Browsing by Author "Khan, Nafisah"

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Decentralized state-space controller design of a large PHWR
(2009-11-01) Khan, Nafisah; Lu, Lixuan
The behaviour of a large nuclear reactor can be described with sufficient accuracy using a nodal model, like the spatial model of a 540 MWe large Pressurized Heavy Water Reactor (PHWR). This model divides the reactor into divisions or nodes to create a spatial model in order to control the xenon induced oscillations that occur in PHWRs. However, being such a large scale system, a 72nd-order model, it makes controller design challenging. Therefore, a reduced order model is much more manageable. A convenient method of model reduction while maintaining the important dynamics characteristics of the process can be done by decoupling. Also, due to the nature of the system, decentralized controllers could serve as a better option because it allows each controller to be localized. This way, any control input to a zone only affects the desired zone and the zones most coupled with, thus not causing a respective change in neutron flux in the other zones. In this thesis, three decentralized controllers were designed using the spatial model of a 540 MWe large PHWR. A decoupling algorithm was designed to divide the system into three partitions containing 20, 27, and 25 states each. Reduced order sub-systems were thus created to produce optimal decentralized controllers. An optimal centralized controller was created to compare both approaches. The decentralized versus centralized controllers’ system responses were analyzed after a reactivity disturbance. A fail-safe study was done to highlight one of the advantages of decentralized controllers.
A new approach to neutron spectrometry with multi-element scintillators
(2014-02-01) Khan, Nafisah; Machrafi, Rachid
The combined effects of the nuclear industrial renaissance, the events of 9/11, and the Fukushima disaster have had a significant impact on the research and development of radiation detection instrumentation. Notably, there is ample worldwide scientific research effort into a better understanding of the material properties and nuclear interactions with a view to support the improvement of radiation detection and measurement. These improvements are spurred by the heightened security requirements that entail the monitoring of contraband material including explosives in transit, and the need to enhance occupational safety as well as environmental radiation protection in respect to nuclear power generation. Moreover, the regulatory authorities require routinely employed radiation detection and measuring devices in nuclear installations to meet the revised standard specifications in terms of their design and performance. Currently, there is no neutron dosimeter/spectrometer that can meet the requirements in terms of size and performance. In particular, the requirements for high sensitivity, spectroscopic features, and determination of operational quantities to enable radiation protection decision making have resulted in a closer examination of the basic physics of the radiation interactions in detector materials. Neutron dosimetry is regarded as the last frontier in radiation protection. Due to the large span of neutron energy and the strong energy dependence of the dose to fluence coefficient, neutron dosimetry requires the knowledge of the neutron spectra for any accurate neutron dose quantification. As a result, spectrometry is a precursor to determine dosimetry quantities and spectrometers are therefore vital to determine and characterize radiation fields present to individuals as they provide information about the radiation intensity and energy spectra. However, current spectrometers have many drawbacks and limitations in different aspects. From one side, fast spectrometry currently uses the scattering process on hydrogen rich materials and uses complicated unfolding techniques to extract the energy spectra. From another side, neutron fields are inherently mixed with a gamma component and therefore, it is paramount to distinguish each component since their contribution to the dose equivalent is weighted differently. More specifically, the challenge becomes more profound with neutrons in the energy range between 10 keV and few MeV. These challenges are mainly due to: • The drastic change in the dose-to-fluence conversion coefficient that increases by a factor of 40; • The low sensitivity of the neutron sensors used in neutron spectroscopy (low cross section); • The poor resolution of the detectors, which makes accurate neutron spectrometry difficult to achieve. However, by exploiting new developments and high sensitivity scintillators, in this thesis, a new approach has been adopted using different nuclear reaction processes with different contents of scintillating material. More specifically, two nuclear reactions, i.e. (n,α) and (n,p), on two different elements have been used to carry out neutron spectrometry. In addition, this thesis aims to investigate the spectrometric properties of scintillating materials as a first step to establish a platform for developing a neutron spectrometer/dosimeter. In terms of methodology, the thesis has taken an empirical approach in studying potential sensors that can be used for neutron spectrometry. Four scintillators have been explored and studied. Each scintillator corresponds to a particular energy region. The first part of the thesis consists of extensive Monte Carlo calculations to optimize the sensor’s isotope contents, while the second part consists of conducting a series of experiments using three main facilities, namely an AmBe source of 120 mCi, a neutron generator of 2.5 MeV neutrons at the University of Ontario Institute of Technology, a KN Van De Graaff accelerator at McMaster University, and gamma ray sources with different energies. All sensors have been used in conjunction with a miniature data acquisition system that consists of a multi-channel analyzer, mounted on a photomultiplier, and controlled by software to operate, control and analyze the output data. The time characteristics of the output pulse, such as integration time and rising time, have been optimized for each sensor. The thesis presents a thorough literature review, a comprehensive methodology of the study, a description of the used facilities and the results of the simulation data with four different sensors along with the experimental work carried out at the aforementioned facilities. The response functions of each scintillator to a given radiation type and energy has been analyzed and discussed. Furthermore, the sensor’s potential for use in neutron spectrometry/dosimetry has been assessed for future work.