Thesis (Nucl.E. and S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, February 2002. / Includes bibliographical references (v. 1, leaves 152-171). / The applicability of the Monte Carlo code MCNP4B to the neutronic modeling of pebblebed reactors was investigated. A modeling methodology was developed based on an analysis of critical experiments carried out at the HTR-PROTIEUS and ASTRA facilities, and the critical loading of the HTR-10 reactor. A body-centred cubic lattice of spheres with a specified packing fraction approximates the pebble bed, and exclusion zones offset the contribution of partial spheres generated by the geometry routines in MCNP4B at the core boundaries. The coated fuel particles are modeled in detail and are distributed over the fuelled region of the fuel sphere using a simple cubic lattice. This method predicted the critical core loading accurately in all cases. The calculation of control-rod worths in the more decoupled tall annular ASTRA core gave results within 10% compared to the reported experiments. An approximate method was also developed for the MCNP4B modeling of pebble-bed reactors with burnup. The nuclide densities of homogenized layers in the VSOP94 reactor model are transferred to the corresponding MCNP4B model with the lattice of spheres represented explicitly. The method was demonstrated on the PBMR equilibrium core, and used for a parallel study of burnup k- and isotopics on a single pebble. Finally, a study was carried out of the proliferation potential of a modular pebble-bed reactor for both normal and off-normal operation. VSOP94 analysis showed that spent fuel from pebble-bed reactors is proliferation resistant at high discharge burnup, because of its unfavourable plutonium isotopic composition and the need to divert -157,000 pebbles to accumulate sufficient 239Pu for a nuclear weapon. The isotopics of first-pass fuel pebbles are more favourable, but even more pebbles (-258,000) would be needed. However, a supercell MOCUP model was used to demonstrate that -20,000 pebbles would be needed if loaded with depleted uranium. But the associated reactivity loss would necessitate a compensatory increase in core height of approximately 50 cm. Such a change in core loading, as well as the properties of the special pebbles, would be noticed in a safeguarded facility. / by Julian Robert Lebenhaft. / Nucl.E.and S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001. / Includes bibliographical references (p. 203-216). / This thesis is a study of atomistic measures of strength and deformation of ceramic materials, utilizing molecular dynamics (MD) simulation that incorporates newly developed theoretical models and computational algorithms to probe the microstructural effects in crystalline, amorphous, and nanocrystalline media. Specific issues of a materials property or mechanics of materials nature are addressed in the context of characterizing the limits to strength and mechanisms of structural failure in two physically rather different model ceramics, SiO₂ described by pairwise ionic interactions and ZrC with many-body covalent interactions. These range from stress-strain responses to various applied loading, determination of fracture toughness, structural relaxation effects, to scaling with grain size. Additionally, a study of the thermal conductivity of ZrC is presented. On the computational side, thesis contributions consist of improved MD algorithms for finding neighbors and integrating the Newton's equations of motion, extension of Ewald summation to a binary ionic lattice for phonon dispersion, elastic constant, and heat current calculations, and a stand-alone scheme for coupled MD-continuum simulations based on domain decomposition and control feedback. The onset of structural instability in MD simulation of an initially defect-free lattice gives not only the theoretical or ideal strength of the deforming material, but also the mode of deformation and details of the structural defects nucleated afterwards. This provides a systematic basis for determining the effects of temperature on mechanical response, and stoichiometry effects in the case of carbon vacancies in ZrCl[sub]1-z. / (cont.) In tensile deformation of quartz, the structural transition from to β phase is observed and analyzed using a pseudo-critical phase transition model, leading to a new interpretation of the structure of the β-quartz. In uniaxial compression of α-quartz a local process of nucleation and growth of disordering is observed which appears to be distinct from the essentially homogeneous crystal-to-amorphous transformation that is well known to occur under hydrostatic compression. This finding also leads to a new interpretation of plastic deformation experiments. Fracture toughness is studied by introducing a pre-existing nanocrack in the simulations and following the details of crack tip extension under mode I loading. Simulations are shown to be quite consistent with the Griffith model in elementary fracture mechanics, confirming on the one hand the brittle nature of these two ceramics while also revealing the effects of surface relaxation, energy dissipation, and surface energy at the atomistic level. Similarly, simulations performed with initial structures with microstructural disorder, in the form of prepared amorphous and nanocrystalline specimens, also lead to new results pointing to the particular mechanisms, void nucleation and growth as well as strain-rate dependence in structural failure of an amorphous specimen, and grain-boundary sliding in shear deformation in very fine-grained nanocrystals. Taken together the thesis results demonstrate the feasibility and utility of the investigation of thermal and mechanical behavior of binary solids at the atomistic level ... / by Dongyi Liao. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references. / A number of boron neutron capture therapy (BNCT) clinical trials are currently underway around the world. Due to the small number of patients at each of the individual centers, it is desirable to pool the clinical data from each into one patient database. Before this can be done, a number of differences between the clinical centers must be evaluated. The patients treated at the BNCT centers at Brookhaven National Laboratory and that at Harvard-MIT will be evaluated as a start to the ultimate pooling of all the BNCT centers. One difference involves the difference between the normal tissue composition definition between the institutions. In particular, the difference in weight percent of ¹⁴N must be evaluated. This particular component of tissue is of importance due to the dose to tissue resulting from the ¹⁴N(n,p)¹⁴ C reaction. The difference between the 1.8% ¹⁴N composition used at BNL and the 2.2% used at MIT has a negligible effect on the total dose. Most importantly, differences in dosimetry techniques between the different centers must be computed. Once these differences are quantified, the patients can be pooled, and a better estimate of the normal brain tolerance to BNCT can be determined. / (cont.) The thermal neutron doses calculated from thermal flux measurements are 8% lower when measured by MIT, the gamma dose measurements are 26% lower, and the in-air fast neutron measurements are 27% lower in the same beam. The endpoint used for the tolerance of normal brain to BNCT is somnolence syndrome. A 50% somnolence response can be seen at 5.5 Gy-Eq. / by Julie Catherine Turcotte. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2000. / Includes bibliographical references (p. 151-153). / The critical currents of silver sheathed multifilamentary Bi 2Sr 2Ca2 Cu3010 (BSCCO- 2223/Ag) tapes at temperatures ranging from 77 K to 110 K were measured using a typical four-terminal technique for two different BSCCO-2223/Ag tapes manufactured by Vacuumschmelze (VAC). The critical currents for the two VAC samples were measured as a function of magnetic field strength. The critical current of each sample was well described by the generalized Kim model. The critical current was also measured as a function of angle of the magnetic field with respect to the plane of the tapes, in all cases perpendicular to the transport current flow. Both samples displayed a high sensitivity to the magnetic field orientation with the largest critical currents measured when the magnetic field was in-plane with the broad face of the tapes. The current ramp rate effect on the critical current was investigated and showed no significant effect on the critical current measurements for ramp speeds between 6 A/min and 120 A/min. The energy dissipation or AC losses as a function of peak AC magnetic field, magnetic field sweep frequency, and transport current for a VAC sample and a sample manufactured by IGC - Advanced Superconductors (IGC) were measured at 77 K. The losses were measured using a pick up coil with a lock-in amplifier to measure the magnetization of the superconducting samples. The AC losses of both samples displayed complete electromagnetic coupling of the superconducting filaments at magnetic sweep frequencies of approximately 0.1 Hz. These results are consistent with results measured at magnetic field sweep frequencies of 60 Hz. / by Timothy A. MacDonald. / S.M.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1996. / Includes bibliographical references (p. 157). / by Sarah Ali Abdelkader. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1995. / Includes bibliographical references (187-190). / by Michael A. Graf. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1994. / Includes bibliographical references. / by Kenneth Mitchell Crosswait. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1994. / Includes bibliographical references (p. 293-310). / by Mathias Koch. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1994. / Includes bibliographical references (p. 140). / by Hilaire Mansoux. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1993. / Title as it appears in the Feb. 1993 MIT Graduate List: Atomistic study of stress-induced structural responses of crystals at finite temperatures. / Includes bibliographical references (leaves 208-212). / by Jing Han Wang. / Ph.D.