Showing papers by "Atomic Energy of Canada Limited published in 1997"

Direct measurement of the Si–O bond length and orientational disorder in the high-temperature phase of cristobalite

Abstract: Neutron total scattering measurements from powdered samples of cristobalite have been used to determine the local structure in both the tetragonal and cubic phases. The results for the cubic phase show directly that the Si–O bonds are tilted at an angle of around 17° to the unit cell [111] direction. It is striking that the structure of β-cristobalite over the range 5–10 A is closer to that of silica glass than α-cristobalite, which suggests that the local structure of β-cristobalite is not likely to consist of domains with the structure of α-cristobalite. The measurements show a small thermal expansion of the Si–O bonds over the temperature range 570–950 K.

Microbial studies in the Canadian nuclear fuel waste management program

Abstract: Atomic Energy of Canada Limited (AECL) has developed a concept for permanent geological disposal of used nuclear fuel in Canada. This concept, based on a multibarrier system, would involve disposal of nuclear fuel waste in titanium or copper containers, surrounded by compacted clay-based buffer and backfill materials, in a vault 500–1000 m deep in granitic rock of the Canadian Shield. Subsurface environments will not be sterile and an experimental program was initiated in 1991 by AECL to address and quantify the potential effects of microbial action on the integrity of the disposal vault. This microbial program focuses on answering specific questions in areas such as the survival of bacteria in compacted clay-based buffer materials under relevant radiation, temperature and desiccation conditions; mobility of microbes in compacted buffer materials; the potential for microbially influenced corrosion of containers; microbial gas production in backfill material; introduction of nutrients as a result of vault excavation and operation; the presence and activity of microbes in deep granitic groundwaters; and the effects of biofilms on radionuclide migration in the geosphere. This paper summarizes the results to date from the research activities at AECL.

Occurrence and identification of microorganisms in compacted clay-based buffer material designed for use in a nuclear fuel waste disposal vault

Abstract: A full-scale nuclear fuel waste disposal container experiment was carried out 240 m below ground in an underground granitic rock research laboratory in Canada. An electric heater was surrounded by ...

Depuration and uptake kinetics of I, Cs, Mn, Zn and Cd by the earthworm (Lumbricus terrestris) in radiotracer‐spiked litter

Abstract: The relative depuration and uptake kinetics of contaminants should be known to interpret appropriately the use of organisms such as earthworms in environmental bioassays and monitoring. For example, 14-d earthworm bioassays should be interpreted with the knowledge that some contaminants will continue to accumulate in tissues for months. The radiotracers {sup 125}I, {sup 134}Cs, {sup 54}Mn, {sup 65}Zn, and {sup 109}Cd were applied to deciduous litter and specimens of Lumbricus terrestris were exposed, either to litter alone or to litter on the top of soil columns. Depuration was monitored for 120 d and uptake, in a separate experiment, for 20 d. Both depuration and uptake were described using two-phase, first-order statistical models. Cut clearance had a mean half-time of 1.4 d. The mean half-time for physiological depuration decreased from I (210 d) > Cd (150 d) > Zn (69 d) > Mn (40 d) > Cs (24 d). Both the depuration and the uptake experiments were necessary to resolve even partially the multiphase processes. Earthworm/soil dry weight concentration ratios decreased from Cd > Zn > I {ge} Cs {ge} Mn. The very slow kinetics indicate that tissue concentrations will increase continuously for a long time, with important implications for subsequentmore » food-chain transfers.« less

Evolution of Redox Conditions and Groundwater Composition in Recharge-Discharge Environments on the Canadian Shield

Abstract: The composition of groundwater in fractured crystalline rocks of the Canadian Shield evolves along flow paths from recharge to discharge in response to interactions with bedrock and fracture-filling minerals, and by dissolution of soluble (Cl-rich) salts in the rock matrix. The groundwater redox potential changes from oxidizing to reducing conditions due, initially, to rapid consumption of dissolved oxygen by organics in the upper ∼100 m of bedrock and, subsequently, to interaction with rock minerals. Measured Eh values of deep groundwater in the granitic Lac du Bonnet batholith in southeastern Manitoba indicate that the Fe(II)/Fe(III) couple controls groundwater redox potential. This conclusion is supported by other geochemical characteristics, such as absence of CH4, H2S, H2, NO3 and abundance of SO4. These observations are important in selecting a suitable site for disposal of nuclear fuel wastes.

Simultaneous double sided irradiation

Abstract: An apparatus for delivering a beam of charged particles along two separate beam paths comprising a pulsed charged particle beam source for producing a series of beam pulses along a first beam path, a switching magnet for developing a magnetic field, and a power supply means for selectively applying current pulses to said switching magnet in timed relation to each of a plurality of predetermined beam pulses effective to develop a constant magnetic field throughout the period of each predetermined beam pulse and deflect each entire predetermined beam pulse from said first beam to a second beam path.

Heat-induced thermal tolerance and radiation resistance to apoptosis in human lymphocytes

Abstract: We have investigated heat- and radiation-induced apoptosis in human lymphocytes in vitro. We have previously shown that apoptosis was induced by radiation at doses as low as 0.05 Gy. Here we report that heat induced apoptosis in human lymphocytes in a temperature- and time-dependent manner. Temperatures at or below 42 degrees C, for up to 90 min, did not cause lymphocytes to undergo apoptosis, whereas temperatures at or above 43 degrees C, for 30 min and longer, did induce apoptosis. Lymphocytes were protected against apoptosis induced by 44 degrees C heat by a prior heat shock of 42 degrees C for 30 min. Heat-induced thermal tolerance developed immediately following the inducing heat shock, was greater after 4 h, and persisted for at least 24 h. While heat also induced radiation resistance, this change was minor and not apparent until about 24 h after the heat shock. Prior to the development of radiation resistance, heat shock sensitized lymphocytes to radiation-induced apoptosis. We have previously shown that radiation-induced apoptosis in lymphocytes varies between donors and therefore may be useful in assessing individual radiosensitivity. We report here that heat also induced variable levels of apoptosis in lymphocytes from different donors, although the range of responses was not as large as those observed with radiation-induced apoptosis. In summary, heat shock induces tolerance to heat-induced apoptosis and results first in sensitization and then protection of lymphocytes against radiation-induced apoptosis.

Absence of chiral domains in mixtures of dipalmitoylphosphatidylcholine molecules of opposite chirality

Abstract: We present calorimetric data for liposome mixtures of l-dipalmitoylphosphatidylcholine with perdeuterated hydrocarbon chains (l-DPPC-${\mathrm{d}}_{62}$) and d-DPPC at varying molar concentrations and excess water (\ensuremath{\geqslant}30% by wt.) conditions. The data are consistent with a binary system whose components exhibit complete mutual solid solubility. Therefore, the asymmetric ripples observed in racemic dimyristoylphosphatidylcholine bilayers in the ${\mathrm{P}}_{{\mathrm{\ensuremath{\mathrm{B}}}}^{|\mathrm{I}\mathrm{H}}}$ phase [Katsaras and Raghunathan, Phys. Rev. Lett. 74, 2022 (1995)] are not the result of pure enantiomer domains.

Modeling the Failure of Nuclear Waste Containers

Abstract: A model was developed to predict the failure of Grade-2 titanium (Ti-2) nuclear waste containers. Two major corrosion modes were included: failure by crevice corrosion (CC) and failure by hydrogen-induced cracking (HIC). A small number of containers were assumed to be defective and to fail within 50 years of emplacement. The model is probabilistic in nature, and each modeling parameter was assigned a range of values, resulting in a distribution of corrosion rates and failure times. The crevice corrosion rate (RCC) was assumed to be dependent only upon properties of the material used and the temperature of the vault. CC was assumed to initiate rapidly on all containers and to propagate indefinitely without repassivation. Failure by HIC was assumed to be inevitable once container temperature (T) fell to ≤ 30°C. Depending upon the rate at which they were expected to cool, temperature-time profiles for individual containers were approximated by two-step or single-step temperature-time functions. Thes...

Adaptation to Ionizing Radiation in Mammalian Cells

Abstract: The concept of an ionizing radiation-induced increase in resistance against the effects of a subsequent exposure is an accepted and reasonably well-understood process in both prokaryotes (Walker, 1984, 1985) and nonmammalian eukaryotes (Calkins, 1967; Boreham et al., 1990, 1991; Boreham and Mitchel, 1991, 1993, 1994; Mitchel and Morrison, 1982, 1984, 1987; Koval, 1986, 1988). Such processes however, have been more difficult to demonstrate in mammalian cells, where their existence and/or significance have been very controversial (Olivieri et al., 1984; Olivieri and Bosi, 1990; Wiencke et al., 1987, Wilson, 1989; Wojcik et al., 1992a,b; Wolff, 1992 a,b). This lack of general acceptance, while reflecting the lack of volume of the data as well as the variability noted above and a lack of direct evidence of an influence on whole animal risk, also reflects the fact that the concept challenges long entrenched and widely held beliefs and practices, both scientific and public, on which all radiation protection programs and cancer risk estimates are based. Consequently, demonstration of the existence and quantification of the significance of adaptation to radiation in mammalian cells has, potentially, large social and economic implications.

Evidence of the hydration force in gel phase lipid multibilayers

Abstract: We present applied osmotic pressure vs distance curves constructed from high-resolution neutron and X-ray diffraction scattering density profiles of dipalmitoyl phosphatidylcholine (DPPC) multibilayers in the gel Lβ' phase The data, obtained from three distinct regions of the lipid bilayer, show that at small interbilayer distances the repulsion observed in these bilayers can be accounted for by the so-called hydration force Also, a previous observation of a sharp upward break in the pressure-distance curve of highly ordered Lc' DPPC bilayers is not the result of steric repulsion but rather of a 2D → 3D phase transition The data are consistent with a recent theoretical model which predicts that the repulsive forces in ordered phases (eg, Lβ' and Lc' phases) are dominated by the hydration force

High-Efficiency Structured-Packing Catalysts with Activated Carbon for SO2 Oxidation from Flue Gas

Abstract: Coiled screen or Sulzer CY gauze structured packings, loaded with an activated carbon treated to make it partially hydrophobic, were employed to investigate the removal of SO2 from a stack gas. Measurements were made of SO2 removal and its conversion to H2SO4 and gas-side pressure drop as a function of gas and liquid loadings, temperature, and method of operation (continuous liquid flow vs intermittent liquid flow). Both packings exhibited remarkably high catalytic efficiency in terms of SO2 converted per gram of carbon, as well as high conversions of SO2 to H2SO4 and low-pressure drops. Although the Sulzer CY packing displayed the lowest pressure drop per meter of packing depth, the coiled screen provided higher removal of SO2 per volume of packing. The performances of each were close enough that a choice between them would require an economic analysis.

Advanced candu systems for plutonium destruction

Abstract: High neutron economy, on-line refuelling, and a simple fuel-bundle design result in a high degree of versatility in the use of the CANDU® reactor for the disposition of weapons-derived plutonium. CANDU mixed-oxide (MOX) fuel is a near-term, technically achievable, economic option. Studies led by AECL show that four Bruce A reactors could consume 50 te of plutonium in less than 12.5 years. The symmetry in the simultaneous drawdown of excess weapons-derived plutonium from both the United States and Russia in Canada was an important consideration in the recent US Record of Decision, which includes the CANDU MOX option for further evaluation.

Passive emergency hydrogen mitigation system for water-cooled nuclear reactors

Abstract: An improved system for passively removing hydrogen inside containment in a nuclear reactor in the event of a loss of coolant accident by means of catalytic hydrogen recombination. A baffle wall located inside containment is used to establish within containment an air upflow path and an air downflow path in convective exchange. The air upflow is past the area of the coolant lines and is effective to entrain hydrogen from the break and the air downflow path is in the area adjacent the containment wall. The air upflow path downstream of the coolant lines is ducted so as to confine the entrained hydrogen to the ducted air upflow path and catalytic hydrogen recombiners are located in the ducted air upflow path for recombining said entrained hydrogen with oxygen in said ducted air upflow.

The determination of orientation-dependent stored energy using neutron diffraction

Abstract: A knowledge of the orientation-dependent stored energy is extremely important in understanding the evolution of texture during the recrystallization of materials. Neutron diffraction line-broadening measurements were made on a composite spherical specimen made from the sheets of 80% cold-rolled, interstitial-free (IF) steel. Using the stored energy calculated from the line-broadening measurements, the stored energy distribution function (SEDF) was derived and represented in the Euler angle space. However, the error in the computed energies for a particular orientation, is strongly dependent on how well that orientation is represented in the orientation distribution function (ODF). The Taylor factor, which is often considered as a measure of slip system activity during the deformation, is compared with the measured stored energy.

Protective coating to reduce stress corrosion cracking in zirconium alloy sheathing

Abstract: A protective coating to reduce stress corrosion cracking of zirconium alloy sheathing in nuclear reactors. The coating is graphite based and includes ZrO 2 and ethyl cellulose and acts as a sacrificial layer to immobilize fission products by formation of Zr x I y C type compounds which do not cause stress corrosion cracking.

36Cl and 129I in teeth and bones

Abstract: Measurements of 36 Cl and 129 I in inert biological matrices such as teeth and bone have the potential to provide a record of exposure to high neutron fluxes and direct exposure to these isotopes in air and food. In fossil specimens, measurements of 36 Cl and 129 I atom ratios in bone and teeth may provide a methodology to determine the age of samples that cover a range of ages from ∼ 100 k to ∼ 200 k and 15 M to 75 M years before present, respectively. To demonstrate the feasibility of exploiting these potential records, we have developed and tested a pyrolytic technique to extract the isotopes and stable analogs without introducing contamination. Concentrations of stable chlorine and iodine measured in teeth and bone range from 100 to 3000 ppm and 20 to 700 ppb, respectively and provided sufficient sample for measurement by AMS. 36 Cl and 129 I concentrations measured in samples of teeth and bone that were exposed to elevated concentrations of these two isotopes during weapons testing have elevated concentrations of the isotopes within the expected range of concentrations.

Electrochemical modifications on the surface of a ti film

Abstract: An in situ neutron reflectometry experiment on a Ti film, sputter-deposited on a Si substrate, shows in detail how the native oxide thickens when the film is anodized. The resultant oxide has two distinct regions: the inner region with the same scattering length density (SLD) as the original oxide and the outer region of lower SLD. The latter must therefore be highly porous or contain significant amount of hydrogen, most probably in the form of (OH)− ions. Subsequent measurements under cathodic potential show that the low-SLD region becomes thicker at the expense of high-SLD region but the overall thickness of the oxide layer remains constant. Rapid increase in electrode current begins to occur when the remaining high-SLD region becomes very thin. We conclude that the low-SLD oxide is relatively more conductive and does not provide as much protection against hydrogen ingress as the native oxide.