Nuclear Engineering - Ph.D. / Sc.D.
http://hdl.handle.net/1721.1/7854
2015-12-22T04:52:12ZOptimization of deep boreholes for disposal of high-level nuclear waste
http://hdl.handle.net/1721.1/97968
Optimization of deep boreholes for disposal of high-level nuclear waste
Bates, Ethan Allen
This work advances the concept of deep borehole disposal (DBD), where spent nuclear fuel (SNF) is isolated at depths of several km in basement rock. Improvements to the engineered components of the DBD concept (e.g., plug, canister, and fill materials) are presented. Reference site parameters and models for radionuclide transport, dose, and cost are developed and coupled to optimize DBD design. A conservative and analytical representation of thermal expansion flow gives vertical velocities of fluids vs. time (and the results are compared against numerical models). When fluid breakthrough occurs rapidly, the chemical transport model is necessary to calculate radionuclide concentrations along the flow path to the surface. The model derived here incorporates conservative assumptions, including instantaneous dissolution of the SNF, high solubility, low sorption, no aquifer or isotopic dilution, and a host rock matrix that is saturated (at a steady state profile) for each radionuclide. For radionuclides that do not decay rapidly, sorb, or reach solubility limitations (e.g., 1-129), molecular diffusion in the host rock (transverse to the flow path) is the primary loss mechanism. The first design basis failure mode (DB 1) assumes the primary flow path is a 1.2 m diameter region with 100x higher permeability than the surrounding rock, while DB2 assumes a 0.1 mm diameter fracture. For the limiting design basis (DB 1), borehole repository design is constrained (via dose limits) by the areal loading of SNF (MTHM/km2 ), which increases linearly with disposal depth. In the final portion of the thesis, total costs (including drilling, site characterization, and emplacement) are minimized ($/kgHM) while borehole depth, disposal zone length, and borehole spacing are varied subject to the performance (maximum dose) constraint. Accounting for a large uncertainty in costs, the optimal design generally lies at the minimum specified disposal depth (assumed to be 1200 in), with disposal zone length of 800-1500 m and borehole spacing of 250-360 meters. Optimized costs range between $45 to $191/kgHM, largely depending on the assumed emplacement method and drilling cost. The best estimate (currently achievable), minimum cost is $134/kgHM, which corresponds to a disposal zone length of -900 meters and borehole spacing of 272 meters.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2015.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 223-240).
2015-01-01T00:00:00ZDomain decomposition for Monte Carlo particle transport simulations of nuclear reactors
http://hdl.handle.net/1721.1/97859
Domain decomposition for Monte Carlo particle transport simulations of nuclear reactors
Horelik, Nicholas E. (Nicholas Edward)
Monte Carlo (MC) neutral particle transport methods have long been considered the gold-standard for nuclear simulations, but high computational cost has limited their use significantly. However, as we move towards higher-fidelity nuclear reactor analyses the method has become competitive with traditional deterministic transport algorithms for the same level of accuracy, especially considering the inherent parallelism of the method and the ever-increasing concurrency of modern high performance computers. Yet before such analysis can be practical, several algorithmic challenges must be addressed, particularly in regards to the memory requirements of the method. In this thesis, a robust domain decomposition algorithm is proposed to alleviate this, along with models and analysis to support its use for full-scale reactor analysis. Algorithms were implemented in the full-physics Monte Carlo code OpenMC, and tested for a highly-detailed PWR benchmark: BEAVRS. The proposed domain decomposition implementation incorporates efficient algorithms for scalable inter-domain particle communication in a manner that is reproducible with any pseudo-random number seed. Algorithms are also proposed to scalably manage material and tally data with on-the-fly allocation during simulation, along with numerous optimizations required for scalability as the domain mesh is refined and divided among thousands of compute processes. The algorithms were tested on two supercomputers, namely the Mira Blue Gene/Q and the Titan XK7, demonstrating good performance with realistic tallies and materials requiring over a terabyte of aggregate memory. Performance models were also developed to more accurately predict the network and load imbalance penalties that arise from communicating particles between distributed compute nodes tracking different spatial domains. These were evaluated using machine properties and tallied particle movement characteristics, and empirically validated with observed timing results from the new implementation. Network penalties were shown to be almost negligible with per-process particle counts as low as 1000, and load imbalance penalties higher than a factor of four were not observed or predicted for finer domain meshes relevant to reactor analysis. Load balancing strategies were also explored, and intra-domain replication was shown to be very effective at improving parallel efficiencies without adding significant complexity to the algorithm or burden to the user. Performance of the strategy was quantified with a performance model, and shown to agree well with observed timings. Imbalances were shown to be almost completely removed for the finest domain meshes. Finally, full-core studies were carried out to demonstrate the efficacy of domain-decomposed Monte Carlo in tackling the full scope of the problem. A detailed mesh required for a robust depletion treatment was used, and good performance was demonstrated for depletion tallies with 206 nuclides. The largest runs scored six reaction rates for each nuclide in 51M regions for a total aggregate memory requirement of 1.4TB, and particle tracking rates were consistent with those observed for smaller non-domain- decomposed runs with equivalent tally complexity. These types of runs were previously not achievable with traditional Monte Carlo methods, and can be accomplished with domain decomposition with between 1.4x and 1.75x overhead with simple load balancing.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2015.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 151-158).
2015-01-01T00:00:00ZA methodology for modular nuclear power plant design and construction
http://hdl.handle.net/1721.1/96442
A methodology for modular nuclear power plant design and construction
Lapp, Christopher Warren
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1989.; Includes bibliographical references (leaves 390-391).
1989-01-01T00:00:00ZPulsed field gradient magnetic resonance measurements of lithium-ion diffusion
http://hdl.handle.net/1721.1/95617
Pulsed field gradient magnetic resonance measurements of lithium-ion diffusion
Krsulich, Kevin D
The transport of lithium ions between the electrolyte-electrode interface and the electrode bulk is an essential and presently rate limiting process in the high-current operation of lithium-ion batteries. Despite their importance, few methods exist to experimentally investigate these macroscopic diffusion processes and, as a result, much remains unknown regarding their underlying mechanisms and the resulting macroscopic transport. Gradient nuclear magnetic resonance measurements are a mature and effective means of investigating macroscopic transport phenomena and posses several advantages over competing measures of transport in ionic solids. However, short coherence times, slow diffusion rates and a small gyromagnetic ratio have, to date, limited their usefulness for measurements of room-temperature transport in solid lithium-ion conductors. Recent developments in quantum control have demonstrated methods for extending the coherence times of dipolar-coupled nuclear spins by several orders of magnitude, into a regime enabling gradient measurements of slow lithium-ion diffusion. This thesis proposes and demonstrates, through the utilization of a dipolar refocusing sequence and a strong pulsed magnetic field gradient, a nuclear magnetic resonance method for the direct measurement of the lithium ion self-diffusion coefficient within room-temperature lithium-ion conductors. Magnetic resonance field gradient measurements derive ensemble transport statistics through observation of the residual phase mismatch following two position dependent phase rotations, implemented as DC pulses of a spatially varying gradient field, separated in time by a transport period. Generating sufficiently fine spatial encodings to be sensitive to slow diffusion has proven challenging in solids where strong relaxation due to the homonuclear dipole-dipole interaction drastically shortens coherence times and thus limits the duration of applied gradient pulses. This study utilizes a magic echo based refocusing sequence to nullify the dominant decoherence mechanism allowing effective gradient pulses on the order of one millisecond. Combined with a custom-built pulsed field gradient, spatial encodings on the order of 1 [mu]m are obtained. For a demonstrative sample, the lithium-ion conductor lithium sulfide is chosen both for its favorable NMR properties and for its role in the recent renewal of interest in nanostructured integration cathode materials. Initial sample characterization reveals two ⁷Li NMR lines distinguished by their static line widths and refocusing behavior. A modified version of the 1D EXSY selective inversion experiment is performed to characterize an exchange process between these two lines and extract their intrinsic spin-lattice relaxation rates. Two stimulated echo diffusion measurements are performed to identify the apparent diffusion coefficients of each line in the presence of exchange. The observed diffusion coefficient of the narrow line is determined to be 2.39 +/- 0.34 . 10-⁸ cm²/s. Diffusive attenuation is not observed for the broad line. These results are analyzed through a two bath exchange model parameterized by the results of the earlier exchange experiments. The influence of exchange on the observed diffusion coefficients is determined to be negligible as diffusion times are limited by the inverse of the exchange rates.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 102-119).
2014-01-01T00:00:00Z