Department of Nuclear Engineering
https://hdl.handle.net/1721.1/7852
2020-04-08T12:28:51ZEffect of self-irradiation damage on thermal diffusivity and SAW speed in a thorium-doped lead sulfide thin film
https://hdl.handle.net/1721.1/123420
Effect of self-irradiation damage on thermal diffusivity and SAW speed in a thorium-doped lead sulfide thin film
Sergheyev, Keldin Nehmovitch.
Lead sulfide (PbS) is an important semiconductor for infrared light detection, and use in space necessitates understanding how it evolves when damaged by ionizing radiation. Previous work in chemical bath deposition (CBD) resulted in thin films of epitaxially grown polycrystalline PbS uniformly doped with radioactive thorium 228 (Th-228), permitting convenient study of a self-irradiating sample. This thesis represents a continuation of that work by studying the evolution of thermal diffusivity and surface acoustic wave (SAW) speed in a self-irradiating PbS thin film using the non-contact, non-destructive transient grating spectroscopy (TGS) assay. Radiation damage is allowed to accumulate and TGS is used to take measurements before and after annealing. Damage was presumed to create new phonon-scattering defects, thus decreasing SAW speed and thermal diffusivity. However, after annealing, radiation damage caused a monotonic increase in both. Both parameters asymptotically approach a maximum, which indicates a radiation damage saturation point. Thermal diffusivity does not return to its pre-annealed value, indicating an unknown affect. A longer TGS study is recommended to eliminate latent effects, as well as a band gap time-evolution study and an x-ray diffraction study.
Thesis: S.B., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2019; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 41-43).
2019-01-01T00:00:00ZEfficient and accurate sampling of the thermal neutron scattering law in OpenMC
https://hdl.handle.net/1721.1/123419
Efficient and accurate sampling of the thermal neutron scattering law in OpenMC
Trainer, Amelia J.
Convenient access to accurate nuclear data, particularly data describing low-energy neutrons, is crucial to the quality of thermal nuclear reactor simulations. Obtaining the scattering kernel for thermal neutrons (i.e. neutrons with energy on the order of 1 eV or less) can be a difficult problem, since the neutron energy is not enough to break molecular bonds, and thus the neutrons must often interact with a molecule or lattice structure. The "scattering law" S([alpha] [beta]), which is a function of unitless momentum and energy transfer, is used to relate the frequency distribution (also called "vibrational density of states") of the scattering media, to the scattering kernel. Currently, the most popular method of calculating S([alpha] [beta]) involves running the LEAPR module of the NJOY nuclear data processing code. Several antiquated approximations are used in LEAPR, such as the Einstein-crystal approximation (i.e. discrete oscillator approximation), which represents peaks in the frequency distribution as 6-functions. This project identifies insufficiencies in current thermal scattering data preparation: redundant numerical operations, arbitrary summation cutoffs, the discrete oscillator approximation, and the requirement that input frequency distributions be provided on a uniform energy mesh. Solutions to these shortcomings are identified and discussed. Additionally, a recently developed method of sampling energies and angles of the scattered neutrons is implemented into the OpenMC Monte Carlo neutron transport code to facilitate the testing of better phonon representations and maintain the continuous representation of the scattering kernel in energy and angle..
Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2019; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 76-77).
2019-01-01T00:00:00ZCorrelations in Monte Carlo eigenvalue simulations : uncertainty quantification, prediction and reduction
https://hdl.handle.net/1721.1/123375
Correlations in Monte Carlo eigenvalue simulations : uncertainty quantification, prediction and reduction
Miao, Jilang.
Monte Carlo methods have mostly been used as a benchmark tool for other transport and diffusion methods in nuclear reactor analysis. One important feature of Monte Carlo calculations is the report of the variance of the estimators as a measure of uncertainty. In the current production codes, the assumption of independence of neutron generations in Monte Carlo eigenvalue simulations leads to the oversimplified estimate of the uncertainty of tallies. The correlation of tallies between neutron generations can make reported uncertainty underestimated by a factor of 8 in assembly size tallies in a typical LWR. This work analyzes the variance/uncertainty convergence rate in Monte Carlo eigenvalue simulations and develops different methods to properly report the variance.; To correct the underestimated variance as a post-processing step, a simple correction factor can be calculated from the correlation coefficients estimated from a sufficient number of active generations and fitted to decreasing exponentials. If the variance convergence rate is needed before or during the simulation to optimize the run strategy (number of generations and neutrons per generation), a discrete model can be constructed from the inactive generations that can predict the correlation behavior of the original problem. Since it is not efficient to perform variance correction to all tallies on all problems, a simple correlation indicator is also developed to quickly determine the potential impact of correlations on a given tally in a given problem. This can help decide if more complicated correction analysis or the use of independent simulations should be used to calculate the true variance.; Run strategy to reduce correlations is also investigated by introducing the notion of delayed neutrons. A predictive model for the new source update scheme was developed to help identify optimal delayed neutron parameters before implementing in OpenMC. Optimal run strategies in terms of delayed bank size, frequency of delayed bank sampling and true simulation costs are proposed.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.; Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2018; Cataloged from student-submitted PDF version of thesis.; Includes bibliographical references (pages 323-327).
2018-01-01T00:00:00ZValidation of turbulent transport models on Alcator C-Mod and ASDEX upgrade
https://hdl.handle.net/1721.1/123374
Validation of turbulent transport models on Alcator C-Mod and ASDEX upgrade
Creely, Alexander James.
This thesis developed hardware and analysis techniques to measure two validation constraints experimentally, and then applied these constraints in the validation of plasma turbulent transport models on two tokamaks, Alcator C-Mod and ASDEX Upgrade, resulting in both greater physics understanding of multi-scale turbulent interactions and greater confidence in predictions for future fusion devices. On the path toward the clean, sustainable, and safe energy of a fusion power plant, experiment and modeling each contribute something unique. Before one can in good faith use plasma turbulent transport models to explain turbulent dynamics or predict machine performance, however, one must ensure that these models can correctly reproduce experimentally measured conditions on existing devices. Validation, the process of determining how accurately a model represents reality, has thus become a key endeavor in fusion energy research.; First, this thesis developed an analysis technique to measure the electron perturbative thermal diffusivity based on tracking the propagation of heat pulses generated by partial sawtooth crashes. In addition, correlation electron cyclotron emission (CECE) hardware was constructed on both Alcator C-Mod and ASDEX Upgrade, and analysis techniques were derived, in order to measure turbulent electron temperature fluctuations. These validation constraints were applied to two turbulent transport models, the nonlinear gyrokinetic model and the quasi-linear gyrofluid model. In particular, these constraints were used to study the importance of multi-scale turbulent effects (due to coupling between ion- and electron-scales) in correctly modeling plasma behavior.; The gyrokinetic codes GYRO and GENE were validated on Alcator C-Mod and ASDEX Upgrade respectively, using both constraints developed in this thesis as well as ion and electron heat fluxes from power balance, revealing that in some cases ionscale simulations are sufficient to match experimental constraints, while in other cases multi-scale effects are important. To investigate this discrepancy, a novel type of validation study was performed with the gyrofluid code TGLF, including many discharges from both machines. This study resulted in two physical criteria that determine when multi-scale effects are important, and when ion-scale simulations are sufficient to model the plasma behavior, shedding light on the physical phenomena that govern the importance of multi-scale turbulent effects.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.; Thesis: Ph. D. in Applied Plasma Physics, Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2019; Cataloged from student-submitted PDF version of thesis.; Includes bibliographical references (pages 351-369).
2019-01-01T00:00:00Z