Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 149-162). / Plasma-material interactions (PMI) in magnetic fusion devices such as fuel retention, material erosion and redeposition, and material mixing present significant scientific and engineering challenges, particularly for the next generation of devices that will move towards reactor-relevant conditions. Achieving an integrated understanding of PMI, however, is severely hindered by a dearth of in-situ diagnosis of the plasma-facing component (PFC) surfaces. To address this critical need, this thesis presents an accelerator-based diagnostic that nondestructively measures the evolution of PFC surfaces in-situ. The diagnostic aims to remotely generate isotopic concentration maps that cover a large fraction of the PFC surfaces on a plasma shot-to-shot timescale. The diagnostic uses a compact, high-current radio-frequency quadrupole accelerator to inject 0.9 MeV deuterons into the Alcator C-Mod tokamak. The tokamak magnetic fields in between plasma shots are used to steer the deuterons to PFCs where the deuterons cause high-Q nuclear reactions with low-Z isotopes ~5 [mu]m into the material. Scintillation detectors measure the induced neutrons and gammas; energy spectra analysis provides quantitative reconstruction of surface concentrations. An overview of the diagnostic technique, known as accelerator-based in-situ materials surveillance (AIMS), and the first AIMS diagnostic on the Alcator C-Mod is given; a description of the complementary simulation tools is also provided. Experimental validation is shown to demonstrate the optimized beam injection into the tokamak, the quantification of PFC surfaces isotopes, and the measurement localization provided by magnetic beam steering. Finally, the first AIMS measurements of fusion fuel retention are presented, demonstrating the local erosion and codeposition of deuterium-saturated boron surface films. The finding confirms that deuterium codeposition with boron is insufficient to account for the net fuel retention in Alcator C-Mod. / by Zachary Seth Hartwig. / Ph. D.

Thesis: S.M., 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. Page 105 blank. / Includes bibliographical references (pages 99-104). / The Fukushima Nuclear Power Plant (NPP) accident in Japan has motivated improving the safety of current light water reactors (LWRs). Accident tolerant fuels (ATF) are being developed to enhance the safety of LWRs by tolerating loss of active cooling in the core for a longer duration compared to standard UO₂ and Zirconium-based claddings. In this work, high-temperature steam oxidation characteristics of potential ATF claddings, monolayer iron-chromium-aluminum (FeCrAl) and Cr-coated Zircaloy, are experimentally investigated. Specifically, this work investigates the high-temperature oxidation characteristics of FeCrAl alloy after exposure to 1000-1400 °C steam for 1 hour. A model for oxidation of FeCrAl alloy was developed based on the measured weight gain. The severe degradation of the FeCrAl samples from the steam attack was observed at ~1400 °C. Experimental investigation of ATF claddings also included high-temperature oxidation of Cr-coated Zircaloy pressure tube. Post-test analysis showed that for some regions, the Cr-coating is still present after 90 minutes of exposure to 1200°C steam, protecting the Zircaloy substrate beneath the coated layer. The performances of the FeCrAl and Cr-coated ATF claddings under beyond design basis accidents (BDBA) are modeled with thermal-hydraulics design basis code TRACE. A 3-loop Pressurized Water Reactor (PWR) model is created and the following BDBAs are simulated for this study: large break loss of coolant accident (LOCA) without safety injection systems, short-term station blackout (SBO) without any mitigation actions from the beginning and long-term SBO with auxiliary feedwater flow for the first 24 hours and the no mitigation actions afterwards. Two models are used for high-temperature oxidation of FeCrAl: the MIT model based on the experimental results of this work, and the Oak Ridge National Laboratory (ORNL) model based on experimental results of ORNL's work. The results showed that ATF claddings increase the coping time and produce less hydrogen compared to Zircaloy cladding under the considered BDBAs scenarios. / by Anil Gurgen. / S.M.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 34-36). / Based on extracellular field recordings and stimulations at the Schaeffer collateral-CA1 synapse, the synaptic tagging and capture (STC) model has hypothesized that at synapses that express any form of LTP and LTD (long-term potentiation and depression, respectively) are tagged in a protein synthesis-independent manner, the induction of LLTP/ L-LTD leads to protein synthesis, and all tagged synapses can use the resulting plasticity-related products to express L-LTP/L-LTD. Several models have hypothesized that STC works through somatically synthesized plasticity-related protein products available to synapses throughout the neuron, suggesting that, at the single neuronal level, memory engrams are formed at synapses throughout the dendritic arbor. However, the Clustered Plasticity Hypothesis suggests that neurons store long-term memory engrams at synapses that tend to be spatially clustered within dendritic branches, as opposed to dispersed throughout the dendritic arbor. This hypothesis suggests that the dendritic branch, as opposed to the synapse, is the primary unit for long-term memory storage. Evidence for this hypothesis has come from studies of LTP, however, and there is no such data for LTD. This thesis establishes a single-synapse marker for LTD, namely spine length changes, that can be used to study the role of LTD and dendritic branch-specific plasticity. / by Alexander J. Murphy. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 71-72). / In his MIT Doctoral thesis on an inverted Hydride-fueled reactor concept, Paolo Ferroni (2010) suggested using short-length twisted tape inserts in order to increase the critical heat flux (CHF) at which departure from nucleate boiling occurs. Shortlength twisted tapes were proposed in his study because they offer the benefits of increased heat transfer and CHF, but also introduce a smaller pressure drop than a full-length twisted tape insert. This thesis project investigated the effects of short-length twisted tape inserts on the pressure drop in round tubes. The objectives of the project were two-fold. The first objective was to characterize the development of swirl flow in sequential twisted tapes. The second objective was to create a correlation for the Darcy's friction factor as a function of the Reynolds number (Re), tape twist ratio (y), and tape spacing (s) which could be used for both Ferroni's work and other future work. To characterize developing flow, 6 test sections were constructed. Measurements were collected for the pressure drop at several sequential twisted tapes, and the resulting friction factors were compared for each tape module. The results showed at most ±10% difference in friction factor between tapes and no significant trends between friction factor and the axial sequence location (or index) of the tape. To develop a correlation, 21 test sections were constructed by P. Ferroni and the author. Pressure drop measurements were collected for conditions spanning 10000 < Re < 90000, 1.5 < y < 6, and s = 30, 40, 50. A correlation for the Darcy's friction factor (f) was developed through the separation of the three variables to find f oc Re-- , y 1 87 and s- 67. Tap water at room temperature and nearly atmospheric pressure was used throughout the experiments, and for all short-length twisted tapes the number of revolutions. Nev, was held constant at 1.5. / by Robert E Block. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014. / 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 249-252). / Fine-group whole-core reactor analysis remains one of the long sought goals of the reactor physics community. Such a detailed analysis is typically too computationally expensive to be realized on anything except the largest of supercomputers. Recondensation using the Discrete Generalized Multigroup (DGM) method, though, offers a relatively cheap alternative to solving the fine group transport problem. DGM, however, suffered from inconsistencies when applied to high-order spatial methods. Many different approaches were taken to rectify this problem. First, explicit spatial dependence was included in the group collapse process, thereby creating the first ever set of high-order spatial cross sections. While these cross sections were able to asymptotically improve the solution, exact consistency was not achieved. Second, the derivation of the DGM equations was instead applied to the transport equation once the spatial method had been applied, allowing for the definition of an exact corrective factor to drive recondensation to the exact fine-group solution. However, this approach requires excessive memory to be practical for realistic problems. Third, a new method called the Source Equivalence Acceleration Method (SEAM) was developed, which was able to form a coarse-group problem equivalent to the fine-group problem allowing recondensation to converge to the fine-group solution with minimal memory requirements and little additional overhead. SEAM was then implemented in OpenMOC, a 2D Method of Characteristics code developed at MIT, and its performance tested against Coarse Mesh Finite Difference (CMFD) acceleration. For extremely expensive transport calculations, SEAM was able to outperform CMFD, resulting in speed-ups of 20-45 relative to the normal power iteration calculation. Additionally, to address the growing interest in Krylov based solvers applied to reactor physics calculations, an energy-based preconditioner was developed that is inexpensive to form and can accelerate convergence. / by Matthew S. Everson. / Ph. D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 167-169). / An Inverted Fuel Pressurized Water Reactor (IPWR) concept was previously investigated and developed by Paolo Ferroni at MIT with the effort to improve the power density and capacity of current PWRs by modifying the core geometry. A detailed study was performed to optimize the IPWR design considering mechanics, thermal hydraulics and neutronics design constraints from which it was concluded that the maximum achievable power for the IPWR design was 4078MW, 19 percent higher than the reference PWR (the Seabrook Power Station), limited simultaneously by the core pressure drop and steady state departure from nucleate boiling (DNB) constraints. While the thermal power is already higher than that of typical pressurized water reactors (PWRs), it is still possible to achieve higher power by improving the DNB performance of the design. Unlike the conventional pin geometry in current PWRs, the inverted geometry opens the possibility to improve the core DNB performance by using swirl flow promoters. This thesis further takes advantage of the new core geometry to increase the core power density by using twisted tapes (TTs) as swirl flow promoters inside the IPWR cooling channels. The study focuses on optimizing the cooling channel design with twisted tapes to improve the DNB performance alongside using more powerful reactor coolant pumps to deliver higher core pressure drop limit. Four steady state design constraints, which are core pressure drop, DNB, peak fuel temperature and peak cladding temperature, are considered. As the core power rating is gradually increased from the reference value (3411 MWt), the steady state operating parameters can be calculated using Ferroni's IPWR analyzing tool and Arment's pressure drop and DNB correlations. The maximum achievable core power is determined when one of the design constraint reaches its limit value. Various options of IPWR cooling channel design, including the no TT (E-IPWR), full length TT (F-IPWR) and short length TT at a fixed location in the cooling channels (SF-IPWR), were investigated at different core inlet and outlet temperature conditions. Results show that the SFIPWR design offers the best performance in all cases. When using Ferroni's selected assembly geometry and operating with the AP1000 enthalpy condition, the SF-IPWR design can achieve the maximum core power of 4786 MWt, 140 percent of the reference core power, limited by the peak fuel temperature design constraint. By modifying the assembly geometry, higher power rating is achievable although more safety analyses would be needed to confirm the feasibility of operating at power rating higher than the reference plant full power value. / by Nghia T. Nguyen. / S.B.

Thesis: Sc. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2017. / 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 133-140). / Tungsten undergoes surface morphology changes on the nanometer scale when subjected to low energy helium ion bombardment. This is due in part to the ion bombardment causing tungsten atoms to move on the surface, but also because of helium implantation and bubble development in the near surface at a depth < 30 nm. At high enough surface temperatures, T/TM >/~ 0.2, where TM is the melting temperature, nanoscale tendrils form on the surface and grow longer with additional bombardment by helium, but will decompose at the same temperature without helium bombardment. A tungsten surface that develops a densely packed layer of nano-tendrils over macroscopic areas greater than the grain size is referred to as tungsten fuzz, and is under intense study in fusion energy research, both for better understanding of how tungsten fuzz forms and of how tungsten fuzz affects the performance of plasma-facing components. The necessity of helium irradiation of the surface to induce nano-tendril growth motivates investigation into the dynamic process of helium implantation and accumulation in the surface. In this thesis, in situ elastic recoil detection is developed and used to measure the dynamic concentration of helium within a tungsten surface during the active growth of tungsten fuzz. During the development of in situ elastic recoil detection analysis, a variant of tungsten nano-tendril growth was discovered featuring drastically isolated bundles of nano-tendrils that grow at a higher rate than tungsten fuzz. The variation in nano-tendril morphology is correlated with incident helium ion energy modulation. The dependence on ion energy modulation and isolated nature of the nano-tendril bundles reveals clearly that nano-tendril growth is sensitive to surface kinetic effects. In this thesis, the structure and parameter space of the newly discovered nano-tendril bundle growth is analyzed with a suite of electron-based surface science techniques. / by Kevin Benjamin Woller / Sc. D.

Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 57-60). / Abstract Advanced Zirconium based alloys used in the nuclear industry today, such as ZIRLOTM , M5 contain up to wt 1.2% Niobium [8]. Experimental effort to determine the effect of Nb on corrosion behaviour of these alloys has no clear answer to whether Nb improves or degrades the corrosion resistance[8, 48, 201. Even the charge state of Nb as a defect in zirconia is debated. Experimental findings of Froideval et al [5] indicate charge state between +2 and +4 whereas other authors assume it to be +5 [21, 31, 34]. In order to uncover the role of Nb on the local oxide protectiveness we employed ab initio Density Functional Theory (DFT) calculations, and assessed the effect of Niobium on the point defect equilibria in tetragonal zirconia which is critical in the oxide protectiveness [81 among other phases of zirconia. DFT calculated defect formation energies are adjusted for finite temperature effects by accounting for thermal vibrations. Adjusted defect formation energies are then used to construct Kroger-Vink diagram for defect equilibrium concentrations at applicable p02 levels. The Kr6ger-Vink diagrams for Nb containing zirconia was compared to that of pure t-Zirconia in order to isolate the changes due to Nb. Nb is treated as point defect in the oxide. Among the considered point defects and defect complexes of Nbzr ,Nbi, Nbzr - Vo and Nbzr - Oi, the substitutional point defect Nbzr was found to have the lowest free energy of formation, and the highest equilibrium concentration. Nb substitutional point defect, Nbzr, is found to be stable for Nb+3, Nb+4, Nb+5 charge states while Nb+5 has the highest concentration. The effect of applied external compressive strain on the energetics and stress of different types of defects, and formation energy is quantified as a function of strain. It is observed that the more positively charged the defect, the formation energy increases less as compressive strain is applied. Compared to pure T-ZrO2, ~100 times increase in Zirconium vacancy concentration accompanied by a ~5 times decrease in the doubly charged oxygen vacancy concentration was found due to the presence of Niobium in the high oxygen partial pressure (p02) regime corresponding to oxide/water interface. This change implies slowing down of oxygen diffusion from surface to bulk, while accelerating oxygen exchange on the surface. Diffusion of zirconium ion to the surface will also accelerate as available point defect concentration increases due to Nb. The increased concentration of Nbzr defect with increasing oxygen partial pressure is consistent with our experimental findings in a parallel work in our group[47]. / by Uuganbayar Otgonbaatar. / S.M.and S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007. / Includes bibliographical references (p. 151-161). / Quantum Information Processing (QIP) promises increased efficiency in computation. A key step in QIP is implementing quantum logic gates by engineering the dynamics of a quantum system. This thesis explores the requirements and methods of coherent control in the context of magnetic resonance for: (i) nuclear spins of small molecules in solution and (ii) nuclear and electron spins in single crystals. The power of QIP is compromised in the presence of decoherence. One method of protecting information from collective decoherence is to limit the quantum states to those respecting the symmetry of the noise. These decoherence-free subspaces (DFS) encode one logical quantum bit (qubit) within multiple physical qubits. In many cases, such as nuclear magnetic resonance (NMR), the control Hamiltonians required for gate engineering leak the information outside the DFS, whereby protection is lost: It is shown how one can still perform universal logic among encoded qubits in the presence of leakage. These ideas are demonstrated on four carbon-13 spins of a small molecule in solution. Liquid phase NMR has shortcomings for QIP, like the lack of strong measurement and low polarization. These two problems can be addressed by moving to solid-state spin systems and incorporating electron spins. If the hyperfine interaction has an anisotropic character, it is proven that the composite system of one electron and N nuclear spins (le-Nn) is completely controllable by addressing only to the electron spin. This 'electron spin actuator' allows for faster gates between the nuclear spins than would be achievable in its absence. In addition, a scheme using logical qubit encodings is proposed for removing the added decoherence due to the electron spin. Lastly, this thesis exemplifies arbitrary gate engineering in a le-ln ensemble solid-sate spin system using a home-built ESR spectrometer designed specifically for engineering high-fidelity quantum control. / by Jonathan Stuart Hodges. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 419-429). / The sodium fast reactor (SFR) is currently being reconsidered as an instrument for actinide management throughout the world, thanks in part to international programs such as the Generation-IV and especially the Global Nuclear Energy Partnership (GNEP). The success of these programs, in particular the GNEP, is dependent upon the ability of the SFR to manage actinide inventory while remaining economically competitive. In order to achieve these goals, the fuel must be able to operate reliably at high power densities. However, the power density of the fuel is limited by fuel-clad chemical interaction (FCCI) for metallic fuel, cladding thermal and irradiation strain, the fuel melting point, sodium boiling, and to a lesser extent the sodium pressure drop in the fuel channels. Therefore, innovative fuel configurations that reduce clad stresses, sodium pressure drops, and fuel/clad temperatures could be applied to the SFR core to directly improve the performance and economics. Two particular designs of interest that could potentially improve the performance of the SFR core are the internally and externally cooled annular fuel and the bottle-shaped fuel. In order to evaluate the thermal-hydraulic performance of these fuels, the capabilities of the RELAP5-3D code have been expanded to perform subchannel analysis in sodium-cooled fuel assemblies with non-conventional geometries. This expansion was enabled by the use of control variables in the code. When compared to the SUPERENERGY II code, the prediction of core outlet temperature agreed within 2%. In addition, the RELAP5-3D subchannel model was applied to the ORNL 19-pin test, and it was found that the code could predict the measured outlet temperature distribution with a maximum error of -8%. / (cont.) As an application of this subchannel model, duct ribs were explored as a means of reducing core outlet temperature peaking within the fuel assemblies. The performance of the annular and bottle-shaped fuel was also investigated using this subchannel model. The annular fuel configurations are best suited for low conversion ratio cores. The magnitude of the power uprate enabled by metal annular fuel in the CR = 0.25 cores is 20%, and is limited by the FCCI constraint during a hypothetical flow blockage of the inner-annular channel due to the small diameters of the inner-annular flow channel (3.6 mm). On the other hand, a complete blockage of the hottest inner-annular flow channel in the oxide fuel case results in sodium boiling, which renders the annular oxide fuel concept unacceptable for use in a SFR. The bottle-shaped fuel configurations are best suited for high conversion ratio cores. In the CR = 0.71 cores, the bottle-shaped fuel configuration reduces the overall core pressure drop in the fuel channels by up to 36.3%. The corresponding increase in core height with bottle-shaped fuel is between 15.6% and 18.3%. A full-plant RELAP5-3D model was created to evaluate the transient performance of the base and innovative fuel configurations during station blackout and UTOP transients. The transient analysis confirmed the good thermal-hydraulic performance of the annular and bottle-shaped fuel designs with respect to their respective solid fuel pin cases. / by Matthew J Memmott. / Ph.D.