Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 91-104) and index. / Water is the most ubiquitous substance on earth, and is essential to sustain all known forms of life. However, despite centuries of research, a coherent picture of the unusual phase behavior of water is so far lacking. The most promising theory under scrutiny relies on the hypothetical existence of a liquid-liquid phase transition and an associated liquid-liquid critical point hidden in the region of supercooled temperatures and elevated pressures, where bulk water does not exist in liquid state. Therefore it is a grand experimental challenge to investigate the properties of water, both thermodynamic and dynamic, in the relative region of the phase diagram. A combination of neutron triple-axis spectrometer and small-angle neutron scattering instrument are used to measure the density of water confined in a nanoporous silica matrix MCM-41-S in a Temperature-Pressure range inaccessible for the bulk (300-130 K and 1-2900 bar), namely, the equation of state p(T, P). The measured isobaric density profiles clearly show a change of behavior around 1500 bar. This experiment provides the most direct evidence supporting the existence of a liquid-liquid critical point in such confined water. The experimental result further implies that the nature of the liquid-liquid critical point of water might be of a tricritical type. Moreover, the reported density data of confined water under extreme conditions are valuable for the vast communities in biological, geological and planetary sciences. Parallel to the density measurement, the dynamics of water in confined geometry, such as aged cement paste and the vicinity of protein surfaces, are also investigated using a variety of quasi-elastic and inelastic neutron scattering spectrometers. A wide range of pre-glass-transition phenomena, such as dynamic crossover, dynamic heterogeneity and boson peak, are observed above the conventional glass-transition temperature. Possible explanations are discussed in the frameworks of the liquidliquid critical point scenario of water and the extended mode-coupling theory. Computer simulations are frequently exploited to achieve a unified interpretation. These new findings of confined water may provide new insights to the research on glassy systems of multi-scales as well as innumerable examples in soft condensed matters, where randomness and cooperativity are common and intrinsic. / by Yang Zhang. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 107-110). / Ion collection by dust grains and probes in plasmas with a neutral background is of interest in the study of both space and terrestrial plasmas, where charge-exchange collisions can play an important role in ion collection. Further, background drifts or magnetic fields can significantly affect the ion collection by and the potential structure near such objects, and should therefore also be included. These effects, however, are difficult to include in a theoretical treatment, and thus this problem lends itself to a computational approach. To be able to tackle problems with a neutral background, the 3D3v hybrid particlein- cell code SCEPTIC3D has been upgraded to include charge-exchange collisions. This required the development of a new Monte Carlo based reinjection scheme. The new reinjection scheme and other upgrades are described in detail, and the collisionless operation of the reinjection scheme is validated against the old SCEPTIC3D reinjection scheme, while its collisional operation is validated through comparisons with the reinjection scheme in SCEPTIC (2D). The new reinjection scheme can easily be modified to allow the injection of an almost arbitrary distribution function at the domain boundary, enabling future studies of the sensitivity of ion collection to the injected velocity distribution. Studies of ion collection in magnetized or drifting plasmas using the upgraded code extend earlier stationary, unmagnetized results, which showed an enhancement of ion current at intermediate collisionality. It is found that this enhancement is gradually suppressed with increasing background neutral drift speed, and is entirely absent for speeds above the ion sound speed. Adding a magnetic field rather than a neutral drift appears to in fact increase the collisional ion current enhancement. / by Christian Bernt Haakonsen. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 487-495). / A key challenge for full-core transport methods is reactor agnostic multi-group cross section (MGXS) generation. Monte Carlo (MC) presents the most accurate method for MGXS generation since it does not require any approximations to the neutron flux. This thesis develops novel methods that use MC to generate the fine-spatial mesh MGXS that are needed by high-fidelity transport codes. These methods employ either engineering-based or statistical clustering algorithms to accelerate the convergence of MGXS tallied on fine, heterogeneous spatial meshes by Monte Carlo. The traditional multi-level approach to MGXS generation is replaced by full-core MC calculations that generate MGXS for multi-group deterministic transport codes. Two pinwise spatial homogenization schemes are introduced to model the clustering of pin-wise MGXS due to spatial self-shielding spectral effects. The Local Neighbor Symmetry (LNS) scheme uses a nearest neighbor-like analysis of a reactor geometry to determine which fuel pins should be assigned the same MGXS. The inferential MGXS (iMGXS) scheme applies unsupervised machine learning algorithms to "noisy" MC tally data to identify clustering of pin-wise MGXS without any knowledge of the reactor geometry. Both schemes simultaneously account for spatial self-shielding effects while also accelerating the convergence of the MC tallies used to generate MGXS. The LNS and iMGXS schemes were used to model MGXS clustering from radial geometric heterogeneities in a suite of 2D PWR benchmarks. Both schemes reduced U-238 capture rate errors by up to a factor of four with respect to schemes which neglect to model MGXS clustering. In addition, the schemes required an order of magnitude fewer MC particle histories to converge MGXS for multi-group deterministic calculations than a reference MC calculation. These results demonstrate the potential for single-step MC simulations of the complete heterogeneous geometry as a means to generate reactor agnostic MGXS for deterministic transport codes. The LNS and iMGXS schemes may be valuable for reactor physics analyses of advanced LWR core designs and next generation reactors with spatial heterogeneities that are poorly modeled by the engineering approximations in today's methods for MGXS generation. / by William Robert Dawson Boyd, III. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007. / Includes bibliographical references (p. 33). / As the increasing demand for nuclear energy results in larger spent fuel volume, implementation of longer fuel cycles incorporating higher burnup are becoming common. Understanding the effect of higher burnup on the spent fuel composition and radioactive properties is essential to ensure that spent fuel receives proper cooling in storage before it is sent to a disposal site or proper treatment and reprocessing if its useful content is to be extracted prior to disposal. Using CASMO-4, a standard Westinghouse 4-loop pressurized water reactor model was created and simulated with a three batch fuel cycle. U-235 enrichment was adjusted to achieve fuel burnups of 30, 50, 70 and 100 MWD per kg of initial uranium. These burnups demanded reload enrichments of 3.15%, 4.63%, 6.26% and 9.01% U-235 w/o respectively. The resultant spent fuel transuranic isotopic compositions were then provided as input into ORIGEN to study the decay behavior of the spent fuel. It was found that when burnup increased from 30 MWD/kg to 100 MWD/kg, the activity more than doubled due to the decreased Pu-241 content and the increased Np-239 presence. More importantly, the activity per MWD significantly decreased despite absolute increases in unit mass. The net result is that the half-life of high burnup fuels is greatly increased in comparison to low burnup fuels for the first decade of life. Beginning from day 14 after shutdown and until 10 years later, the 100 MWD/kg fuel has a half-life of 129 days while the 30 MWD/kg spent fuel has a half life of 5 days. Previous work has suggested that different trends dominate decay behavior from years 10 to 100 years following discharge. / by Drew A. Reese. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 31-32). / Next generation nuclear technology calls for more advanced fuels to maximize the effectiveness of new designs. A fuel currently being studied for use in advanced light water reactors (LWRs) is uranium zirconium hydride (UZH), a fuel currently being used in the popular TRIGA research reactors. UZH is being considered because unlike the current fuel of choice, uranium dioxide, it is metal based and therefore better able to transfer the heat out of the fuel that is coming from fission. This can lead to lower operating temperatures which will reduce the amount of fission gas release to negligible quantities, eliminate cracking, and reduce the internal energy of the fuel. Furthermore, it is hoped the UZH will be better able to attain higher burnups, partly because of the presence of the strong moderator hydrogen, and thus will help better utilize resources and reduce the volume of nuclear waste produced. In order for UZH to be viable as a fuel it is recommended that the peak central temperature of the fuel be maintained below 650°C, at which point swelling due to void formation around the uranium atoms becomes a concern. In order to keep temperature below this level it has been proposed that lead-bismuth eutectic (LBE) be used as the gap material instead of helium. In order to ensure that the properties of UZH while using a LBE gap, specifically the thermal conductivity, do not degrade to the point of the fuel not being viable, an experiment was designed and put into the MIT research reactor. The initial results show a decreasing trend in thermal conductivity, albeit with much of this change considered to be because of the many thermal cycles the experiment underwent while in the reactor. / by Brendan M. Ensor. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2006. / Includes bibliographical references (p. 159-166). / Multipactor discharges are a resonant condition in which electrons impact a surface in phase with an alternating electric field. The discharge is sustained by electron multiplication from secondary emission. As motivation, multipactor discharges can adversely affect many different rf systems in vacuum, and this work provides an improved general understanding of multipactor and gives promising results for improved performance and reliability of these systems. The Coaxial Multipactor Experiment (CMX) creates and investigates multipactor discharges in both parallel plate and coaxial geometries at very high frequency (VHF). CMX provides the first detailed investigation of multipactor energy distribution functions for both coaxial and parallel plate geometries with the use of retarding potential analyzers (RPA). A 1-D particle tracking simulation supports these experimental distributions and yields the underlying physics behind the distribution shape. Experimental and simulation energy distributions have a low energy population of defocused electrons due to space charge effects and RPA emission, and a high energy population responsible for sustaining the discharge. Results show a higher energy distribution for the coaxial geometry as compared to the parallel plate geometry with the same electrode spacing, implying that coaxial geometries are more susceptible to multipactor. These results are supported by CMX susceptibility data, which are provided for both coaxial and parallel plate electrodes. Lastly, similar multipactor experiments were performed on Alcator C-Mod rf systems, allowing the discovery of multipactor-induced glow discharge in these systems. / (cont.) Results suggest the onset of this glow discharge causes the observed C-Mod neutral pressure limits. These results are further supported by CMX experiments, and a new, 50 pim sandblasted copper surface treatment has been shown to sufficiently lower 6 < 1 for multipactor prevention on CMX and raise the minimum pressure for glow discharge breakdown. This surface treatment shows no significant degradation of high voltage handling, and it is proposed for implementation on the multipactor-susceptible regions of C-Mod rf systems. / by Timothy P. Graves. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2018. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 39-40). / Reactor Physics is not always an intuitive subject for students to understand. When nuclear engineering was beginning as a field it was common for students to complete measurements on sub-critical reactors, which could not sustain a fission chain reaction, in order to develop student intuition. The Massachusetts Institute of Technology has one such reactor, a graphite exponential pile, which went unused for decades. In this thesis the MIT Graphite Exponential Pile was returned to experimental operation, and a prototypic student experiment was completed. The material buckling was found by indium foil activations completed with a plutonium-beryllium source in the pile. From the experimental results it was calculated the pile would have to be a cube with sides that are 5.42m long to become a critical reactor. This proof of concept experiment makes it possible for mens et manus based education at MIT for reactor physics. / by Micah D. Gale. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Stress corrosion cracking (SCC) susceptibility of Inconel Alloy X-750 in the HTH condition has been evaluated in high purity water at 93 and 288°C under Boiling Water Reactor Normal Water Chemistry (NWC) and Hydrogen Water Chemistry (HWC) conditions. SCC crack growth rates of approximately 1.1 x 10-7 mm/s (K=28 MPa[square root symbol]m) under NWC conditions and 1.4xI 0-8 mm/s (K=28 MPa[square root symbol]m) under HWC in high purity water at 288°C were observed. The environmental conditions were changed from NWC to HWC during constant K loading, and the crack growth rate immediately slowed down by approximately one order of magnitude. The alloy was also tested in HWC at 93°C. No SCC crack growth was observed at K= 35 MPa[square root symbol]m for the length of time tested at 93°C. The fracture mode transitioned from predominantly transgranular cracking under fatigue conditions to a mixture of intergranular, pseudo-intergranular, and a small amount of transgranular fracture in constant stress intensity SCC. Pseudo-intergranular cracking is when a crack propagates directly adjacent to the grain boundary carbides and not actually on the grain boundary. The SCC crack tips were characterized with scanning electron microscopy (SEM) and 3D Atom Probe Tomography (APT). The SEM analysis was focused on the fractographic analysis and crack-propagation mode. The crack was observed to propagate adjacent to grain boundary carbides (pseudo-intergranular) and along a boundary with high coherency where no carbides were present (intergranular). The small and localized areas of transgranular cracking were occasionally seen between two regions of intergranular cracking. The APT reconstructions of the crack tips and crack wall identified several key features contributing to the SCC process: 1) Preferential oxygen transport occurs in either a finger-like or crystallographic morphology extending from the crack tip region. These regions are enriched in both oxygen and oxide with the oxide being a chromium-nickel spinel. 2) The matrix ahead of each finger-like "tunnel" is enriched in oxygen and predominantly chromium oxide. This indicates that oxygen is diffusing ahead of the crack tip into the bulk material. 3) The oxygen that penetrates directly into the base material from the crack walls in an ordered manner suggests that it is controlled by crystallographic features. 4) The main SCC crack tip is full of predominantly oxide phase and, to a lesser extent, metal atoms. The very crack tip forms a spinel of chromium and nickel oxides. Iron oxide begins to contribute to the oxide spinel approximately 25-30 nm from the actual tip. 5) The [gamma] precipitates that are directly adjacent to each crack tip and crack wall were deficient in aluminum content. The aluminum content in the bulk [gamma] was approximately 6.6 at% and the near-crack [gamma] aluminum content ranged from 2.5-3.5 at%. The range of affected [gamma] was approximately 100 nm wide. / by Jonathan Paul Gibbs. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 47-48). / Two research subjects: (1) excitation of "forced ion acoustic waves", and (2) "simultaneous excitation of plasma density fluctuations and geomagnetic field fluctuations" are reported in my M.S. thesis. The data was acquired in our experiments conducted at Gakona, Alaska from summer 2007 to winter 2008, using DoD/NSF-funded HAARP facilities and our own optical (ASIS) and radio instruments (VLF receiving system of IRIS) aided by GPS satellites as well as AMISR radar at Poker Flat, Alaska. We suggest that "Forced ion acoustic waves" detected by MUIR radar on Oct. 29 during 6:20-6:30 UT arise from keV electron precipitation associated with the occurrence of green aurora. Our work shows, for the first time, that MUIR radar is suitable for probing naturally occurring space plasma processes and not limited to HF heater-induced effects. This would extend the usage of MUIR for the investigation of space weather together with AMISR radar at Poker Flat, to advance our knowledge in space plasma turbulence. The research on "simultaneous excitation of plasma density fluctuations and geomagnetic field fluctuations" is an extension of my B.S. thesis research on thermal filamentation instability, which started in our summer Gakona experiments in 2005. Large plasma sheets (also known as sheet-like filaments) can be excited by HF O-mode and X-mode heater waves via thermal filamentation instability. / (cont.) The dominant nonlinearity is provided by the differential Joule heating acting on electrons, which subsequently gives rise to a cross-field thermal pressure force, to concomitantly generate spatially varying plasma density fluctuations and geomagnetic field fluctuations. It is interesting to find that the fractional density fluctuations are approximately equal to the fractional magnetic field fluctuations. This gives us the theoretical basis to use ground-based magnetometer measurements to infer the density fluctuations in space plasma turbulence. Such a remote sensing technique for probing the space plasma is much more effective and economic than using a beacon satellite. / by Joel Cohen. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 359-368). / In-Vessel Retention ("IVR") is a severe accident management strategy that is power limiting to the Westinghouse AP1000 due to critical heat flux ("CHF") at the outer surface of the reactor vessel. Increasing the CHF level by altering the cooling fluid would increase the safety margin of current design power or allow for higher power. The modification to current licensed design to implement a new cooling fluid would not require significant changes to the containment and associated systems. Previous research at MIT and other institutions has demonstrated that CHF of water on a heated metal surface can be increased from 30% to 200% with the introduction of nanoparticles. Alumina has shown the best CHF enhancement of the nanoparticles tested to date at MIT. Alumina nanoparticles and water based nanofluids have also shown long term stability in solution, which is important for the long time frame (hours to days) of IVR. To measure the CHF of geometry and conditions relevant to IVR for the AP1000, a two-phase flow loop has been designed and built. The test section designed to have hydrodynamic similarity to the AP 1000 and allows for all angles that represent the bottom surface of the reactor vessel. Research completed herein measured CHF for varied conditions of orientation angle, pressure, mass flux, fluid type, and surface material. Results for stainless steel with water based alumina 0.001% by volume nanofluid indicate an average 70% CHF enhancement with a range of 17% to 108% for geometry and conditions expected for IVR. Experiments also indicate that only about thirty minutes of boiling time is needed to obtain CHF enhancement. Implementation could involve storage tanks of high concentration nanofluids installed in containment. Once the IVR strategy is initiated with flooding of the vessel cavity with water from the In-containment Refueling Water Storage Tank ("IRWST"), the nanofluids would be released to mix as the natural circulation flow sets up along the gap between the vessel and the insulation mounted to the concrete wall in the vessel cavity. Boiling then plates nanoparticles onto the surface enhancing CHF. / by Gregory Lee DeWitt. / Ph.D.