Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2002. / Includes bibliographical references (leaves 185-188). / In this thesis, a self-consistent model for analyzing the transport performance of a Reversed Field Pinch (RFP)-type of thermonuclear fusion reactor has been developed. The study has been focused on determining equilibrium configurations which describe a plasma evolution dominated by particular instabilities originated by plasma electrical resistivity (tearing-, or resistive interchange, modes). The ultimate goal is to provide a model of transport scaling in an RFP which can evaluate the global parameters describing the plasma confinement performance. Starting from a magnetic field configuration essentially given by Taylor's relaxation model, the self-consistent pressure profile is determined by assuming that the ohmic heating source raises the plasma pressure until the profile is locally marginally stable to tearing modes. A critical point here is the long held belief that an RFP, because of its bad curvature, would always be unstable to tearing or resistive interchange modes; that is, no marginally stable state exists. This belief turns out to be untrue. The basis for this statement is a careful ordering of the resistive layer dynamics, showing that thermal conductivity dominates over convection and compressibility. Thus, the use of the adiabatic equation of state in earlier work is not accurate for an RFP. / (cont.) As a result, tearing and interchange modes can indeed be stabilized in an RFP. In this model, a proper, selfconsistent definition of tearing-mode marginality has been used as a prescription for building the pressure profile. The actual numerical determination of the marginally stable profiles can be solved by using state-of-the-art personal computers. It is worth emphasizing that there are no free parameters in the model. Point checks indicate reasonable agreement with typical experimental data. Parametric numerical studies are also shown, spanning the operational space of RFP experiments, and finally providing the tearing mode transport scaling relations for the global confinement parameters. Comparisons with experiments as well as other transport models are shown. / by Antonio Bruno. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2002. / Includes bibliographical references (leaves 107-115). / The myocardial structure-function relation is the key to understand the functional design of the ventricular myocardium. While the function of myocardial fibers has been extensively studied, the recently observed laminar organization (sheets) of myocardial fibers is not well understood. This thesis establishes noninvasive MRI methods, registered cardiac diffusion and strain MRI, to acquire information about myocardial sheet structure and myocardial strain under identical in vivo conditions, and thus defines the functional role of myocardial sheets. This methodology solves limitations of existing methods that require postmortem dissection, and can be applied to living humans at multiple time horizons in the cardiac cycle. To establish valid MRI methods to map myocardial structure, we present a new method for diffusion MRI in the beating heart that is insensitive to cardiac motion and strain. Using phantom and in vivo validations, we demonstrate that this method addresses the problem of motion sensitivity of diffusion MRI in the beating heart. We also map myocardial sheet and fiber structure during systole in normal humans and find evidence that sheet architecture undergoes remarkable changes during contraction. To establish valid MR methods to quantify myocardial strain, we present a new 3D phase contrast strain imaging using single-shot 3D EPI. Compared to previous phase contrast 3D strain methods, the new method realizes potential sensitivity of phase contrast EPI. It significantly improves image quality regarding noise and artifact, requires much shorter acquisition time, and can be quickly and automatically processed without operator supervision. Following validation using a strain phantom, / (cont.) we demonstrate the validity of our 3D single-shot strain method in healthy in vivo human heart and brain. Using these methods, we acquire registered diffusion and strain MRI to provide quantitative maps of myocardial structure-function relations in living humans. From these quantitative maps, we are able to define for the first time accurate images of the functional role of myocardial sheets. We find that myocardial sheets contribute to ventricular thickening through all three cross-fiber strain components: sheet shear, sheet extension, and by previously undocumented sheet-normal thickening. Each of these mechanisms demonstrates remarkable spatial non-uniformity as well as inter-subject variability. In all cases, the contributions to thickening of fiber strains are small. Sheet function in normal humans is found to be heterogeneous and variable, contrasting with the previously demonstrated uniformity of fiber shortening. Future studies on myocardial structure-function relations must investigate the causes and extent of such heterogeneous properties of the myocardium. This thesis shows that MRI is a valid and effective tool for noninvasive study of myocardial mechanical function in humans. / by Jiangang Dou. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (p. 194-200). / The final outcome of a nuclear Probabilistic Risk Assessment (PRA) is generally inaccurate and imprecise. This is primarily because not all risk contributors are addressed in the analysis, and there are state-of-knowledge uncertainties about input parameters and how models should be constructed. In this thesis, we formulate two measures, risk significance (RS) and risk change significance (RCS) to examine these drawbacks and assess the adequacy of PRA results used for risk-informed decision making. The significance of an event within a PRA is defined as the impact of its exclusion from the analysis on the final outcome of the PRA. When the baseline risk is the final outcome of interest, we define the significance of an event as risk significance, measured in terms of the resulting percentage change in the baseline risk. When there is a proposed change in plant design or activities and risk change is the final outcome of interest, we define the significance of an event as risk change significance, measured in terms of the resulting percentage change in risk change. These measures allow us to rank initiating events and basic events in terms of relative importance to the accuracy of the baseline risk and risk change. / (cont.) This thesis presents general approaches to computing the RS and RCS of any event within the PRA. Our significance measures are compared to traditional importance measures such as Fussell-Vesley (FV), Risk Achievement Worth (RAW), and Risk Reduction Worth (RRW) to gauge their effectiveness. We investigate the use of RS and RCS to identify events that are important to meet the decision maker's desired degree of accuracy of the baseline risk and risk change. We also examine the use of 95th confidence level acceptance guideline for assessing the adequacy of the uncertainty treatment of a PRA. By comparing PRA results with the desired accuracy and precision level of risk and risk change, one can assess whether PRA results are adequate enough to support risk-informed decisions. Several examples are presented to illustrate the application and advantages of using RS and RCS measures in risk-informed decision making. We apply our frame- work to the analysis of the component cooling water (CCW) system in a pressurized water nuclear reactor. This analysis is based upon the fault tree for the CCW system presented in the plant's PRA analysis. / (cont.) One result of our analysis is an estimate of the importance of common cause failures of the CCW pumps to the accuracy of plant core damage frequency (CDF) and change in CDF. / by Yingli Zhu. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (p. 189-193). / Fuel performance models have been developed to assess the performance of internally and externally cooled LWR annular fuel. Such fuel may be operated at 30-50% higher core power density than the current operating LWRs, and to a burnup of 80-100MWd/kgU. The models are used to optimize the fuel design so that it is able to achieve high power density and high burnup, and to identify the features of this fuel that will impact its operation limits. The annular fuel performance codes have been developed based on the NRC licensed FRAPCON-3 code with major modifications to the code structure and with implementation of new fuel performance models. A heat split calculation was enabled by adding a heat flux iteration loop. The radial power peaking and the rim effects at both the inner and outer fuel surfaces have been modeled by a modified radial power/burnup fit to the neutronic calculations. The temperature profile calculation method was updated with new boundary conditions and meshing scheme to capture the internal cooling and the double power peaking at the rims. The annular fuel performance codes are able to simulate both sintered annular fuel and Vibration- Packing (VIPAC) fuel with internal and external cooling. / (cont.) For the sintered annular fuel, the anchor ring location of fuel thermal expansion is determined to be the innermost ring, and the fuel dimensions are calculated considering the effects of thermal expansion, swelling and densification. Fuel relocation is assessed via a new empirical model that has been implemented in the code. A fuel cladding mechanical interaction model has been developed with three regimes: the free standing cladding regime, the single closure regime and the fuel cladding full contact regime. The interaction mechanisms for each regime are analyzed and solutions are provided. A low temperature fission gas release model is implemented for sintered annular fuel by taking into account the double surface effects. It is found that the sintered annular fuel rod has lower fission gas release than that of a solid IPWR rod at the same power density. The cladding hydrogen concentration and the oxide accumulation of the annular fuel are comparable to those of the solid fuel due to comparable cladding heat flux and irradiation. Fuel gap conductance asymmetry caused by outward thermal expansion has been identified as a major concern due to its potential effects on NMDNBR. / (cont.) A sensitivity study has been performed to evaluate the impact of fuel parameters on fuel performance. The gap asymmetry problem can be circumvented by combining several approaches including: (1) allowing a larger outer gap and a smaller inner gap, (2) enlarging the fuel and cladding surface roughness, and (3) spattering the fuel surface with ZrO₂ particles. The optimized sintered annular fuel shows great potential for achieving high burnup (up to 86MWd/kgU rod average) and operating at 150% power density without compromising fuel safety. A VIPAC annular fuel performance model has also been developed and implemented. An empirical VIPAC fuel thermal conductivity formula has been developed as a function of temperature, burnup, porosity and gas pressure. A fuel-clad interface thermal conductance model has been developed incorporating the effects of fuel particle size, gas pressure and interfacial pressure. The VIPAC fuel bulk dimensional changes have been calculated assuming the same bulk thermal expansion and densification coefficient as the sintered fuel, but the fission product swelling is assumed to be accommodated by porosity. / (cont.) The VIPAC fuel- cladding mechanical interaction model applies the full gap closure regime of the pelletized annular fuel model except that fuel cladding "slippage" is assumed. An athermal fission gas release model for VIPAC is developed incorporating the surface and resolution effects of the fuel particles. Compared with the sintered fuel, the gap conductance imbalance problem is completely eliminated for VIPAC fuel but the fission gas release fuel is higher. The VIPAC annular fuel EOL cladding oxide thicknesses and hydrogen concentrations are comparable with those of the sintered annular fuel but the VIPAC annular rod cladding strains are significantly reduced. A sensitivity study of the important parameters of the VIPAC fuel identified that the optimum initial helium gas pressure is within 1.4-2.0 MPa and the optimum particle size is within 300- 600 [mu] m. The optimum smear density is in the range of 85%-90%. A preliminary study of Reactivity Initiated Accidents behavior involving the annular fuel has been made. During a RIA, the annular fuel peak enthalpy is found to be comparable to the solid fuel. However the permanent hoops strain of the sintered annular fuel is lower, and of the VIPAC annular fuel is higher, than that of the solid fuel. / (cont.) Analyses of the annular fuel testing at the MIT reactor and planning of the annular fuel post irradiation examination are also presented. / by Yi Yuan. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (leaves 117-124). / The nuclear industry has relied on the concept of Defense in Depth (DID) and traditional safety margins to deal with the uncertainties associated with the design and operation of nuclear facilities. These concepts were formulated in the early days of development of nuclear power when these uncertainties could not be quantified. The subsequent development of Probabilistic Risk Assessment (PRA) has provided the analytical tools that allow the quantification of uncertainties associated with accident initiation and progression. But, while the impact of redundancy has been explicitly modeled and quantified, the role of safety margins is still not explicitly taken into account. The present work identifies the impact of safety margins in the PRA and proposes a methodology to quantify them. Practical examples are developed and discussed in two case studies. In the first study, we analyze the passive cooling of a gas-cooled fast reactor and we use an importance sampling Monte Carlo technique to propagate the epistemic uncertainties and to calculate the overall probability of failure. A comparison with an alternative active design is considered also. The results show that the active system can have, for this particular application, better reliability than the passive one. / (cont.) An approach to derive the uncertainty distribution on the capacity is presented in the second case study, where computer simulations are performed to propagate uncertainties and to derive a probabilistic failure limit for high burnup fuel in a PWR rod ejection accident. / by Lorenzo P. Pagani. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005. / Includes bibliographical references (leaves 59-60). / This paper discusses a possible approach to ranking geographic regions that can influence multiple infrastructures. Once ranked, decision makers can determine whether these regions are critical locations based on their susceptibility to terrorist acts. We identify these locations by calculating a value for a geographic region which represents the combined values to the decision makers of all the infrastructures crossing through that region. These values, as well as the size of the geographic regions, are conditional on a minor destructive threat of a given size, e.g,. a bomb that can affect objects within 15 feet of it. This approach first requires an assessment of the users of the system. During this assessment, each user is assigned a performance index (PI) based on the disutility of the loss of each infrastructure's resource via multi-attribute utility theory (MAUT). A Monte Carlo network analysis is then performed to develop importance measures (IM) for the elements of each infrastructure for their ability to service each user. We combine the IMs with the user PIs to a value that we call valued worth (VW) for each infrastructure's elements independently. / (cont.) Then we use spatial analysis techniques within a Geographic Information System (GIS) to combine the VWs of each infrastructure's elements in a geographic area, conditional on the threat, into a total value we call geographic valued worth (GVW). The GVW is graphically displayed in the GIS system in a color scheme that shows the numerical ranking of these geographic areas. The map and rankings are then submitted to the decision makers to better allocate anti-terrorism resources. A case study of this methodology is preformed on the Massachusetts Institute of Technology's (MIT) campus. The results of the study show how the methodology can bring attention to areas that may be ignored through individual infrastructure analysis. The intersections of major infrastructures on the campus prove to be of the most importance to the stakeholders of the campus. / by Sean Albert Patterson. / S.M.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1986. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Includes bibliographies. / by David Andrew Petti. / Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1988. / by Peter A. Donis. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001. / Includes bibliographical references (leaves 161-167). / Boron neutron capture therapies using the 10B(n,a)7Li reaction have been proposed as treatments for glioblastoma multiforme, metastatic melanoma, rheumatoid arthritis, and other debilitating conditions. This thesis presents the first combined biological and physical dosimetry interbeam comparison data of three neutron beams used in boron neutron capture therapies: the Massachusetts Institute of Technology (MIT) and Brookhaven National Laboratory epithermal neutron beam facilities previously used in Phase I/II human clinical trials of boron neutron capture therapy (BNCT), and the boron neutron capture synovectomy (BNCS) facility at MIT. The biological dosimetry methodology developed included in vitro irradiation of rodent cells at various depths in a water-filled phantom that simulated healthy tissue. These experiments evaluated the biological effectiveness of the neutron and photon components since no boron was present. Cell survival at a given dose was dependent upon the depth in the phantom as a result of moderation and attenuation of the beam components by overlying water. Results were compared with 250 kVp X-ray irradiations to determine relative biological effectiveness (RBE) values of the beams; neutron RBE values were calculated from the beam RBE values. / by Susan Marie White. / Ph.D.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001. / Includes bibliographical references (p. 281-289). / Remote sensing techniques employed to diagnose ionospheric modification experiments are intrinsically ambiguous, uncorrelated with "ground truth." To overcome this limitation, laboratory experiments are performed in the model ionosphere of the Versatile Toroidal Facility (VTF). The VTF contains a thermionically produced, weakly magnetized ( wce < wpe) background plasma of either hydrogen or argon. The HF "pump" wave of ionospheric experiments is modeled by 2.45 GHz microwaves, launched perpendicular to the magnetic field and the density gradient of the VTF in the ordinary mode. The peak plasma density is several times greater than the critical density (nc ~/= 7.4xI0 16 m-3 ), and the microwaves reflect, forming a standing wave Airy pattern. Wave spectra produced near reflection are measured using a miniature double probe and microwave receiver along with a fast oscilloscope. This combination is capable of simultaneously measuring spectra in two 250 MHz bands, one near DC and the other near the 2.45 GHz pump, to μs resolution. In addition, absolute electric field strengths and wavenumber spectra can be estimated. To explore the extent to which the VTF experiments simulate ionospheric heating, similarity rules are derived from the governing equations and applied to the two plasmas. A set of ten dimensionless parameters results, six of which match satisfactorily between the two plasmas. Three others can be neglected, leaving only one unmatched parameter: the ratio T/Ti, which in the VTF is about 12 and in the ionosphere is near unity. Consideration of boundary conditions limits the scope of the simulation to the first Airy maximum. The main observational results of VTF heating experiments are: (1) Langmuir wave sidebands both up- and down-shifted from the pump frequency that decrease monotonically to the noise floor in tens of MHz, (2) lower hybrid waves in a broad band from 35 - 150 MHz, with maximum power occurring at 50 - 90 MHz, (3) both Langmuir and lower hybrid waves appear in bursts of duration and period in the 2- 100 ms range, depending upon radius, (4) Langmuir and lower hybrid bursts are anti-correlated at the edge of the plasma but become uncorrelated in the core, and (5) the electric field, both of the pump and the plasma sidebands, varies by a factor of 100 in a burst period, from 1.3 to 130 kV /m for the pump (expected: 10.8 kV/m). The main features of ionospheric heating were reproduced in these experiments: down- and up-shifted high frequency sidebands, extreme time-variability of electric field amplitude, large pump wave absorption, and significant electron heating. The observed spectral bursts suggest the concentration of electric field into small time-varying regions. The periods and parameter dependencies of the bursts resemble results of three-dimensional simulations of Langmuir turbulence. However, the upshifted Langmuir waves predicted by strong Langmuir turbulence (SLT) and nonlinear scattering theory are not observed in the VTF. A consistent account of the VTF observations is obtained by combining the caviton collapse cycle of SLT and the parametric production of lower hybrid waves by energetic Langmuir waves. As the high frequency electric field concentrates in cavitons, the threshold for the Langmuir decay instability is exceeded, generating lower hybrid waves in anti-correlated bursts. Because of the similarity of the VTF experiments to ionospheric heating, the observation of lower hybrid wave production during heating may also be borne out by future field experiments with diagnostics capable of viewing field-aligned modes. / by Nathan E. Dalrymple. / Sc.D.