Showing papers by "Atomic Energy of Canada Limited published in 2004"

Nanometer scale chemomechanical characterization of antiwear films

Abstract: We report the first nanometer scale chemical and mechanical (chemomechanical) characterization of selected features of a tribologically derived zinc dialkyl-dithiophosphate (ZDDP) antiwear film. AFM permits identification of the features responsible for preventing wear. These features are identified by nearby microscale fiducial marks, and their mechanical properties are determined by imaging nanoindentation. The same features are then studied by X-ray photoelectron emission microscopy (X-PEEM), which provides both elemental and chemical information at ∼200 nm spatial resolution. The mechanical properties are then determined for the same features, which are formed of a polyphosphate glass. This information provides new insights into the mechanisms by which ZDDP antiwears films are effective at inhibiting asperity contact between two metal surfaces

Upper dose thresholds for radiation-induced adaptive response against cancer in high-dose-exposed, cancer-prone, radiation-sensitive Trp53 heterozygous mice.

Abstract: Mitchel, R. E. J., Jackson, J. S. and Carlisle, S. M. Upper Dose Thresholds for Radiation-Induced Adaptive Response against Cancer in High-Dose-Exposed, Cancer-Prone, Radiation-Sensitive Trp53 Heterozygous Mice. Radiat. Res. 162, 20–30 (2004). Trp53 heterozygous mice are radiation-sensitive and cancer-prone. Groups of 7–8-week-old female Trp53 heterozygous mice were exposed to 4 Gy of 60Co γ radiation at high (0.5 Gy/min) or low (0.5 mGy/min) dose rate. Other groups received 10 or 100 mGy at low dose rate 24 h prior to the 4-Gy dose. Tumor frequency and latency were measured over the animals' life span. Exposure to 10 mGy prior to 4 Gy resulted in a small (∼5%) but significant life-span regain and increased latency (∼9%) for all malignant tumors taken together, but 100 mGy further reduced life span slightly (∼7%). Latency responses were tumor type-specific. The prior 10-mGy exposure resulted in a small (∼7%) regain in latency for lymphomas but no change in latency for spinal osteosarcomas. Increa...

Damping of Heat Exchanger Tubes in Two-Phase Flow: Review and Design Guidelines

Abstract: Two-phase flow exists in many shell-and-tube heat exchangers such as condensers, evaporators, and nuclear steam generators. Some knowledge on tube damping mechanisms is required to avoid flow-induced vibration problems. This paper outlines the development of a semi-empirical model to formulate damping of heat exchanger tube bundles in two-phase cross flow. The formulation is based on information available in the literature and on the results of recently completed experiments. The compilation of a database and the formulation of a design guideline are outlined in this paper. The effects of several parameters such as flow velocity, void fraction, confinement, flow regime and fluid properties are discussed. These parameters are taken into consideration in the formulation of a practical design guideline.

Measurement of 222 Rn dissolved in water at the Sudbury Neutrino Observatory

Abstract: The technique used at the Sudbury Neutrino Observatory (SNO) to measure the concentration of 222 Rn in water is described. Water from the SNO detector is passed through a vacuum degasser (in the light water system) or a membrane contact degasser (in the heavy water system) where dissolved gases, including radon, are liberated. The degasser is connected to a vacuum system which collects the radon on a cold trap and removes most other gases, such as water vapor and N2. After roughly 0.5 tonnes of H2O or 6 tonnes of D2O have been sampled, the accumulated radon is transferred to a Lucas cell. The cell is mounted on a photomultiplier tube which detects the α-particles from the decay of 222 Rn and its progeny. The overall degassing and concentration efficiency is about 38% and the single-α counting efficiency is approximately 75%. The sensitivity of the radon assay system for D2O is equivalent to ∼3×10 −15 g U/g water. The radon concentration in both the H2O and D2O is sufficiently low that the rate of background events from U-chain elements is a small fraction of the interaction rate of solar neutrinos by the neutral current reaction.

An examination of radiation hormesis mechanisms using a multistage carcinogenesis model.

Abstract: A multistage cancer model that describes the putative rate-limiting steps in carcinogenesis is developed and used to investigate the potential impact on cumulative lung cancer incidence of the hormesis mechanisms suggested by Feinendegen and Pollycove. In the model, radiation and endogenous processes damage the DNA of target cells in the lung. Some fraction of the misrepaired or unrepaired DNA damage induces genomic instability and, ultimately, leads to the accumulation of malignant cells. The model explicitly accounts for cell birth and death processes, the clonal expansion of initiated cells, malignant conversion, and a lag period for tumor formation. Radioprotective mechanisms are incorporated into the model by postulating dose and dose-rate-dependent radical scavenging. The accuracy of DNA damage repair also depends on dose and dose rate. As currently formulated, the model is most applicable to low-linear-energy-transfer (LET) radiation delivered at low dose rates. Sensitivity studies are conducted to identify critical model inputs and to help define the shapes of the cumulative lung cancer incidence curves that may arise when dose and doserate-dependent cellular defense mechanisms are incorporated into a multistage cancer model. For lung cancer, both linear no-threshold (LNT-), and non-LNT-shaped responses can be obtained. If experiments demonstrate that the effects of DNA damage repair and radical scavenging are enhanced at least three-fold under low-dose conditions, our studies would support the existence of U-shaped responses. The overall fidelity of the DNA damage repair process may have a large impact on the cumulative incidence of lung cancer. The reported studies also highlight the need to know whether or not (or to what extent) multiply damaged DNA sites are formed by endogenous processes. Model inputs that give rise to U-shaped responses are consistent with an effective cumulative lung cancer incidence threshold that may be as high as 300 mGy (4 mGy per year for 75 years) for low-LET radiation.

Depleted uranium dust from fired munitions: physical, chemical and biological properties.

Abstract: This paper reports physical, chemical and biological analyses of samples of dust resulting from munitions containing depleted uranium (DU) that had been live-fired and had impacted an armored target. Mass spectroscopic analysis indicated that the average atom% of U was 0.198 +/- 0.10, consistent with depleted uranium. Other major elements present were iron, aluminum, and silicon. About 47% of the total mass was particles with diameters 35 microg DU dust/injection site about 2% appeared in urine within 7 d. Natural uranium (NU) ore dust was instilled into rat lungs for comparison. The fraction dissolving in lung showed a pattern of exponential decline with increasing initial burden similar to DU. However, the decline was less steep, with about 14% appearing in urine for lung burdens up to about 200 microg NU dust/lung and 5% at lung burdens >1,100 microg NU dust/lung. NU also showed both a fast and a more slowly dissolving component. At the higher lung burdens of both DU and NU that showed lowered urine excretion rates, histological evidence of kidney damage was seen. Kidney damage was not seen with the muscle burdens tested. DU dust produced kidney damage at lower lung burdens and lower urine uranium levels than NU dust, suggesting that other toxic metals in DU dust may contribute to the damage.

Thermo-Hydro-Mechanical (T-H-M) Impacts of Glaciation and Implications for Deep Geologic Disposal of Nuclear Waste

Abstract: The thermo-hydro-mechanical impacts of extreme climate change on the lithosphere down to depths at which deep repositories might be sited have been simulated. The effects of glaciation, including ice sheet and permafrost development, have been studied using site-specific data by combining four models. A climate model provides the forcing function, and ice sheet, permafrost, and coupled hydromechanical models are used to assess impacts. It is concluded that glaciation occurs on a timescale and has impacts on a depth scale that require it to be analysed in a safety analysis for deep lithosphere disposal of long-lived radionuclides in areas that have been prone to glaciation in the past. The simulations have provided valuable insight about processes and mechanisms likely to influence the long-term performance of a repository, the geosphere, or both. The key impacts are discussed, and appropriate methods identified.

The Bystander Effect: Recent Developments and Implications for Understanding the Dose Response

Abstract: The bystander effect refers to the biological response of a cell resulting from an event in an adjacent or nearby cell. Such effects depend on intercellular communication and amplify the consequences of the original event. These responses are of particular interest in the assessment of ionizing radiation risk because at public or occupational exposure levels not every cell receives a radiation track. Current radiation protection regulations and practices are based on the assumption of a linear increase in risk with dose, including low doses where not all cells are hit. Mechanisms that amplify biological effects are inconsistent with these assumptions. Evidence suggests that there are two different bystander effects in mammalian cells. In one type, a radiation track in one cell leads to damaging, mutagenic, and sometimes lethal events in adjacent, unhit cells. In the other type, a radiation track in one cell leads to an adaptive response in bystander cells, increasing resistance to spontaneous or radiation-induced events. This paper describes some of the data for radiation-induced bystander effects in vitro and correlates that data with in vitro and in vivo observations of risk at low doses. The data suggest that protective effects, including beneficial bystander effects, outweigh detrimental effects at doses below about 100 mGy, but that the reverse is true above this threshold.

Wetproofed catalysts for hydrogen isotope exchange

Abstract: A method to catalyze hydrogen isotope exchange between water and hydrogen in which the water and hydrogen are contacted with a wetproofed catalyst, the catalyst having a hydrophobic porous matrix which has dispersed therein catalytically active platinum and at least one other metal which is chromium or titanium.

The Thermal Hydraulics of Tube Support Fouling in Nuclear Steam Generators

Abstract: It is hypothesized that the thermal-hydraulic environment plays a role in the fouling of tube supports in nuclear steam generators. Experiments were performed to simulate the thermal-hydraulic environment near various designs of supports. Pressure loss, local velocity, turbulence intensity, and local void fraction were measured to characterize the effect of the support. Fouling mechanisms specific to supports were inferred from these experimental data and from actual steam generator inspection results. An analytical model was developed to predict the rate of particulate deposition on the supports, to better understand the complex processes involved.This paper presents the following set of tools for assessing the fouling propensity of a given support design: (1) proposed fouling mechanisms, (2) criteria for support fouling propensity, (3) correlation of fouling with parameters such as mass flux and quality, (4) descriptions of experimental tools such as flow visualization and measurement of pressure-loss profiles, and (5) analytical tools.An important conclusion from this and our previous work is that the fouling propensity is greater with broached support plates, both trefoil and quatrefoil, than with lattice bar supports and formed bar supports, in which significant cross flows occur.

Radiation and cardiovascular diseases.

Abstract: Both epidemiological and experimental evidence emphasize the connection between radiation exposure and cancer. Little effort has been directed toward finding an association between radiation and cardiovascular diseases. Lately, studies on the A-bomb survivors and Chernobyl accident victims have indicated that radiation doses as low as 0.05-1.0 Gy could be responsible for an increase in the incidence of cardiovascular diseases. Exposures to high doses of radiation (approximately 10-40 Gy) have also been reported to induce atherosclerotic lesions in cancer patients undergoing radiotherapy. Earlier studies in experimental animals have shown that radiation, mostly at high doses (>5 Gy), could accelerate the formation of atherosclerotic lesions. This article provides an up-to-date review of the literature connecting cardiovascular diseases to radiation exposures, particularly at low doses, and the potential implications of this connection in radiation risk assessment.

The Effect of Alpha-Radiolysis on UO2 Dissolution Determined from Batch Experiments with 238Pu-Doped UO2

Abstract: Dissolution experiments were performed with 238Pu-doped UO2 containing doping levels of ~1–100 Ci/kg UO2, in 0.1M Perchlorate with or without 0.1M carbonate, at pH 9.5, to investigate the effects of α dose-rate on UO2 dissolution rates and on Geff, the fraction of radiolytically produced H2O2 supporting dissolution. Dissolution rates increased by only about 10 times for a ~100-fold increase in doping level in both solutions, but increased between 10 and 100 times in 0.1M carbonate, compared to dissolution rates in 0.1M perchlorate. Geff values increased with decreasing doping level, suggesting that the dissolution reaction is limited by the reaction rate between UO2 and H2O2 at high dose rates, but becomes limited by the rate of production of H2O2at lower dose rates.

The Probability and Management of Human Error

Abstract: Embedded within modern technological systems, human error is the largest, and indeed dominant contributor to accident cause. The consequences dominate the risk profiles for nuclear power and for many other technologies. We need to quantify the probability of human error for the system as an integral contribution within the overall system failure, as it is generally not separable or predictable for actual events. We also need to provide a means to manage and effectively reduce the failure (error) rate. The fact that humans learn from their mistakes allows a new determination of the dynamic probability and human failure (error) rate in technological systems. The result is consistent with and derived from the available world data for modern technological systems. Comparisons are made to actual data from large technological systems and recent catastrophes. Best estimate values and relationships can be derived for both the human error rate, and for the probability. We describe the potential for new approaches to the management of human error and safety indicators, based on the principles of error state exclusion and of the systematic effect of learning.Copyright © 2004 by ASME

Upscaling the Thermohydromechanical Properties of a Fractured Rock Mass Using a Modified Crack Tensor Theory

Abstract: A methodology based on the modified crack tensor theory of Oda (1986) was developed for the purposes of upscaling the thermohydromechanical properties for fractured rock mass. The methodology was applied to the Bench Mark Test Case 2 (BMT2) of DECOVALEX III, which is based on a hypothetical site with three fractured rock formations. It was found that the upscaled equivalent rock mass permeability, hydraulic and mechanical porosities, and Young's modulus approached respective asymptotic values at an upscale length of less than 10 meters. The equivalent properties are highly dependent on effective stress. The theoretically determined values of Young's modulus and permeability are generally very similar to, or on the same order of magnitude as, those obtained by Nirex and cited in the BMT2 Problem Definition and Description document.