Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Engineering, 1987 / Bibliography: p. 241-245. / by Peter Haesung Yoon. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Nuclear Engineering

No description available.

Canadian Nuclear Laboratories conceptualized a new reactor design to fulfill the goals set forth by the Generation IV International Forum. Unlike the traditional CANDU™, the Canadian Supercritical Water-cooled Reactor (SCWR) stands vertically and uses the batch fueling technique. Despite the CANDU-like assembly design, the traditional code system is not adequate to model this concept. Moreover, the current design lacks reactivity control mechanisms to remove the initial excess reactivity and manage the core power distribution. Fuel-integrated burnable absorber and cruciform control rods were designed for the Canadian SCWR as part of this work. A new genetic algorithm technique has been applied to optimize the control rod pattern by trying to minimize the maximum cladding surface temperature (MCST) and the maximum fuel centerline temperature (MFCLT) through the use of unique surface responses. The results of the study showed that the MCST and MFCLT did not meet the safety margins during normal operation. Therefore, the concept was enhanced by optimizing the fuel enrichment and burnable absorber concentration along the fuel assembly, shortening the cycle length, reducing the reactivity worth of the individual control rod, and finally lowering the core thermal power. The control rod pattern was re-evaluated and considerably improved the results. PARCS and RELAP5/SCDAPSIM/MOD4 were coupled through a series of external scripts to simulate 3D neutron kinetics/thermal-hydraulics control rod drop accidents. Every assembly was individually modeled in the coupling to accurately represent the evolution of these localized transients. Fuel melting occurred when the reactor SCRAM was not available. On the contrary, the safety margins were largely met when the reactor tripped on the overpower signal. However, this detection method demonstrated its limitation when one power pulse did not reach the SCRAM signal and led to fuel melting. A local neutron overpower signal could be implemented to ensure the reactor SCRAM in every control rod drop accident. / Thesis / Doctor of Philosophy (PhD)

Thesis: Nuc. E., Massachusetts Institute of Technology, Department of Nuclear Engineering, 1984 / Includes bibliographics references. / by Luca Ottinetti. / Nuc. E. / Nuc. E. Massachusetts Institute of Technology, Department of Nuclear Engineering

No description available.

<p>We present a study of non-uniform pumping effects in semiconductor lasers. The first portion of this work is a theoretical undertaking, in which a physical model of a split-electrode, ridge-waveguide, InGaAsP/InP laser is developed. This model is based on the time-dependent solution of the carrier and photon rate equations, and is capable of describing such device features as multiple electrical contacts, illumination of the facets via an external light source, multiple optical cavity modes, and continuous variations in the carrier and photon concentrations. The simulations generated from this model demonstrate: (1) output power vs. bias characteristics that include threshold control, discontinuities, and bistability, (2) wavelength tuning via gain peak shifts and varying refractive index, (3) self-sustained pulsations caused by repetitive Q-switching, (4) external injection effects such as injection locking, all-optical switching, and optical self-pulsation control, and (5) current modulation characteristics. The effects of varying the device bias and geometric parameters on these phenomena are presented. The second part of the thesis consists of an experimental investigation of a series of multi-quantum well InGaAsP/InP split-contact lasers. By varying the bias currents and contact lengths, we have studied their effects on the output power and spectra of these devices. The experimental results we present are in good agreement with the simulations based on our theoretical model.</p> / Doctor of Philosophy (PhD)

<p>An experimental and theoretical study of the dynamics of NH₃ lasers is presented. The significant achievements of this work are summarized below. In initial experiments, a pulsed transversely-excited CO₂ laser operating on the 9R(30) transition is used to optically pump mixtures of NH₃ in buffer gas. A simple oscillator/amplifier system allows the performance of the NH₃ amplifier in the 11 μm region to be characterized. Small-signal gain coefficients of >10%/cm are measured on the aQ(3,3) transition at 10.8 μm, while pump conversion efficiencies of ~50% are shown to occur under saturation conditions. The NH₃ laser system is described by a rate-equation model, which is validated by comparison with experiment over a wide range of operating conditions. Measurements are made for NH₃ concentrations ranging from 0.05 to 0.2%, for Ar, N₂ and He buffer gas pressures from 170 to 700 Torr, and for gas temperatures from 200 to 300 K. Optically pumped NH₃ is shown to be a versatile and efficient system for the amplification of mid-infrared radiation. The rate-equation model is used to aid in the design of a simple and efficient NH₃ laser. This laser is tuned over more than 70 vibrational band transitions between 10.08 and 14.14 μm. Output energies greater than 1 J per pulse are achieved on several of the strongest lines. In a non-selective cavity an energy conversion efficiency of greater than 35% is obtained with a maximum output energy of 4.6 J. Optically pumped NH₃ is shown to be a flexible and efficient system for the downconversion of CO₂ radiation to the 10-14 μm region. In other experiments, it is shown that optically pumped high pressure mixtures of NH₃ in N₂ are efficient, broadband amplifiers of pulsed CO₂ radiation. In a dilute NH₃ mixture at 6 atmospheres and 200 K, a single pass gain of 150 (21.8 dB) is measured for the 10P(34)CO₂ transition. Gain is observed in NH₃ at pressures as high as 10 atmospheres. Experimental measurements are made for a range of wavelengths in the 10.7 μm region, and the results compared with calculations based on a simplified rate-equation model. The operation of a two-step optically pumped NH₃ laser (using two CO₂ lasers) is examined. Output is obtained from 16-22 μm. Experimental measurements made at 200 K and 300 K are compared to calculations based on an extended rate-equation model. As a result of this work it was shown that a single CO₂ pump laser can be used to obtain 16-22 μm lasing at 200 K.</p> / Doctor of Philosophy (PhD)

<p>A chemical kinetic model has been constructed to predict the pressure and gas and electron temperature dependency of the neutral and ionic species composition in Ar-CO₂ mixtures under thermal plasma conditions. Pure Ar and Ar-C models have also been constructed as a part of this investigation. The models include electron impact, thermal impact, ion-molecule, and recombination reactions as well as accounting for diffusion. Important metastable and excited states of species have been accounted for as well as the presence of neutral molecules, radicals, and atoms, positive and negatively charged atoms and molecular ions as well as electrons. All relevant electron temperature, gas temperature and pressure terms have been included, and primarily experimentally derived reaction rate constants were utilized. Electron and gas temperature dependent species concentrations were obtained under both thermodynamic equilibrium and non-equilibrium conditions for gas temperatures from 300 to 15000 K, electron temperatures from 300 to 20000 K, and pressures from 1 Torr to 15200 Torr. Percentage mixtures of C and CO₂ in Ar were varied between 0.1 and 40%. Results indicate that the neutral and excited species Ar, Ar*, Ar**, C, CO, CO₂, O, O₂ and O₃, positive ions Ar⁺, Ar₂⁺, C⁺, CArO⁺, CO⁺, CO₂⁺, CO₄⁺, C₂O₂⁺, O⁺, O₂⁺, O₄⁺, and O₅⁺ and negative ions CO₃⁻, CO₄⁻, O⁻, O₂⁻, and O₃⁻ and electrons are observed. Under thermodynamic equilibrium conditions, Ar, C, CO, CO₂, and O were found to be the dominant neutral species, C⁺, CO₄⁺, O⁺, and O₂⁺ the dominant positive ions, and O⁻ and electrons the dominant negatively charged species under certain gas temperature ranges. The introduction of thermodynamic non-equilibrium conditions, changes in gas pressure, and percentage mixture of C or CO₂ were observed to have a significant influence on the temperature dependent concentrations of these species. The results were found to be in good agreement with previous experimental and theoretical results. Comparison of the present results with the results of the Saha equation seem to indicate that the simple Saha equation type approach may mispredict the plasma density of Ar, Ar-C, and Ar-CO₂ mixtures under thermal plasma conditions.</p> / Doctor of Philosophy (PhD)

<p> Accurate prediction of evaporative losses from light water reactor nuclear power plant (NPP) spent fuel storage pools (SFPs) is important for activities ranging from sizing of water makeup systems during NPP design to predicting the time available to supply emergency makeup water following severe accidents. Existing correlations for predicting evaporation from water surfaces are only optimized for conditions typical of swimming pools. This new approach modeling evaporation as a diffusion process has yielded an evaporation rate model that provided a better fit of published high temperature evaporation data and measurements from two SFPs than other published evaporation correlations. Insights from treating evaporation as a diffusion process include correcting for the effects of air flow and solutes on evaporation rate. An accurate modeling of the effects of air flow on evaporation rate is required to explain the observed temperature data from the Fukushima Daiichi Unit 4 SFP during the 2011 loss of cooling event; the diffusion model of evaporation provides a significantly better fit to this data than existing evaporation models.</p>

<p> Formation of a high density of Mn-Ni-Si nanoscale precipitates in irradiated reactor pressure vessel steels could lead to severe, unexpected embrittlement, which may limit the lifetimes of our nation&rsquo;s light water reactors. While the existence of these precipitates was hypothesized over 20 years ago, they are currently not included in embrittlement prediction models used by the Nuclear Regulatory Commission. This work aims to investigate the mechanisms and variables that control Mn-Ni-Si precipitate (MNSP) formation as well as correlate their formation with hardening and embrittlement. </p><p> A series of RPV model steels with systematic variations in Cu and Ni contents, two variables that have been shown to have a dominant effect on hardening, were irradiated in a series of test reactor and power reactor surveillance irradiations. Atom probe tomography (APT) measurements show that large volume fractions (f_v) of MNSPs form in all the steels irradiated at high fluence, even those containing no added Cu, which were previously believed to have low sensitivity to embrittlement. It is demonstrated that while Cu enhances the rate of MNSP formation, it does not appear to significantly alter their saturation f_v or composition. The high fluence MNSPs have compositions consistent with known intermetallic phases in the Mn-Ni-Si system and have f_v very near those predicted by equilibrium thermodynamic models. In addition, X-ray diffraction experiments by collaborators shows that these precipitates also have the expected crystal structure of the predicted Mn-Ni-Si phases. </p><p> Post irradiation annealing experiments are used to measure the hardness recovery at various temperatures as well as to determine if the large f_v of MNSPs that form under high fluence neutron irradiation are thermodynamically stable phases or non-equilibrium solute clusters, enhanced or induced by irradiation, respectively. Notably, while post irradiation annealing of a Cu-free, high Ni steel at 425&deg;C results in dissolution of most precipitates, a few larger MNSPs appear to remain stable and may begin to coarsen after long times. A cluster dynamics model rationalizes the dissolution and reduction in precipitate number density, since most are less than the critical radius at the annealing temperature and decomposed matrix composition. The stability of larger precipitates suggests that they are an equilibrium phase, consistent with thermodynamic models. </p><p> Charged particle irradiations using Fe³⁺ ions are also used to investigate the precipitates which form under irradiation. Two steels irradiated to a dose of 0.2 dpa using both neutrons and ions show precipitates with very similar compositions. The ion irradiation shows a smaller f_v, likely due to the much higher dose rate, which has been previously shown to delay precipitation to higher fluences. While the precipitates in the ion irradiated condition are slightly deficient in Mn and enriched in Ni and Si compared to neutron irradiated condition, the overall similarities between the two conditions suggest that ion irradiations can be a very useful tool to study the susceptibility of a given steel to irradiation embrittlement. </p><p> Finally, the large f_v of MNSPs that are shown to form in all steels, including those low in Cu, at high fluence, even those without added Cu, result in large amounts of hardening and embrittlement. A preliminary embrittlement prediction model, which incorporates MNSPs at high fluence, is presented, along with results from a recent test reactor irradiation to fluences representative of extended lifetimes. This model shows very good agreement with the data.</p>