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University of New Mexico

1. Ortega, Mario Ivan. Fission Multiplicity Distribution Sampling in MCNP6 Criticality Calculations.

Degree: Nuclear Engineering, 2016, University of New Mexico

URL: http://hdl.handle.net/1928/31686

In the solution of the neutron transport equation, the k-effective eigenvalue is related to the average number of neutrons emitted in fission of the system. It can be shown that if the average number of neutrons emitted in fission and the average neutron energy spectrum is conserved, the criticality of a system remains the same without regard to the actual physical fission process. However, the fissioning of a nucleus leads to the emission of any number of neutrons with some probability with correlated emission energies that is a function of the incident neutron energy. In general, Monte Carlo codes used for criticality calculations do not use explicit fission multiplicity sampling instead opting for the expected-value outcome approach. As computational methods and resources advance, there is growing interest in high fidelity modeling, including nuclear fission physics modeling. Extensive criticality benchmarks have been established to verify and validate Monte Carlo calculations versus analytic solutions and benchmarked experiments using the expected-value outcome method and no verification-validation work has been done to date on using explicit fission neutron multiplicity models in MCNP6. To determine the effect of sampling fission multiplicity probability distributions during criticality (KCODE) calculations, it was necessary to modify MCNP6 to allow for the use of neutron fission multiplicity models during criticality calculations along with correlation of neutron emission energies. Previously, MCNP6 did not allow for the use of these models during criticality calculations and only allowed their use in fixed-source problems. It was found that explicit fission multiplicity sampling agreed within two standard deviations of expected-value outcome sampling calculated k-effective values. Various benchmark suites used to test MCNP6 k-effective criticality calculations demonstrated good agreement using explicit fission multiplicity sampling and confirmed the validity of using explicit fission multiplicity sampling in Monte Carlo criticality calculations.
*Advisors/Committee Members: Prinja, Anil, Busch, Robert, Brown, Forrest.*

Subjects/Keywords: Monte Carlo; criticality; fission multiplicity; fission neutron energy correlation; nuclear data uncertainty

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^{th} Edition):

Ortega, M. I. (2016). Fission Multiplicity Distribution Sampling in MCNP6 Criticality Calculations. (Masters Thesis). University of New Mexico. Retrieved from http://hdl.handle.net/1928/31686

Chicago Manual of Style (16^{th} Edition):

Ortega, Mario Ivan. “Fission Multiplicity Distribution Sampling in MCNP6 Criticality Calculations.” 2016. Masters Thesis, University of New Mexico. Accessed July 20, 2019. http://hdl.handle.net/1928/31686.

MLA Handbook (7^{th} Edition):

Ortega, Mario Ivan. “Fission Multiplicity Distribution Sampling in MCNP6 Criticality Calculations.” 2016. Web. 20 Jul 2019.

Vancouver:

Ortega MI. Fission Multiplicity Distribution Sampling in MCNP6 Criticality Calculations. [Internet] [Masters thesis]. University of New Mexico; 2016. [cited 2019 Jul 20]. Available from: http://hdl.handle.net/1928/31686.

Council of Science Editors:

Ortega MI. Fission Multiplicity Distribution Sampling in MCNP6 Criticality Calculations. [Masters Thesis]. University of New Mexico; 2016. Available from: http://hdl.handle.net/1928/31686

University of New Mexico

2. Gonzales, Matthew. A moment-preserving single-event Monte Carlo model of electron and positron energy-loss straggling.

Degree: Nuclear Engineering, 2013, University of New Mexico

URL: http://hdl.handle.net/1928/22002

Analog simulation of energy straggling of electrons and positrons is computationally impractical because of long-range Coulomb forces resulting in highly peaked cross sections about small energy-losses and extremely small collision mean free paths. The resulting transport process is dominated by very frequent small energy transfer collisions but a significant contribution to the overall energy-loss distribution comes from the infrequent high energy-losses. Sufficient resolution in energy-loss spectra and dose profiles using single-event Monte Carlo methods would then require a large number of particle samples. In this thesis, we demonstrate that a pseudo-differential cross section designed to approximately yet accurately preserve energy-loss moments is capable of yielding accurate energy-loss spectra and dose distributions in a single-event Monte Carlo formulation. A benchmark solution for the analog problem for incident electrons and positrons is developed in order to provide an exact solution in which our approximation is evaluated against. A ""random walk"" sequence is used to randomly sample a distance to collision followed by a sampled energy-loss at the distance traveled by the particle. This process was completed until specific boundaries or cutoffs were met. Due to the non-linearity of the probability distribution functions for the electron and positron energy-loss differential cross sections, analog energy-loss sampling is simulated using the rejection method. The Landau straggling distribution is examined in detail and its accuracy is quantitatively assessed. Under the constraints in the formulation of Landau, we show that the number of energy-loss moments preserved is equal to the number of energy-flux moments preserved. More specifically, when the energy-losses within a given distance are sufficiently small so that the mean free path can be considered constant, the number of energy-loss moments preserved up to order N is equal to the number of energy-flux moments preserved up to order N. Energy-flux moments of Landau and the analog solution are compared. This moment-preserving theory provides the foundation in which a pseudo-transport model based on the Landau energy-loss distribution is then constructed. Next, the Landau distribution is used in formulating a pseudo-transport model. The energy-dependent mean free paths of this model are exponentially sampled while the energy-loss will be sampled using the Landau energy-loss distribution in terms of the respective mean free path, incident energy and mean energy-loss of the particle. The Landau Pseudo-Transport(LPT) model allows longer mean free paths and smoother distributions increasing the efficiency of electron and positron transport. Extensive numerical comparisons of the LPT model against the benchmark are conducted for energy-loss spectra and depth-dose profiles. It is shown that while high fidelity dose distributions can be obtained at a fraction of the cost of the analog calculation, energy spectra are difficult to resolve…
*Advisors/Committee Members: Prinja, Anil, Brown, Forrest, Cooper, Gary, Hughes, Grady.*

Record Details Similar Records

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^{th} Edition):

Gonzales, M. (2013). A moment-preserving single-event Monte Carlo model of electron and positron energy-loss straggling. (Masters Thesis). University of New Mexico. Retrieved from http://hdl.handle.net/1928/22002

Chicago Manual of Style (16^{th} Edition):

Gonzales, Matthew. “A moment-preserving single-event Monte Carlo model of electron and positron energy-loss straggling.” 2013. Masters Thesis, University of New Mexico. Accessed July 20, 2019. http://hdl.handle.net/1928/22002.

MLA Handbook (7^{th} Edition):

Gonzales, Matthew. “A moment-preserving single-event Monte Carlo model of electron and positron energy-loss straggling.” 2013. Web. 20 Jul 2019.

Vancouver:

Gonzales M. A moment-preserving single-event Monte Carlo model of electron and positron energy-loss straggling. [Internet] [Masters thesis]. University of New Mexico; 2013. [cited 2019 Jul 20]. Available from: http://hdl.handle.net/1928/22002.

Council of Science Editors:

Gonzales M. A moment-preserving single-event Monte Carlo model of electron and positron energy-loss straggling. [Masters Thesis]. University of New Mexico; 2013. Available from: http://hdl.handle.net/1928/22002

University of New Mexico

3. Dixon, David. A Computationally Efficient Moment-Preserving Monte Carlo Electron Transport Method with Implementation in Geant4.

Degree: Nuclear Engineering, 2015, University of New Mexico

URL: http://hdl.handle.net/1928/27772

The subject of this dissertation is a moment-preserving Monte Carlo electron transport method that is more efficient than analog or detailed Monte Carlo simulations, yet provides accuracy that is statistically indistinguishable from the detailed simulation. Moreover, the Moment-Preserving (MP) method is formulated such that it is distinctly different than Condensed History (CH) methods making the MP method free of the limitations inherent to CH and proving a viable alternative for transporting electrons. Analog, or detailed, Monte Carlo simulations of charged particle transport is computationally intensive; thus, it is impractical for routine calculations. The computational cost of analog Monte Carlo is directly attributed to the underlying charged particle physics characterized by extremely short mean free paths (mfp) and highly peaked differential cross sections (DCS). As a result, a variety of efficient, although approximate solution methods were developed over the past 60 years. The most prolific method is referred to as the Condensed History method. However, CH is widely known to suffer from inconsistencies between the underlying theory and the application of the method to real, physical problems. Therefore, it is of interest to develop an alternative method that is both efficient and accurate, but also a completely different approach to solving the charged particle transport equation that is free of the limitations inherent to CH. This approach arose from the development of a variety of reduced order physics (ROP) methods that utilize approximate representations of the collision operators. The purpose of this dissertation is the theoretical development and numerical demonstration of an alternative to CH referred to as the Moment-Preserving method. The MP method poses a transport equation with reduced order physics models characterized by less-peaked DCS with longer mfps. Utilizing pre-existing single-scatter algorithms for transporting particles, a solution to the aforementioned transport equation is obtained efficiently with analog level accuracy. The process of constructing ROP models and their properties are presented in detail. A wide variety of theoretical and applied charged particle transport problems are studied including: calculation of angular distributions and energy spectra, longitudinal and lateral distributions, energy deposition in one and two dimensions, a validation of the method for energy deposition and charge deposition calculations, and response function calculations for full three-dimensional detailed detector geometries. It is shown that the accuracy of the MP method is systematically controllable through refinement of the ROP models. In many cases, efficiency gains of two to three orders of magnitude over analog Monte Carlo are demonstrated, while maintaining analog level accuracy. That is, solutions generated sufficient ROP DCS models are statistically indistinguishable from the analog solution. To maintain analog level accuracy under strict problem conditions, small efficiency gains…
*Advisors/Committee Members: Prinja, Anil, Franke, Brian, Brown, Forrest, de Oliviera, Cassiano, Luan, Sean.*

Subjects/Keywords: Monte Carlo; Electron Transport; Moment-preservation

Record Details Similar Records

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^{th} Edition):

Dixon, D. (2015). A Computationally Efficient Moment-Preserving Monte Carlo Electron Transport Method with Implementation in Geant4. (Doctoral Dissertation). University of New Mexico. Retrieved from http://hdl.handle.net/1928/27772

Chicago Manual of Style (16^{th} Edition):

Dixon, David. “A Computationally Efficient Moment-Preserving Monte Carlo Electron Transport Method with Implementation in Geant4.” 2015. Doctoral Dissertation, University of New Mexico. Accessed July 20, 2019. http://hdl.handle.net/1928/27772.

MLA Handbook (7^{th} Edition):

Dixon, David. “A Computationally Efficient Moment-Preserving Monte Carlo Electron Transport Method with Implementation in Geant4.” 2015. Web. 20 Jul 2019.

Vancouver:

Dixon D. A Computationally Efficient Moment-Preserving Monte Carlo Electron Transport Method with Implementation in Geant4. [Internet] [Doctoral dissertation]. University of New Mexico; 2015. [cited 2019 Jul 20]. Available from: http://hdl.handle.net/1928/27772.

Council of Science Editors:

Dixon D. A Computationally Efficient Moment-Preserving Monte Carlo Electron Transport Method with Implementation in Geant4. [Doctoral Dissertation]. University of New Mexico; 2015. Available from: http://hdl.handle.net/1928/27772