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Improvements, Validation, and Applications of a Metaheuristic Optimization Method for Neutron Spectra Tailoring at the National Ignition Facility


Gnowee/COEUS v1.0 is a metaheuristic software package that has been developed at the University of California, Berkeley (UC Berkeley), in collaboration with Lawrence Livermore National Laboratory (LLNL), to design optimized Energy Tuning Assemblies (ETA). ETAs are designed in order to modify the neutron source spectrum and produce an objective spectrum given a large set of constraints and changeable variables. The software package is based on a general-purpose metaheuristic optimization algorithm, Gnowee, which uses COEUS to couple the algorithm to the the Monte Carlo N-Particle (MCNP) radiation transport package. The initial successful application, of the software package, was the design of a conical ETA to spectrally shape a simplified monoenergetic 14.1 MeV neutron point source to a thermonuclear and prompt fission neutron spectrum for technical nuclear forensics (TNF) purposes. Like TNF, many other applications in the nuclear engineering field require neutron energy distributions that cannot be obtained with currently available neutron sources. With an increased demand in neutron beam applications, numerous facilities, including the NIF, are interested in employing an accurate and efficient optimization design methodology, such as Gnowee/COEUS, to tailor available neutron spectra and intensity for specific requirements.

The dissertation research included the following steps: (a) development of a fast and efficient optimization methodology and software package for tailoring neutron energy, (b) identification of the neutron transport simulation code package to be coupled with the optimization part, (c) experimental validation of optimization software package (d) applying the optimization package for specific ETA designs, and (e) performing simple experiments on specific ETA designs.

This dissertation describes further efforts made in developing and efforts that have been made in generating a generalized, problem independent Gnowee/COEUS v2.0. The new software package includes the ability to design an ETA within a high fidelity model of a neutron producing facility, with realistic source configurations, a wider range of possible optimization functions, and a larger set of possible variables and constraints. The first part of the dissertation describes the improved COEUS v2.0 and the possibilities of applying the code to design ETAs within LLNL's National Ignition Facility (NIF) Target Chamber (TC). The improved modeling of the NIF TC environment by a set of Monte Carlo and deterministic codes is described, including the modeling and characterization of its system components and instrumentation, including Diagnostic Instrument Manipulator (DIM) 90-78, Target and Diagnostic Manipulator (TANDM) 90-348, SNOUT and large Target Option Activation Device (HTOADs) used for validation and ETA experiments. In addition, a detailed description of the activation foils used to cover a large range of neutron energy spectra is included, as well as the analysis and the unfolding techniques of the obtained experimental results. The second part of the dissertation focuses on the improvements and experimental validation of the full 3-D Monte Carlo model of the NIF Target Chamber. A discussion about various system errors and material uncertainties that could explain certain differences in modeling and experimental results is included. This thesis shows the ability of the newly developed software package to shape the NIF's high neutron flux output of a mainly monoenergetic 14.1 MeV neutron source peak, to an energy range including 8 to 10 MeV for integral benchmarking applications, or to highly peaked 8 or 10 keV spectra for a Boron Neutron Capture Therapy (BNCT) application. The final results provide a powerful demonstration of the tailoring capabilities of Gnowee/COEUS v2.0. This could allow various neutron producing facilities, such as the NIF, to expand their user base in various nuclear science and engineering applications, including detector characterization and calibration, study of radiation damage to various materials, cross section measurements for neutron activation studies, or medical applications such as BNCT or isotope separation.

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