Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1996. / Includes bibliographical references (p. 113-116). / by Tammy L. Stoops. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1996. / Includes bibliographical references (leaves 97-101). / by James Roger Gilmore. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1997. / Includes bibliographical references. / by Terry Tak-Keon Lee. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1997. / Includes bibliographical references (p. 144-146). / by Kent J. Riley. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1997. / Includes bibliographical references (p. 225-234). / This Ph.D. thesis has covered the scientific issues of defect reactions involving dopants and impurities in Si, and the applications of this knowledge to reactive ion etching (RIE) and Fe gettering processes. The reactions among self-interstitials (Sii), vacancies (V), impurities (C, 0), and dopants (B, P) in Si produce undesirable defects which affect device operation and control transport processes such as dopant diffusion. Electron beam irradiation has been used to generate Sii and V to initiate the defect reactions. Deep level transient spectroscopy (DLTS) has been used to identify specific defects and to measure defect concentrations. The experimental results can be summarized in terms of a complex hierarchy diagram of defect reactions. We describe the defect reactions as a three-step process: (i) the displacement reaction to generate Sii and V, (ii) the Watkins replacement reaction to generate C and B interstitials (Ci, Bi), and (iii) the diffusion limited pairing and clustering processes to generate defect pairs and clusters. On the basis of reaction kinetics, we have simulated the reaction processes. The interstitial migration enthalpy (Him) and capture radius (r,) are two parameters used in the model to describe the long range migration and the near neighbor capture of mobile interstitials. The calculated defect reaction rates are in good agreement with the experimental data. We conclude that the diffusion limited pairing reactions are the predominant processes in the defect reactions. The reaction kinetics are determined by Him, r,, and the background dopant and impurity concentrations. The model supports the defect assignments by DLTS. The model can be improved by including pair breakup processes and large interstitial clusters. / (cont.) RIE causes substrate surface contamination, substrate damage, and induces defect reactions at depths extending to um range. We have applied the defect reaction model to RIE and developed a model describing interstitial injection for the defect reaction region to evaluate the defect depth profile. The reaction kinetics is formulated as a series of coupled 1-D interstitial diffusion-reaction partial differential equations (PDEs) with a moving boundary. The model predicts the profiles which are consistent with that measured in the photoluminescence (PL) experiments. We conclude that the depth profile is determined by the interstitial diffusion coefficient, the etch rate, the etch time, the interstitial defect reaction rate, and the background dopant and impurity concentrations in the Si substrate. The um range depth profile can be explained as: (i) fast diffuser Sii injection to ,um depth range; (ii) the generation of Bi and Ci by the Watkins replacement reactions, and (iii) the formation of Bi- and Ci-related defects through diffusion limited pairing reactions. The injection of Bi or Ci is extremely shallow during a typical RIE process. Fe is incorporated into Si as a highly mobile and soluble interstitial species (Fei) during device processing or in the starting materials. Fei and Group III impurities (B, Al, Ga, In) form Fe-acceptor (FeiAs) pairs in Si. Both Fei and the FeiAs pairs introduce deep levels in the bandgap which act as recombination centers. The long range Coulomb interaction between Fei and As is the driving force for the FeiAs pair formation. The short range near neighbor interactions determine the specific FeiAs pair energetics and structures. We have studied the FeiAs pairs within the framework of an ionic model. ... / by Song Zhao. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1994. / Includes bibliographical references. / by Ronald Daniel Rogus. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1991. / Includes bibliographical references (leaves 260-263). / by Gang Fu. / Sc.D.

Massachusetts Institute of Technology. Dept. of Nuclear Engineering. Thesis. 1973. Ph.D. / Includes bibliographical references. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1991. / Includes bibliographical references (leaves 104). / by Christine Marie Martin. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003. / Includes bibliographical references (p. 254-264). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / This work is focused on the strategy for utilizing high-burnup fuel in pressurized water reactors (PWR) with special emphasis on the full array of neutronic considerations. The historical increase in batch-averaged discharge fuel burnup, from ~30 MWd/kg in the 1970s to ~50 MWd/kg today, was achieved mainly by increasing the reload fuel enrichment to allow longer fuel cycles: from an average of 12 months to about 18 months. This also reduced operating costs by improving the plant capacity factor. Recently, because of limited spent fuel storage capacity, increased core power output and the search for increased proliferation resistance, achieving burnup in the 70 to 100 MWd/kg range has attracted considerable attention. However the implications of this initiative have not been fully explored; hence this work defines the practical issues for high-burnup PWR fuels based on neutronic, thermal hydraulic and economic considerations as well as spent fuel characteristics. In order to evaluate the various high burnup fuel design options, an improved MCNP-ORIGEN depletion program called MCODE was developed. A standard burnup predictor-corrector algorithm is implemented, which distinguishes MCODE from other MCNP-ORIGEN linkage codes. Using MCODE, the effect of lattice design (moderation effect) on core design and spent fuel characteristics is explored. Characterized by the hydrogen-to-heavy-metal ratio (H/HM), the neutron spectrum effect in UO2/H2O lattices is investigated for a wide range of moderation, from fast spectra to over-thermalized spectra. It is shown that either wetter or very dry lattices are preferable in terms of achievable burnup potential to those having an epithermal spectrum. Wet lattices are the preferred high burnup approach due to improved proliferation resistance. The constraint of negative moderator temperature coefficient (MTC) requires that H/HM values (now at 3.4) remain below ~6.0 for PWR lattices. Alternative fuel choices, including the conventional solid pellets, central-voided annular pellets, Internally- & eXternally-cooled Annular Fuel (IXAF), and different fuel forms are analyzed to achieve a wetter lattice. Although a wetter lattice has higher burnup potential than the reference PWR lattice, the requirement of a fixed target cycle energy production necessitates higher initial fuel enrichments to compensate for the loss of fuel mass in a wetter lattice. Practical issues and constraints for the high burnup fuel include neutronic reactivity control, heat transfer margin, and fission gas release. Overall the IXAF design appears to be the most promising approach to realization of high burnup fuel. High-burnup spent fuel characteristics are compared to the reference spent fuel of 33 MWd/kg, representative of most of the spent fuel inventory. Although an increase of decay power and radioactivity per unit mass of initial heavy metal is immediately observed, the heat load (integration of decay power over time) per unit electricity generation decreases as the fuel discharge burnup increases. The magnitude of changes depends on the time after discharge. For the same electricity production, not only the mass and volume of the spent fuel are reduced, but also, to a lesser extent, the total heat load of the spent fuel. Since the heat load in the first several hundred years roughly determines the capital cost of the repository, a high burnup strategy coupled with adequate cooling time, may provide a cost-reduction approach to the repository. High burnup is beneficial to enhancing the proliferation resistance. The plutonium vector in the high-burnup spent fuel is degraded, hence less attractive for weapons. For example, the ratio of Pu-238 to Pu-239 increases with burnup to the 2.5 power. However, the economic benefits are uncertain. Under the current economic conditions, the PWR fuel burnup appears to have a shallow optimum discharge burnup between 50 and 80 MWd/kg. The actual minimum is influenced by the financing costs as well as the cost of refueling shutdowns. Since the fuel cycle back-end benefits will accrue to the federal government, the current economic framework, such as the waste fee based on the electricity produced rather than volume or actinide content, does not create an incentive for utilities to increase burnup. Different schemes exist for fuel management of high burnup PWR cores. For the conventional core design, a generalized enrichment-burnup correlation (applicable between 3 w/o and 20 w/o) was produced based on CASMO/SIMULATE PWR core calculations. Among retrofit cores, increasing the number of fuel batches is preferred over increasing the cycle length due to nuclear fuel cycle economic imperatives. For future core designs, a higher power-density core is a very attractive option to cut down the busbar cost. The IXAF concept possesses key design characteristics that provide the necessary thermal margins at high core power densities. In this regard, the IXAF fuel deserves further investigation to fully exploit its high burnup capability. / by Zhiwen Xu. / Ph.D.