Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2006. / Includes bibliographical references (leaves 36-39). / Currently there are a growing number of radiation workers. In order to ensure the safety of the employees, regulations have been established by the federal government and state governments to limit the dose equivalent to radiation workers. The most well known strategy for reducing radiation doses in the work place is the ALARA principle which stands for "as low as reasonably achievable". Within the phrase, "reasonably achievable" there is an implied element of subjectivity. Because "reasonably achievable" can vary in meaning for different people, this paper will analyze the ALARA principle in detail. Also, the manner in which inconclusive data on low dose radiation are treated in the court rooms will be evaluated. A secondary part of the paper will deal with what happens when accidents occur to radiation workers. Specifically, this paper will deal with the accidents at Kerr-McGee, Three Mile Island and SONGS. The thesis will delve into the litigation that followed the radiation accidents and analyses of the rulings, and will look at where current radiation litigation is heading. / by Enrique Esparza. / 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 (p. 161-167). / The need for large-size detectors for long-range active interrogation (Al) detection has generated interest in water-based detector technologies. AI is done using external radiation sources to induce fission and to detect, identify, and characterize special nuclear material (SNM) through the gamma rays and neutrons emitted. Long-range applications require detectors with a large solid angle and an ability to significantly suppress lowenergy background from linear electron accelerators. Water Cherenkov Detectors (WCD) were selected because of their transportability, scalability, and an inherent energy threshold. The main objective of this thesis was to design a large-size WCD capable of detecting gamma rays and to demonstrate particle energy discrimination ability. WCD was modeled in detail using Geant4 for optimization purposes. The experimental detector is composed of an aluminum body with a high efficiency (98.5%) diffuse reflector. Cherenkov photons are detected with six 8" hemispherical Hamamatsu photomultiplier tubes (PMT). PMTs are calibrated using two monoenergetic LEDs. The detector was shown to successfully detect gamma rays of energies above the Cherenkov threshold. The detector was able to discriminate between various sources, such as ⁶⁰Co and ²³²Th, even though WCD are known for their poor energy resolution. The detector design and analysis was completed, and it was demonstrated both computationally and experimentally that it is possible to use WCD to detect and characterize gamma rays. One of the accomplishments of this thesis was demonstration of event reconstruction capability of the detector system. A full-detector model was created using Geant4 simulation toolkit. The performance of the detector was predicted using the model and then experimentally verified. The qualitative agreement between the model and the experiment was observed. The event reconstruction was an important part of the detector performance analysis. Post-experimental data processing was done using ROOT. / by Anna S. Erickson. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 175-182). / In this thesis, the effect of the lower hybrid current drive on ion toroidal rotation in a tokamak is investigated theoretically. Lower hybrid frequency waves are utilized to drive non-inductive current for steady state tokamaks and ion toroidal rotation is used to control disruptions and improve confinement. It has been observed in many tokamaks that lower hybrid waves can change the ion toroidal rotation. These measurements indicate that it may be possible to control rotation with lower hybrid waves, but to do it, it is necessary to understand the mechanisms underlying the rotation change. The toroidal angular momentum injected by the lower hybrid waves initiates acceleration in the the counter-current direction. The parallel and perpendicular components of the toroidal angular momentum are transferred from the waves to ions through electrons via two different channels, and the ions obtain the full toroidal angular momentum injected by the lower hybrid waves after several ion collision times. The momentum transferred to the ions is transported out by turbulent radial transport. The radial transport of toroidal angular momentum is evaluated using gyrokinetics corrected to the higher order in poloidal rhostar. The higher order corrections lead to momentum redistribution even in the absence of rotation, which is called intrinsic momentum transport. The intrinsic momentum transport due to diamagnetic effects is an important piece of the radial momentum transport. The change in the steady state rotation due to lower hybrid waves is estimated theoretically by evaluating the momentum source, the momentum pinch and diffusion, and the intrinsic momentum transport. The effect of the current profile on the intrinsic momentum transport, which is modified by the lower hybrid wave, may explain the reversal of the rotation change from counter-current direction to co-current direction observed in low plasma current discharges in Alcator C-Mod. / by Jungpyo Lee. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 166-175). / Performing nuclear reactor core physics analysis is a crucial step in the process of both designing and understanding nuclear power reactors. Advancements in the nuclear industry demand more accurate and detailed results from reactor analysis. Monte Carlo (MC) eigenvalue neutron transport methods are uniquely qualified to provide these results, due to their accurate treatment of space, angle, and energy dependencies of neutron distributions. Monte Carlo eigenvalue simulations are, however, challenging, because they must resolve the fission source distribution and accumulate sufficient tally statistics, resulting in prohibitive run times. This thesis proposes the Low Order Operator (LOO) acceleration method to reduce the run time challenge, and provides analyses to support its use for full-scale reactor simulations. LOO is implemented in the continuous energy Monte Carlo code, OpenMC, and tested in 2D PWR benchmarks. The Low Order Operator (LOO) acceleration method is a deterministic transport method based on the Method of Characteristics. Similar to Coarse Mesh Finite Difference (CMFD), the other acceleration method evaluated in this thesis, LOO parameters are constructed from Monte Carlo tallies. The solutions to the LOO equations are then used to update Monte Carlo fission sources. This thesis deploys independent simulations to rigorously assess LOO, CMFD, and unaccelerated Monte Carlo, simulating up to a quarter of a trillion neutron histories for each simulation. Analysis and performance models are developed to address two aspects of the Monte Carlo run time challenge. First, this thesis demonstrates that acceleration methods can reduce the vast number of neutron histories required to converge the fission source distribution before tallies can be accumulated. Second, the slow convergence of tally statistics is improved with the acceleration methods for the earlier active cycles. A theoretical model is developed to explain the observed behaviors and predict convergence rates. Finally, numerical results and theoretical models shed light on the selection of optimal simulation parameters such that a desired statistical uncertainty can be achieved with minimum neutron histories. This thesis demonstrates that the conventional wisdom (e.g., maximizing the number of cycles rather than the number of neutrons per cycle) in performing unaccelerated MC simulations can be improved simply by using more optimal parameters. LOO acceleration provides reduction of a factor of at least 2.2 in neutron histories, compared to the unaccelerated Monte Carlo scheme, and the CPU time and memory overhead associated with LOO are small. / by Lulu Li. / Ph. D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009. / 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 (p. 99-101). / Pursuant to a 1986 NRC ruling, the MIT Reactor (MITR) is planning on converting from the use of highly enriched uranium (HEU) to low enriched uranium (LEU) for fuel. Prior studies have shown that the MITR will be able to operate using monolithic U-Mo LEU fuel while achieving neutron fluxes close to that of an HEU core. However, to date, detailed studies on fuel management and burnup while using LEU fuel have not been performed. In this work, a code package is developed for performing detailed fuel management studies at the MITR that is easy to use and is based on state-of-the-art computational methodologies. A wrapper was written that enables fuel management operations to be modeled using MCODE, a code developed at MIT that couples MCNP to the point-depletion code ORIGEN. To explicitly model the movement of the control blades in the MITR as the core is being depleted, a criticality search algorithm was implemented to determine the critical position of the control blades at each depletion timestep. Additionally, a graphical user interface (GUI) was developed to automate the creation of model input files. The fuel management wrapper and GUI were developed in Python, with the PyQt4 extension being used for GUI-specific features. The MCODE fuel management wrapper has been shown to perform reliably based on a number of studies. An LEU equilibrium core was modeled and burned for 640 days with the fuel being moved in the same pattern every 80 days. The control blade movement and nuclide concentrations were shown to be in agreement with what one would intuitively predict. The fuel management capabilities of REBUS-PC and the MCODE fuel management wrapper were compared by modeling the same refueling scheme using an HEU core. The element power peaking factors for the two models showed remarkable agreement. Together, the fuel management wrapper and graphical user interface will help the staff at the MITR perform in-core fuel management calculations quickly and with a higher level of detail than that previously possible. / by Paul K. Romano. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 319-325). / A Motional Stark Effect (MSE) diagnostic system has been installed on the Alcator C-Mod tokamak to measure the plasma internal magnetic pitch angle profile. The diagnostic utilizes polarization patterns from Doppler-shifted Balmer-alpha decay emission from an energetic neutral beam injected into a magnetically confined plasma. This dissertation consists of three parts: (1) the current status of the C-Mod MSE diagnostic which includes major upgrades in the hardware and calibration techniques; (2) the elimination of the spurious drift in the polarization measurements due to thermal-stress induced birefringence; and (3) the measurement of current density profiles in Lower Hybrid Current Drive (LHCD) experiments. The major hardware upgrades include replacement of photomultiplier tubes (PMT's) with avalanche photodiodes (APD's) which enhanced the quantum efficiency; installation of a wire-grid polarizer to verify small Faraday rotation in the diagnostic; installation of steep edge filters to minimize pollution by the thermal Balmer-alpha signals; rotation of the Diagnostic Neutral Beam (DNB) which significantly reduced the anomalous effect from the secondary beam neutrals during the beam-into-gas calibrations. The new calibration techniques include two plasma calibrations: plasma current sweeping and the plasma size sweeping whose feasibility was experimentally proven; and an absolute intensity calibration which measured the real optical throughput of the system. A large database study indicates the signal-to-background ratio larger than 100 is required to have the measurement uncertainty under 0.1 degrees. / (cont.) The spurious drift in the measurement has been identified as the thermals tress induced birefringence imposed on the in-vessel lenses. By modeling this effect as a single wave plate, an in-situ calibration method has been proposed and its feasibility was experimentally verified. Based on the experiments that characterized the thermal response of the system, a single-layer heat shield with gold plating and a lens holder which reduces the thermal conduction path to the lens have been designed and fabricated. A more rigorous model that includes an intrinsic phase shift by mirrors reveals the thermal phase shift can be greatly magnified by the intrinsic phase shift. The current density profiles from LHCD experiments have been obtained from the MSE data corrected by a baseline magnetic equilibrium whose internal profile is constrained by the sawtooth inversion radius. The resultant profiles successfully demonstrate several standard predictions of LHCD theory such as the dependence of efficiency on the parallel refractive index and the off-axis current drive. / by Jin-Seok Ko. / Ph.D.

Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2015. / 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 153-160). / The US Navy has long used highly enriched uranium (HEU) in naval reactors for a variety of technical reasons. In a series of studies, the Department of Naval Reactors determined that switching to low enriched uranium (LEU) was impossible using current fuel designs, but may be possible with a dedicated program to investigate new fuel materials. This thesis simulated an HEU fueled submarine reactor using a uranium oxide-zirconium dispersion fuel, and compared it to an LEU reactor using a uranium-molybdenum alloy fuel. The required energy output of an attack submarine was used to set the burnup requirement of the HEU (333 MWd/kg) and LEU (93.5 MWd/kg) fueled reactors, and each reactor was depleted to the end of life. The results showed that naval reactors could be switched to LEU without sacrificing the lifetime submarine core or increasing reactor volume. Even if unstudied technological details render this impossible, an LEU core would require only a single refueling over the life of an attack submarine. This would necessitate a 3.25% increase in submarine fleet size, which is small compared to the average Department of Defense project cost overrun. / by Brendan Patrick Hanlon. / S.M.

Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 75-78). / Since the establishment of nuclear physics in the early 1900's and the development of the hydrogen bomb in the 1950's, inertial confinement fusion (ICF) has been an important field in physics. Funded largely though the U.S. National Nuclear Security Agency (NNSA), the Laboratory for Laser Energetics (LLE), Sandia National Laboratories (SNL) and Lawrence Livermore National Laboratory (LLNL) have advanced ICF as a platform for stockpile stewardship and weapons physics, but also have contributed to basic science in high energy density regimes and for pursuing fusion an energy source. One of the primary goals of the ICF research program is to produce a thermonuclear burn in an ICF capsule where the power balance of the reaction is net positive. This criterion is often referred to as ignition. One of the most common metrics for gauging progress towards ignition in an ICF implosion is the ITFX parameter (similar to the Lawson Criterion) and is primarily a function of the implosion areal density (pR) and fusion yield. An ITFX value greater than one indicates net energy production. In deuterium/tritium fuel mixtures the yield is determined by measuring the reactant 14.0 MeV neutrons. Subsequently, the ability to obtain highly accurate absolute neutron yield measurements is vital to determining the ITFX and hence progress toward ignition. Although ignition implosions all use deuterium/tritium fuel mixes, other capsule fuel mixes such as pure deuterium and deuterium/helium 3 are also used to improve understanding of capsule performance. At the LLE and LLNL, neutron time-of-flight (nTOF) detectors routinely measure the absolute neutron yield from laser-driven ICF implosions. Although originally calibrated through a series of cross-calibrations with indium and copper neutron activation systems, an alternative method has been developed for measuring the DDn yield that provides a more accurate calibration by directly calibrating nTOF in situ to CR-39 range filter (RF) proton detectors. A neutron yield can be inferred from the CR-39 RF proton measurement since the DD proton and DD neutron branching ratio is well characterized and close to unity. By obtaining highly accurate DDp yields from a series of exploding pusher campaigns on OMEGA, an excellent absolute DDn yield measurement was obtained and used to calibrate the 3m nTOF detector. Data obtained suggest the existing OMEGA nTOF calibration coefficient to be low by 9.0 1.5 % based on the inferred CR-39 DD neutron yield. In addition, comparison across multiple exploding pusher campaigns indicate that significant reduction in charged particle flux anisotropies can be achieved on shots where capsule bang time occurs significantly (on the order of 500ps) after the end of the laser pulse. This is important since the main source of error in the RF DDp yield measurement is due to particle flux anisotropies. Results indicate that the CR-39 RF/nTOF in situ calibration method can serve as a valuable platform for measuring the DDn yield from ICF implosions and for calibrating and reducing the uncertainty of calibration coefficients of nTOF detector systems on OMEGA and other larger facilities such as the National Ignition Facility (NIF). / by Caleb J. Waugh. / S.M.

Thesis: Ph. D., 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 147-153). / Precise knowledge and understanding of the multiphase flow distribution is essential for light water reactor design. Multiphase Computational Fluid Dynamics (M-CFD) modeling and simulation techniques provide three-dimensional resolution of complex flow structures, which can be used to improve operation and safety in current systems, while driving optimization and performance enhancement in next generation designs. Introducing bubbles into liquid flow dramatically modifies the turbulent kinetic energy profile. Examination of experimental and Direct Numerical Simulation (DNS) research reveals a complicated, incomplete, and conflicting picture of bubble-induced turbulence (BIT). Incorporating these physical mechanisms into a BIT model compatible within the Eulerian-Eulerian framework remains a formidable challenge. Two-equation BIT models share a general formulation, manifesting as additional source terms in traditional turbulence models to account for production and dissipation of bubble-induced turbulence. Existing formulations struggle with reliably predicting the turbulent kinetic energy profile, routinely yielding non-physical results that subsequently worsen mean flow predictions. The present work encompasses two research objectives that include (1) advancing the understanding of the complex effects bubbles pose on liquid turbulence, and (2) proposing an approach to incorporate these physical phenomena into a BIT closure relation. Greater understanding of two-phase turbulent mechanisms is advanced through statistical analysis of upward bubbly channel flow DNS data generated by Bolotnov and Lu/Tryggvason. The impact of bubble deformability on the resulting turbulent distributions, energy budgets, and scales are quantified and examined. A methodology that incorporates these fundamental mechanisms into a new BIT model is proposed. The closure comprises five components that include new turbulent viscosity and time-scale formulations in addition to optimized values for the modulation parameter, dissipation coefficient, and newly proposed turbulent viscosity multiplier. Model performance and improvement is confirmed through simulation of the entire Liu (1989) experimental database and comparison with existing closures. The model is incorporated into the Bubbly And Moderate void Fraction (BAMF) formulation (Sugrue et al., 2017) in order to deliver best practices and guidelines for application of momentum closures with BIT modeling. This is accomplished through redefinition of the Wobble number, calibration of expressions for the turbulent dispersion coefficient and lift inversion function, and assessment via simulation of experimental databases. / by Benjamin Lawrence Magolan. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012. / "June 2012." Cataloged from PDF version of thesis. / Includes bibliographical references (p. 39-40). / The global political climate has called for reductions to nuclear arsenals around the world. This thesis researches how potential deep cuts to the United States' large strategic nuclear arsenal would affect its current nuclear deterrence goals. First, case studies on pre-1960 United States, 1964-2012 France, and 1964-2012 China are conducted to understand how a small nuclear arsenal should be constructed in order to prevent nuclear attack from countries with large nuclear arsenals. The lessons learned from these case studies, the current United States deterrence requirements, and the destructive effects from different warheads are then used to propose a potential composition of a small nuclear arsenal for the United States. The proposal consists of only around 500 warheads (in comparison to the current 2,000 the US has on deployment) and achieves United States deterrence goals through its vast destructive capability, variability, and survivability if targeted against in a first nuclear strike. / by Sarah Laderman. / S.B.