Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007. / Includes bibliographical references (p. 118-121). / Rising natural gas prices and growing concern over CO₂ emissions have intensified interest in alternative methods for producing hydrogen. Nuclear energy can be used to produce hydrogen through thermochemical and/or electrochemical processes. This thesis investigates the feasibility of high temperature steam electrolysis (HTSE) coupled with an advanced gas-cooled fast reactor (GFR) utilizing supercritical carbon dioxide (S-CO₂) as the coolant. The reasons for selecting this particular reactor include fast reactor uranium resource utilization benefits, lower reactor outlet temperatures than helium-cooled reactors which ameliorate materials problems, and reduced power conversion system costs. High temperature steam electrolysis can be performed at conditions of 8500C and atmospheric pressure. However, compression of the hydrogen for pumping through pipes is unnecessary if electrolysis takes place at around 6 MPa. The reactor coolant at 6500C is used to heat the steam up to temperatures ranging between 2500C and 3500C, and the remaining heat is provided by thermal recuperation from product hydrogen and oxygen. Several different methods for integrating the hydrogen production HTSE plant with the GFR were investigated. The two most promising methods are discussed in more detail: extracting coolant from the power conversion system (PCS) turbine exhaust to boil water, and extracting coolant directly from the reactor using separate water boiler (WB) loops. Both methods have comparable thermal to electricity efficiencies (-43%) at 6500C. This relates to an overall hydrogen production efficiency of about 47%. The approach which utilizes separate WB loops has the added advantage of being able to provide emergency cooling to the reactor, and also the benefit of not interfering with the operation of the PCS. / (cont.) This makes the separate WB loop integration method a more desirable scheme for hydrogen production using HTSE. The HTSE electrolysis unit adopted for the present analysis was designed by Ceramatec in coordination with INL. In this unit the steam flows into an electrolytic cell. It is separated by electron flow from a nickel-zirconium cathode to a strontium-doped lanthanum manganite anode. The optimal conditions for stack operation have been found by INL using various modeling and experimental techniques. These conditions include a 10% by volume flow of hydrogen in the feed, a stack operating temperature of 8000C, and an operating voltage of 1.2 V. The GFR integrated with the HTSE plant via separate water boiler loops was modeled in this work using the chemical engineering code ASPEN. The results of this model were benchmarked against the Idaho National Lab (INL) process, modeled using HYSIS. Both models predict a hydrogen production rate of -10.2 kg/sec (+ 0.2 kg/sec) for a 600 MWth reactor with an overall efficiency ranging between 47%-50%. The highly recuperated HTSE plant developed for the GFR can in principle be used in conjunction with a variety of other nuclear reactors, without requiring high reactor coolant outlet temperatures. / by Matthew J. Memmott. / S.M.

Thesis: S.B., 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 51-53). / Intensity Modulated Radiation Therapy (IMRT) is a type of external beam radiation therapy that has proven effective at treating many cancers. A related therapy type, Volumetric Modulated Arc Therapy (VMAT), has the potential to provide comparable dose coverage to tumor sites while better sparing nearby organs at risk (OARs). Multi-criteria Optimization (MCO) is an algorithm that is used to optimize a patient's personalized IMRT treatment plan. VMAT treatment plans cannot be optimized using early versions of the MCO algorithm. The purpose of this study was to construct a model for the automated generation of VMAT treatment plans for prostate cancers using a knowledge base of previously implemented IMRT-MCO treatment plans. An initial model configuration was iteratively refined to produce VMAT plans that represent a quality 'first pass" that can be further optimized by trained treatment planners. The clinical implementation of a model like this one could significantly improve the timeliness of standard non-MCO VMAT optimization methods. / by Caroline M. Colbert. / S.B.

Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 85). / The effect of lateral power distribution of the MITR LEU fuel design was analyzed using Computational Fluid Dynamics. Coupled conduction and convective heat transfer were modeled for uniform and non-uniform lateral power distributions. It was concluded that, due to conduction, the maximum heat flux ratio on the cladding surface is 1.16, compared to the maximum volumetric power generation ratio of 1.23. The maximum cladding temperature occurs roughly 0.5 inches from the edge of the support plate, while the peak volumetric power generation is located at the end of the fuel meat, about 0.1 inches from the edge of the support plate. Although the heat transfer coefficient is lower in the corner of the coolant channel, this has a negligible effect on the peak cladding temperature, i.e. the peak cladding temperature is related to heat flux only and a "channel average" heat transfer coefficient can be adopted. Moreover, coolant temperatures in the radial direction are reasonably uniform, which is indicative of good lateral mixing. Finally, a quasi-DNS study has been performed to analyze the effect of the fuel grooves on the local heat transfer coefficient. The quasi-DNS results bring useful insights, showing two main effects related to the existence of the grooves. First, the increased surface leads to an increase in the pressure drop and further, the flow aligned configuration of the grooves limits the ability of the near wall turbulent structures to create mixing, leading to a noticeable reduction in the local heat transfer coefficient at the base of the grooves. Overall, this leads to an effective decrease in the local heat transfer coefficient, but due to the increased heat transfer surface the global heat transfer is enhanced in comparison to the flat plate configuration. The improved understanding of the effects of grooves on the local heat transfer phenomena provides a useful contribution to future fuel design considerations. For example, the increase in pressure drop, together with the reduction in the local heat transfer coefficient indicated that the selection of a grooved wall channel instead of a smooth wall channel might not necessarily be optimal, particularly if fabrication issues are taken into account, together with the concern that grooved walls may promote oxide growth and crud formation during the life of the fuel. / by Mihai Aurelian Diaconeasa. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 130-131). / In the constant effort to increase efficiency, safety margins, and lower cost, a new breed of nuclear reactors, Generation IV, is being developed in which supercritical carbon dioxide (SCO₂) is a prime coolant candidate. SCO₂ allows for higher efficiencies, reduced pumping power, lower plant temperature, and more compactness compared to the other gas coolants currently being examined. However, limited corrosion data currently exists for materials that are potential pressure boundary candidates under high pressures and temperatures in SCO₂ environments. The goal of this investigation was to understand the effect of temperature on corrosion on potential structural materials in a SCO₂ environment A total of 7 different alloys, 6 nickel based and 1 austenitic stainless steel (AUSS), were examined in three sets of experiments. The experiments exposed the specimens to SCO₂ at temperatures ranging from 650 "C to 750 °C, pressures from 12.5 to 20 MPa, and for durations of up to 1000 hours. The nickel based alloys demonstrated very promising results as the weight gain rates were almost an order of magnitude lower than the stainless steel. The average nickel based sample exposed to SCO₂ at a temperature of 750°C and a pressure of 12.5 MPa showed a weight gain rate of 0.0063 mg/cm 2*day, while the stainless steel sample had a weight gain rate of 0.096 mg/cm 2*day after a duration of 1000 hours. This was expected as the combination of nickel and chromium forms a higher integrity and more stable passive film than iron and chromium. Additionally, nickel has a lower oxygen affinity than iron and therefore the migration of cations into the scale is lower. / (cont.) The chromium content for the AUSS 316L was also the lowest, which most likely contributed to the high oxidation rates. The tests conducted at 750 °C and 12.5 MPa showed the highest weight gain rates for the nickel based alloys, which was expected as the corrosion rate should follow an Arrhenius trend. The effect of pressure was small compared to the effect of temperature as a 43% reduction in pressure and a 5% increase in temperature produced significantly higher corrosion rates in the nickel based alloys. The AUSS 316L, behaved counter-intuitively as the highest temperature experiment, 750 °C and 12.5 MPa, had the lowest weight gain rate. This behavior may be explained by the increase in temperature, which caused an increase in the diffusion rate within the alloy. This facilitated a faster growth rate of an inner "healing" layer composed of chromium rich oxide, which may have restricted the outward diffusion of cations and inward diffusion of anions. / by Michael William Dunlevy. / S.M.

Thesis: Ph. 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 75-79). / The mechanical properties of cement pastes depend strongly on their porosities. In a wet paste, the porosity links to the free water volume after hydration. Structural water which presents in the solid phase, constrained water absorbed on the surface of the pores and free water in the center of the pores have different dynamical behavior. Hence, it should be possible to extract information on pore system by exploiting the water dynamics. We investigated the dynamics of hydration water confined in calcium- and magnesium-silicate-hydrate (C-S-H and M-S-H) gels using high-resolution quasi-elastic neutron scattering (QENS). C-S-H and M-S-H are the chemical binders present in calcium rich and magnesium rich cement. To analyze the cement QENS data, we developed a new global model which is numerically more stable than previous models for cement QENS analysis. With this model, we can correctly quantify the structural water index (SWI) and the confining radius. We also established the relation between the constrained to liquid water ratio and the temperature dependence of translational relaxation time. We analyzed two different sets of synthetic cement using this method: (1) C-S-H with different water to cement ratio (w/c) and (2) M-S-H with various additives. For the first set, SWI and confining radius are both controlled by w/c with a linear relation. For the second set, we show that by adding ASN-COOH additive, M-S-H becomes similar to C-S-H in all parameters. We also analyzed the small angle x-ray scattering data of M-S-H gel with a polydisperse cylinder model which fits better than previously published polydisperse sphere model and will be studied further in future work. The result indicates that C-SH and M-S-H have similar globule shape and fractal structure. The evidence from QENS and SAXS experiments suggest that the weak compressive strength of M-S-H compares to C-S-H is due to the high porosity rather than the globule shape. / by Peisi Le. / 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. / Includes bibliographical references (pages 64-68). / The 2011 Fukushima Daiichi accident highlighted the weakness of the current nuclear fuel and motivated R&D of accident tolerant fuels. Accident tolerant fuels (ATF) are fuels that can tolerate loss of active cooling in the core of light water reactors (LWRs) for a considerably longer period of time while maintaining or improving the fuel performance during normal operations. Fully Ceramic Microencapsulated (FCM) fuel is an ATF concept aimed at significantly increasing the fission product retention capability of nuclear fuel at high temperatures. The FCM concept is made up of fuel particles surrounded by multilayers of ceramic material similar to the TRISO fuel concept. The fuel particles are embedded in a SiC matrix in cylindrical pellet geometry which gives the fuel its high temperature corrosion resistance. However, when implementing the FCM concept in a conventional PWR fuel geometry, it is not possible to maintain an 18 month fuel cycle length and remain below the proliferation enrichment limit of 20 w/o U₂₃₅. This is a critical challenge that needs to be overcome in order to benefit from the high temperature fission product retention capability of FCM-type ATF concepts. Therefore, this work aims at investigating the potential benefits of a new accident tolerant fuel, Fuel-in-Fibers (F-in-F) concept. The Fuel-in-Fibers concept was created by Free Form Fibers, a laser chemical vapor deposition direct manufacturing company. It aims to combine the same robust fission product retention and high temperature stability as the FCM fuel concept while drastically decreasing the necessary fuel enrichment. This is done by designing a fuel fiber in cylindrical geometry as opposed to spherical particles to increase the packing fraction within a cylindrical pellet. The direct manufacturing allows for minimization of the volume occupied by the SiC matrix as well as direct deposition of high density fuels like uranium nitride (UN). Assembly level calculations in the Monte Carlo code SERPENT determined that the Fuel-in-Fibers concept could maintain a typical PWR cycle length with less than 20 w/o U₂₃₅ (LEU) enrichment. The fibers in the fuel pellet were then homogenized for use in lattice physics code CASMO and core simulator code SIMULATE3. The SIMUALTE full core simulation showed that the Fuel-in- Fibers design required enrichments of 8% and 6% for UO2 and UN as fuels, respectively. Overall, the full core analysis of a standard 4-loop Westinghouse PWR showed Fuel-in-Fibers concept has similar behavior as the conventional fuel. Due to the high fissile enrichments, the calculated radial power peaking factors were higher in Fuel-in-Fibers concept. This may result in decrease of the coolant outlet temperature by 5 K in order to maintain safety margins. The shutdown margin analysis showed that using B4C instead AgInCd control rods is needed. A design optimization was also performed to calculate the ideal geometry for Fuel-in-Fibers concept. An in-house MATLAB single channel code, built to evaluate PWR Thermal Hydraulic and Structural performance, was used to vary the fuel pin Pitch and Pitch-to-Diameter ratio (P/D Ratio). The results showed that a smaller pitch and larger diameter of 13.2 mm and 12 mm, respectively will improve the Fuel-in-Fibers concept enrichment requirements. A simplified economic analysis based on highly uncertain fabrication cost estimates was performed. The economics analysis determined that the fuel in fiber design is estimated to cost more than current UO₂ fuel by 1.25x - 15x due to the increased enrichment and fabrication costs but may be offset by the additional safety margins provided by the Fuel-in-Fibers concept. / by Briana Hiscox. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 303-314). / Structural aspects of the performance of light water reactor (LWR) fuel rod with triplex silicon carbide (SiC) cladding - an emerging option to replace the zirconium alloy cladding - are assessed. Its behavior under accident conditions is examined with an integrated approach of experiments, modeling, and simulation. High temperature (1100°C~1500°C) steam oxidation experiments demonstrated that the oxidation of monolithic SiC is about three orders of magnitude slower than that of zirconium alloys, and with a weaker impact on mechanical strength. This, along with the presence of the environmental barrier coating around the load carrying intermediate layer of SiC fiber composite, diminishes the importance of oxidation for cladding failure mechanisms. Thermal shock experiments showed strength retention for both [alpha]-SiC and [beta]-SiC, as well as A1₂O₃ samples quenched from temperatures up to 1260°C in saturated water. The initial heat transfer upon the solid - fluid contact in the quenching transient is found to be a controlling factor in the potential for brittle fracture. This implies that SiC would not fail by thermal shock induced fracture during the reflood phase of a loss of coolant accident, which includes fuel-cladding quenching by emergency coolant at saturation conditions. A thermo-mechanical model for stress distribution and Weibull statistical fracture of laminated SiC cladding during normal and accident conditions is developed. It is coupled to fuel rod performance code FRAPCON-3.4 (modified here for SiC) and RELAP-5 (to determine coolant conditions). It is concluded that a PWR fuel rod with SiC cladding can extend the fuel residence time in the core, while keeping the internal pressure level within the safety assurance limit during steady-state and loss of coolant accidents. Peak burnup of 93 MWD/kgU (10% central void in fuel pellets) at 74 months of in-core residence time is found achievable with conventional PWR fuel rod design, but with an extended plenum length (70 cm). An easier to manufacture, 30% larger SiC cladding thickness requires an improved thermal conductivity of the composite layer to reduce thermal stress levels under steady-state operation to avoid failure at the same burnup. A larger Weibull modulus of the SiC cladding improves chances of avoiding brittle failure. / by Youho Lee. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 127-136). / Small-angle scattering (SAS) has been widely applied to study the microstructure of colloidal systems. Although colloids cover a wide range of materials, in general they can simply be viewed as basic building particles arranging themselves, according to their interaction, in a continuous medium. In this study, three seemingly very different systems were investigated under various conditions. They are the calcium-silicate-hydrate (C-S-H) gel, magnesium-silicate-hydrate (M-S-H) gel, and micellar solution formed by Pluronics triblock copolymers. C-S-H is the main binding phase of ordinary Portland cements. An elaborate analytical model for the form factor of C-S-H basic building particles was established for the first time. This model has ability to integrate two different models together by taking two different limits of the form factor formula. Essential structural parameters of C-S-H gels prepared at various conditions were extracted through model fitting. It was found in this study that microstructure of C-S-H gels changes from continuous planar pore structure to discrete colloidal structure when increasing water content or adding methylhydroxyethyl cellulose additive. Open microstructure or small globule size leads to higher flowability or facilitates the extrusion process as macroscopic properties. Much attention has been paid recently to the MgO-based green cements due to the little CO₂ generated during their production process compared with the ordinary cements. However, the poor mechanical properties prevent them from implementing widescale use. This current study on microstructure of both C-S-H and M-S-H gels indicates that the primary unit at the nanoscale level of C-S-H to be a multilayer disk-like globule, whereas for M-S-H it is a spherical globule. This prominent difference at the nanoscale also reflects in gel structure at micrometer lengthscale. The surface contact between the basic particles found in C-S-H gels leads to better mechanical properties than M-S-H gels which interact through point contact. This study therefore gives essential insight to design future robust and eco-friendly binders. Pluronics is a class of amphiphilic copolymers which aggregate to form micelle particles in water. Small-angle neutron scattering contrast variation measurements were conducted to extract the microstructure, especially the solvent distribution within the micelle particles, under several conditions. It is suggested in this study that high water content found in the micelles formed by short copolymer chains but same PO/EO ratio promotes composition fluctuation within the micelles and in turn stabilizes the liquid-like micelle phase. In addition, the dehydration of core region of the micelles due to increasing concentration or temperature leads to phase transition from liquid-like to crystalline micelle state. These results can deepen the current understanding of the complicated phase behaviors of amphiphilic copolymers. Although the three systems studied have very different features, this work demonstrates that they can all be tackled by similar SAS analysis. Furthermore, structure-property relationships and structure-phase behavior relationships are established based on the results. / by Wei-Shan Chiang. / Ph. D.

Thesis: Ph. 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. / Corrosion fatigue is a multivariate challenge that threatens the lifetime of service of nuclear power plant materials, especially austenitic stainless steels. Both enhancement and retardation of crack growth have been observed in laboratory tests. This thesis work performs high temperature autoclave testing, post-test characterization and mechanistic modeling to understand the corrosion fatigue behavior of austenitic stainless steels in simulated light water reactor (LWR) environments. Crack growth rate (CGR) data were generated from the autoclave testing on low (0.001 wt.%) and high (0.03 wt.%) sulfur content heat 1T compact tension (CT) specimens. Tests were controlled under constant K (22-35 MPa [square root of]m) with load ratio of 0.7 and sawtooth waveform (85% rise vs. 15% fall), and at pH =10 and 288 °C with system pressure of 9.54 MPa. Crack enhancement was observed in low sulfur content heat specimens, and the CGR increases as the loading rise time increases. The fracture surfaces of low sulfur content heat specimens showed transgranular features with facets ("river pattern") and few oxide particles. Crack retardation was observed in high sulfur content heat specimens, and the CGR decreases as the loading rise time increases. The fracture surfaces of high sulfur content heat specimens showed distinct features at different rise time step. Transgranular features ("river pattern") were observed at short rise time step, while non-descript surfaces with fine octahedral oxide particles were observed at long rise time step. Additionally, tests in deuterium water were performed to enable measurements on hydrogen/deuterium concentrations in specimens using ToF-SIMS and hot vacuum extraction techniques. Deuterium pick-up from the testing environment was observed, and the enrichment of deuterium/hydrogen ahead of crack tip was also observed. Controlled experiments were also conducted, where specimens were baked prior to the autoclave testing to remove the residual internal hydrogen. Such heat treatment removing the internal hydrogen was found to not affect the crack growth behavior. Dissolved gases, hydrogen and argon respectively, were bubbled into system during the autoclave tests, and they resulted in similar crack growth behaviors. Modeling indicates that there exists an enhancement mechanism other than corrosion mass removal driving the crack growth in simulated LWR environments. Possibly it comes from the effect of corrosion-generated hydrogen. Retardation behavior and experimental observations could be understood and explained by concept and modeling of corrosion blunting. The results suggest excess conservatism of current ASME standards N-809 for high sulfur content austenitic stainless steels. / by Lun Yu. / Ph. D.

Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007. / Includes bibliographical references (p. 101-107). / Multipactor discharges consists of electron multiplication between two surfaces by secondary electron emission in resonance with an alternating electric field. They are detrimental to the performance of radio frequency (RF) systems, such as the ICRF (ion cyclotron range of frequencies) antennas for heating of plasmas in the Alcator C-Mod tokamak and other nuclear fusion devices. This work investigates multipactor discharges in the coaxial geometry in the presence of a constant and uniform magnetic field transverse to the direction of electromagnetic wave propagation. Studies on the Coaxial Multipactor Experiment (CMX) show that the magnetic field decreases the degree to which the discharge detunes the RF circuit. However, it enhances the susceptibility of the system to multipactor-induced gas breakdown at low pressures, which appears to cause the observed neutral pressure limits on antenna performance in Alcator C-Mod. Different surface treatment methods involving roughening and in-situ cleaning failed to suppress the multipactor discharges in a consistent and reliable way in experiments on CMX, despite the success of similar techniques in the parallel-plate geometry. Electron trajectories are significantly more complicated in the presence of magnetic fields of different strengths, as shown by a three-dimensional particle-tracking simulation using Monte Carlo sampling techniques. The trends in electron path length, time of flight, impact energy, secondary emission yield and population growth do not account for the experimental observations between the low and high field limits. These appear to be better explained by collective effects not included in the simulations, such as the effect of the magnetic field on charged particle diffusion. / by Gabriel E. Becerra. / S.M.and S.B.