated that the total S content in the sediments played an important role in defining the Pb accumulation in the mussel tissues.

Radiation-curable prepreg composites

Abstract: Advanced composites, specifically carbon-fiber-reinforced epoxies, are used extensively for a variety of demanding structural applications, primarily because of their high strength-to-weight and stiffness-to-weight ratios, corrosion resistance, and damage tolerance characteristics. Electron beam (EB) treatment can be used to produce useful physical and/or chemical changes in plastics and composites by initiating various polymerization and crosslinking reactions. The advantages of using EB rather than thermal curing for carbon-fiber-reinforced epoxy composites include curing at ambient temperature, reduced curing times, and fewer volatiles. An EB-curable carbon fiber-acrylated epoxy composite is being developed for various applications. The tensile properties of the 14-ply EB-cured epoxy laminate were comparable with the properties of the thermally cured laminates used in the aircraft industry. Research is continuing to develop resin formulations and select coupling agents to improve the compression properties of EB-cured laminates.

Passive indirect shutdown cooling system for nuclear reactors

Abstract: A nuclear reactor system including a main heat transport path around which a first coolant is pumped by main circulation pumps to transport heat from the reactor core to a steam generator. A heat exchanger is located in the main heat transport path after the outlet from the steam generator, the secondary side of that heat exchanger being in a decay heat removal loop which contains a second liquid coolant. A vapor separator in the decay heat removal loop is connected to an outlet of the heat exchanger with the vapor separator's outlet being connected to an inlet of a further heat exchanger located in a large tank of water which forms a heat sink. The further heat exchanger's outlet is connected to the heat exchanger's inlet forming a closed loop. The further heat exchanger is located at a higher elevation than the heat exchanger whereby a natural convection flow can occur in the decay heat removal path. Normally, the vapor/liquid interface in the loop is at a higher elevation than the heat sink which prevents any significant natural convection flow until boiling of the second liquid coolant occurs in the heat exchanger. If the steam generator is lost as a heat sink, this causes boiling of the secondary coolant to occur in the heat exchanger starting a natural convection flow in the decay heat removal loop. This natural convection starts automatically without the need of any valves being opened or any other type of intervention.

Hyperthermic inactivation, recovery and induced thermotolerance of human natural killer cell lytic function

Abstract: Natural killer (NK) cytotoxic activity in human peripheral blood mononuclear cells or preparations of large granular lymphocytes was assayed after a hyperthermia treatment Human NK cells are very sensitive to hyperthermic inactivation; 30 min at 42°C reduced NK lytic activity toward K562 target cells by 40–50%, 43 °C by 85–90% and 44°C or 45°C by 100%, but similar treatment of the target cells did not alter their sensitivity to lysis However, holding at 37°C allowed heated NK cells to recover their lytic activity The extent of the recovery was inversely correlated with the temperature as well as the recovery time Heated and subsequently recovered NK cells were more thermotolerant to loss of lytic function by a further hyperthermia exposure About 065°C increase in temperature was required for a 50% loss of lysis ability in the NK cells made thermotolerant by a previous hyperthermic exposure at 43°C for 30 min The Vmax for NK lytic activity of cells heated at 42°C for 30 min was reduced by 70% compar

Continuous containment monitoring with containment pressure fluctuation

Abstract: A method of determining the rate of air leakage from or into a containment is described. The internal pressure of the containment is maintained at a positive or negative value with reference to ambient air pressure and the air mass changes are continuously monitored through measurements of pressure, temperature, relative humidity and free volume. Air leakage is measured through a mass balance equation and a regression analysis carried out to yield constants which characterize the various features of the leakage flow and give an indication of the accuracy of the results.

The Effect of Weathering on Charcoal Filter Performance: I. The Adsorption and Desorption Behavior of Contaminants

Abstract: This paper reports on triethylenediamine (TEDA) impregnated charcoals, used in nuclear reactors to safeguard against the release of airborne radioiodine, which show high efficiency under various reactor operation and accident conditions when the are new. However, during normal operation, charcoal filters are continuously degraded (or weathered) due to the adsorption of moisture and other air contaminants. The effect of weathering on the efficiency of charcoal for removing radioiodine is of great interest. The results of a study on the adsorption behavior of various contaminants NO{sub 2}, SO{sub 2} 2-butanone (methyl-ethyl ketone (MEK)) and NH{sub 3} on TEDA charcoal are presented. This study is an attempt to characterize and quantify the weathering process of TEDA charcoal by these contaminants. The adsorption and desorption of characteristics of these contaminants range from completely irreversible (NO{sub 2}) to completely reversible (NH{sub 3}). The effect of absorbed water (or humidity) on absorption is different for each contaminant. Absorbed water increases the absorption rate and capacity of TEDA charcoal for NO{sub 2}. However, it appears that SO{sub 2} is absorbed as H{sub 2}SO{sub 4} on the wet charcoal. Absorbed water slightly reduces the adsorption capacity of the charcoal for MEK, but does not affect the absorptionmore » of NH{sub 3}.« less