Investigation of the impacts of deploying reactors fueled by high-assay low enriched uranium

Bachmann, Amanda M

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https://hdl.handle.net/2142/121987 Copy

Description

Title

Investigation of the impacts of deploying reactors fueled by high-assay low enriched uranium

Author(s)

Bachmann, Amanda M

Issue Date

2023-11-20

Director of Research (if dissertation) or Advisor (if thesis)

Munk, Madicken

Doctoral Committee Chair(s)

Munk, Madicken

Committee Member(s)

• Abelson, John R
• Feng, Bo
• Kozlowski, Tomasz
• Stubbins, James F

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• nuclear fuel cycle
• HALEU
• advanced reactors
• optimization

Abstract

The United States is considering the deployment of advanced reactors that require uranium enriched between 5-20% 235U, often referred to as High Assay Low Enriched Uranium (HALEU). At the present, there are no commercial facilities in the US to produce HALEU, prompting questions of how to create a dependable supply chain of HALEU to support these reactors. HALEU can be produced through two primary methods: downblending High Enriched Uranium (HEU) and enriching natural uranium. The amount of HEU available and impurities present in the HEU limit downblending capabilities. The Separative Work Unit (SWU) capacity and amount of natural uranium available limit enriching natural uranium capabilities. To understand the resources necessary to commercially produce HALEU with each of these methods, one can quantify the material requirements of transitioning to HALEU-fueled reactors. In this dissertation, we model the transition from Light Water Reactors to different advanced reactors, considering once-through and closed fuel cycles to determine material requirements for supporting these fuel cycles. Material requirements of interest across this work include the mass of enriched uranium, mass of HALEU, feed uranium, SWU capacity, and the mass of used fuel sent for disposal. We use CYCLUS and publicly-available information about Light Water Reactors, the X-energy Xe-100, the Ultra Safe Nuclear Corporation Micro Modular Reactor, and the NuScale VOYGR to model potential transition scenarios and demonstrate the methodologies developed in this work. To more accurately model the closed fuel cycles, we develop a new CYCLUS archetype, called OpenMCyclus, that couples with OpenMC to dynamically model fuel depletion in a reactor and provide more accurate used fuel compositions. The results of this transition analysis show how the characteristics of the advanced reactors deployed drive the materials required to support the fuel cycle. Closing the fuel cycle reduces the materials required, but the reduction in materials is driven by the amount of material available for reprocessing. To gain more insight into how transition parameters not considered in the transition analysis affect material requirements, we perform sensitivity analysis on one of the once-through transitions by coupling CYCLUS with Dakota. The results of the sensitivity analysis highlight some of the trade-offs between different reactor designs. One such tradeoff is the increased HALEU demand but decreased used fuel discharged when increasing the Xe-100 deployment and decreasing the VOYGR deployment. Additionally, these results identify the Xe-100 discharge burnup as consistently being one of the most impactful input parameters for this transition, because of how the deployment scheme in this work affects the number of Xe-100s built no matter which advanced reactor build share is specified. To identify potential transitions that minimize material requirements, we then use the CYCLUS-Dakota to optimize a once-through transition using the genetic algorithms in Dakota. In single-objective problems to minimize the SWU capacity required to produce HALEU and minimize the amount of used nuclear fuel, the algorithm finds solutions that are consistent with the results of the sensitivity analysis. The results cannot be taken at face value, because the algorithm did not fully converge and the genetic algorithms do not enforce the applied linear constraint for the advanced reactor build shares to sum to 100%. However, the results provide guidance on how to adjust the input parameters to optimize the transition for a minimal HALEU SWU or the used fuel mass. Parameter adjustments include maximizing the number of Light Water Reactors that receive license extensions to operate for 80 years. Similar results occur when using this method for a multi-objective problem to minimize both the HALEU SWU capacity and the used fuel mass. Finally, we use neutronics models of the Xe-100 and Micro Modular Reactor reactor designs to evaluate the steady-state reactor physics performance of downblended HEU in these two designs. We compare the performance of the downblended HEU to nominally enriched fuel, based on the k-eff, βeff, energy- and spatially-dependent neutron fluxes, as well as the fuel, moderator, coolant, and total reactivity temperature feedback coefficients. The differences in the fuel compositions leads to differences in each of the metrics. However, these differences are within error of the results of the nominally enriched fuel, or would not prevent the reactor from meeting stated design specifications or operating in a safe state. The work completed in this dissertation develops and demonstrates a methodology for modeling fuel cycle transitions and understanding the effects of deploying HALEU-fueled reactors in the US. The effects investigated in these example scenarios include various materials and resources required to support these reactors, and how the parameters of the transition affect these requirements. The information generated from this new methodology can be used to develop the necessary infrastructure and supply chains for support a transition to HALEU-fueled reactors. Furthermore, this work explores how the HALEU production method (enriching compared with downblending) affects reactor performance.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright by Amanda M. Bachmann. All rights reserved.

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