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No description available.

Nuclear reactor safeguards are how the peaceful use of nuclear material is ensured. Safeguards consist of a broad array of techniques, such as video surveillance and tamperproof seals, to ensure that nuclear material is not diverted from declared activities. Safeguards research is conducted to ensure that safeguards techniques are capable of meeting the challenges posed by future reactor designs and operating conditions. One such technique that has broad applicability to novel reactor designs, including small modular reactors, is the method of standoff neutron detection using large area neutron detectors. In this method, neutron detectors are employed to detect neutrons which have escaped from the core, which are representative of the flux inside the core. Because the flux required to achieve a given power is dependent on the isotopes being fissioned, due to their different cross sections and fission energies, the state of the core can be assessed using the neutron detectors. Prior research has demonstrated that it is possible to correlate kilogram changes in fissile inventory using neutron detectors by employing the standoff neutron detector method. This work at the McMaster nuclear reactor details additional experiments to support prior work. First, the apparatus and procedure to collect neutron detector data are detailed, along with persistent challenges to the collection. Next, simulations using the OSCAR-5 code to determine the fissile inventory are described. These two sections are subsequently combined, to compare changes in detector signal to the simulated core inventory. It was found that the uncertainty was too large to correlate changes in detector signal with changes in core inventory. Lastly, a method of detecting malicious interference is derived and tested. / Thesis / Master of Applied Science (MASc)

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<p>The refilling and rewetting of a directly heated horizontal channel has been studied experimentally. The quenching characteristics and rewetting rates were obtained under different well-defined initial and boundary conditions. The parametric effects of initial wall temperature, inlet water flow rate and inlet water subcooling as well as effects of residual power input, tube insulation and dissolved air and ions in the inlet water were investigated.</p> <p>The results show that, in horizontal channels, the transverse gravity forces significantly affect the hydraulic, and consequently thermal, behaviour of the system. The heat transfer mechanisms were found to vary during transients, both axially and circumferentially. This results in large differences in the pre-quench characteristics with regard to the bottom and top of the channel.</p> <p>A simple physical model which can account for most of the observed characterisitcs was developed. The model consists of an inclined rewetting front and an entrained "liquid tongue" extending downstream from the rewetting front. The model was also supported by photographic studies.</p> <p>The propagation of the rewetting front appears to be largely controlled by hydrodynamic mechanisms. It was found that surface quenching can occur at very different wall temperatures along the tube. Thus, there is no well defined rewetting temperature. This is contrary to the predictions of the generally accepted conduction controlled rewetting model.</p> <p>Since hydrodynamic mechanisms dominate the thermohydraulic processes in the present flow situation, a simplified two-fluid model was used to analyze the processes. The wall temperature was obtained after the hydraulic equations of the liquid phase were solved. The wall was assumed to be in stable film boiling before quenching.</p> <p>A quench model based on a critical water level, (hʟ)crit' determined the transition of heat transfer mode from film boiling to transition boiling and subsequent quenching of the surface. (hʟ)crit was obtained using a model based on initiation of a Kelvin-Helmholtz type instability at the vapor film-liquid interface.</p> <p>The initiation of the interfacial instability is believed to be the governing mechanism that leads to surface rewetting. When the instability starts, local regions of enhanced heat transfer are assumed to form on the heater surface. This provides the necessary conditions for surface rewet. However, if the surface is highly conductive and has a high thermal capacity, these rewet spots may not grow and surface rewet may not occur. Therefore, the sufficient conditions would require that the rewet spots could grow or spread on the heating surface. For thin-walled tubes, because of low thermal capacity, rewet spots may spread once they are formed. Hence, interfacial instability is postulated to be both necessary and sufficient to quench thin-walled systems of the type studied.</p> <p>The simplified two-fluid model, together with the quench model were found to be quite successful in predicting the rewetting rates and details of the quenching characteristics for the refilling and rewetting experiments.</p> / Doctor of Philosophy (PhD)

<p>The role of nuclear energy systems while produce fissile materials has become an important and essential part of scenarios for the future development of nuclear energy. Definitive analyses and nucleonic comparisons of these breeder systems have been impeded by the lack of a rigorous and consistent methodology for describing fissile fuel production and fertile fuel utilization. The research reported here therefore addresses itself to this problem and is based on a synthesis of three concepts: (1) the lumped parameter formulation of reactor physics, (2) the establishment of symbiotic relationships between breeders and converters and (3) the use of material stockpile inventories.</p> <p>In the lumped parameter synergetic analysis the temporal variation of stockpile inventories and net electrical output of a selected system are established. The system is taken to consist of a general breeder reactor coupled to a fission converter reactor and fuel reprocessing-fabrication plant. By including the converter and processing plant and examining the temporal and nuclear behaviour of the entire system, various types of nuclear breeders can be analysed and subjected to a comparative analysis in a consistent manner. The use of lumped parameters, based on the integration of detailed space and energy dependent effects into single-valued parameters, has facilitated survey calculations and analysis of the conceptual systems defined herein. The temporal variation of the stockpile inventory is used to describe fissile fuel production and fertile fuel utilization since information, such as minimum inventory requirements and material replacement times, is provided. This approach eliminates the ambiguities involved in a single figure of merit description, such as, for example, the doubling time, and includes pre-steady-state effects and reprocessing lags and losses. To assess the net electrical production of the system, the consumption of electricity by the reactors and the processing plant is explicitly included.</p> <p>The synergetics of fast-fission, symbiotic fusion, hybrid fusion and spallation breeders are then investigated. In these analyses, the fissile and fertile inventories and power output are calculated over the system lifetime for a specific breeder power. The effects on the system inventories of varying breeder thermal power are also examined. Since the mathematical-physical formulations are specified in terms of lumped parameters, the results of changing these on the system can be easily dealt with. Four fissile fuel breeding systems are then compared using current economic data.</p> <p>On the basis of this study, it is evident that there exists no single breeder system which consistently outperforms the others in all aspects. The fast-fission breeder, while not a good fissile fuel producer, has the best power generating efficiency and, due to its relatively low capital costs, can produce electricity at the lowest cost. The symbiotic fusion breeder system has the shortest fissile fuel replacement time and requires less initial fuel investment but it produces electricity at the highest cost. For combined fissile fuel production and electrical generation, the hybrid fusion breeder excells and it is also a good conserver of fertile fuel. The spallation breeder outranks all others in fissile fuel production.</p> <p>While no single breeder was found to be superior in a general sense, the synergetics method of analysis has been shown to be effective in several specific respects. The temporal variations of the stockpile inventories and net power derived here have a physically reasonable basis and are mathematically tractable. The pre-steady-state effects can be described with great accuracy by two functions determined by the fuel management scheme. Processing lags and losses are also explicitly incorporated. The flexibility and usefulness of the developed methodology are enhanced by the fact that any material stockpile inventory in the system can be calculated. Essential to this procedure is the inclusion at the outset of all system components in a synergetic analysis.</p> / Doctor of Philosophy (PhD)

<p>The propagation of weak shock waves through a horizontal stratified two-phase system has been investigated both theoretically and experimentally to demonstrate the various facets of its interaction with the phases, fuel bundles and flow network branches. Two types of shock tubes are used: a lucite shock tube is utilized in monitoring the mechanism at the interface of the phases by high speed photography and the other one is an aluminum shock tube of 101.6 mm inner diameter with flow network branches is used in investigating the shock wave behaviour inside a pressure tube during blow-down and loss of coolant accidents.</p> <p>This dissertation can be divided in to three broad categories. First, the inclusion of the interfacial roughness factor in the analysis of the shock wave propagation through a two-phase system inside a pressure tube weakens the strength of the shock waves, because of the energy loss due to frictional resistance at the interface. The interface is subjected to a combined effect of the waves propagating both in the gas and liquid phases of the system. High speed photography of the interface is considered to estimate the parameters pertaining to the generation of the ripples at the interface and the coupling of this parameter to a quasi-steady energy balance for the system can provide the values of the magnitude of the overpressures in the system.</p> <p>Secondly, the interaction of weak shock waves with three different types of fuel bundles used in CANDU reactors is presented. Depending on the percentage of the flow area available, different fuel bundles produce different magnitudes of the overpressure. For the transmitted waves, the choking is markedly observed even for moderate range of the shock waves. Presence of liquid phase enhances the strengths of overpressure for the reflected and transmitted waves. Inside the fuel bundles, the shock waves cause unusual vibrational effects which may be detrimental to the life of the fuel elements.</p> <p>Around the network, the two-phase propagation velocity is observed to be same as the gas phase propagation velocity is observed as the gas phase propagation velocity. Though tap and distilled water exhibit variations in the maximum overpressures, the time averaged magnitude under these two systems agree very well everywhere in the network branches. The vertical pressure profiles in the tap water has dispersive and high oscillatory nature whereas in the distilled water the rise in overpressure is dispersive, but smooth in nature. In distilled water, the pre-pulses moving under a free surface travel at the speed of sound in water and for those in tap water, this velocity is influenced by the presence of air bubbles in the tube walls.</p> / Doctor of Philosophy (PhD)

<p>Reflux condensation in vertical inverted U-tube steam generators forms an important heat removal mechanism for nuclear reactors in certain accidents. Reflux condensation phenomena in single vertical tubes were studied for two well defined boundary conditions to develop an improved understanding of the mechanisms governing heat removal and liquid holdup. A correspondence between an imposed boundary condition and its resulting flow regime has been made where total reflux condensation occurs for an imposed drop across the tube and fill and dump cycling occurs for an imposed steam flow rate at the tube inlet.</p> <p>Total reflux condensation is characterized by a flow pattern made by a single-phase region oscillating over a two-phase region. This flow region can be maintained indefinitely while the average single-phase and two-phase region lengths remain constant. It is also characterized by the complete condensation of the injected steam with all the condensate flowing back to the tube inlet.</p> <p>Fill and dump cycling is characterized by a cyclical operation where, during one cycle, the length of the single-phase region increases at the expense of the two-phase region until a point where the system becomes unstable and the single-phase region is ejected from the top of the tube. All the injected steam is condensed and contrary to total reflux condensation, not all the condensate flows back to the tube inlet. Instead, part of the condensate is carried over the condensation length to form the single-phase region. This flow regime could be qualified as dynamic as opposed to quasi-static for total reflux condensation.</p> <p>Experimental measurements were made in three single vertical tubes with a cooling jacket. These data show fill and dump cycling to be more efficient than total reflux condensation in condensing steam and to allow the condensate not to be trapped in the tube as in total reflux condensation. In addition, the experimental data suggest that total reflux condensation could be well the limiting flow regime for fill and dump cycling as the cycle period becomes very long and consequently it could define conservative lower bounds for the values of total heat removal.</p> <p>An analysis of counter-current film-wise condensation was conducted to model total reflux condensation, central to this model is an extended Nusselt's model of film-wise condensation, a linearized stability analysis of the condensate film flow and the use of three concepts: critical layer, maximum mechanical energy and transfer and film instability. The agreement between the experimental data and the prediction of total heat removal (via condensation rates) and liquid holdup is satisfactory. Both the present model and the experimental data show that flooding occurs at the tube inlet and plays a key role in defining the heat removal and the distribution of condensate in the tube. In particular, it is shown that for a given inlet cooling water temperature, the flooding flow rates, in terms of the Kutateladze variable, are nearly independent of the tube size and system pressure. In general, for a given tube size and system pressure, the inlet cooling water temperature has a notable influence on the value of the flooding flow rates, except for the smaller tube size, and it does not affect the amount of condensate holdup in the tube.</p> <p>The results of the present work could be used in small-break LOCA analyses were the present model could estimate the heat removal capabilities of steam generators and the amount of coolant (condensate holdup) trapped in the steam-generator tubes that would be available for core cooling.</p> / Doctor of Philosophy (PhD)