Validation of the Use of Low Enriched Uranium as a Replacement for Highly Enriched Uranium in US Submarine Reactors by Brendan Patrick Hanlon B.S., Physics (2013) United States Naval Academy Submitted to the Department of Nuclear Science & Engineering in partial fulfillment of the requirements for the degree of Master of Science in Nuclear Science and Engineering at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2015 c Massachusetts Institute of Technology 2015. All rights reserved. Author.................................................................... Department of Nuclear Science & Engineering May 19, 2015 Certified by . R. Scott Kemp Assistant Professor, Department of Nuclear Science & Engineering Thesis Supervisor Certified by . Benoit Forget Associate Professor, Department of Nuclear Science & Engineering Thesis Reader Accepted by . Mujid S. Kazimi TEPCO Professor of Nuclear Engineering Chair, Department Committee on Graduate Students 2 Validation of the Use of Low Enriched Uranium as a Replacement for Highly Enriched Uranium in US Submarine Reactors by Brendan Patrick Hanlon Submitted to the Department of Nuclear Science & Engineering on May 19, 2015, in partial fulfillment of the requirements for the degree of Master of Science in Nuclear Science and Engineering Abstract The US Navy has long used highly enriched uranium (HEU) in naval reactors for a variety of technical reasons. In a series of studies, the Department of Naval Reactors determined that switching to low enriched uranium (LEU) was impossible using current fuel designs, but may be possible with a dedicated program to investigate new fuel materials. This thesis simulated an HEU fueled submarine reactor using a uranium oxide-zirconium dispersion fuel, and compared it to an LEU reactor using a uranium-molybdenum alloy fuel. The required energy output of an attack submarine was used to set the burnup requirement of the HEU (333 MWd/kg) and LEU (93.5 MWd/kg) fueled reactors, and each reactor was depleted to the end of life. The results showed that naval reactors could be switched to LEU without sacrificing the lifetime submarine core or increasing reactor volume. Even if unstudied technological details render this impossible, an LEU core would require only a single refueling over the life of an attack submarine. This would necessitate a 3.25% increase in submarine fleet size, which is small compared to the average Department of Defense project cost overrun. Thesis Supervisor: R. Scott Kemp Title: Assistant Professor, Department of Nuclear Science & Engineering Thesis Reader: Benoit Forget Title: Associate Professor, Department of Nuclear Science & Engineering 3 4 Acknowledgments Primarily, I would like to thank R. Scott Kemp, my thesis adviser, for not only starting me on the road that led to this thesis, but for his constant and consistent encouragement, patience, and support that have helped me to accomplish something I am truly proud of. I can’t imagine having a better mentor and adviser for a research project. I’d also like to thank Benoit Forget for providing more guidance than has ever been required of a thesis reader. It was his constant push to add “just one more thing” that, I think, made this thesis complete. Thanks are also due to Stephanie MacDougall for reading my thesis, “Just one more time, I promise!” Her work catching my myriad errors and insisting I use real words was an unanticipated but invaluable aid to creating a readable document. I would never have been able to make it to this point without the unending support of professors, teachers, and mentors at the MIT Nuclear Engineering Department, the United States Naval Academy Physics Department, and Lake Oswego High School. Thank you for keeping my curiosity alive, and always challenging me to reach for the next step. Finally, I’d like to thank my family. My parents, Roger and Jacinta, for their bottomless love and endless sacrifices, which have made me the person I am today. And my brother, D´onal,for (in the early years) putting up with me and (later) for refusing to allow me to become complacent with what I’ve achieved. I can’t wait to read your thesis in a few years. Disclaimer The opinions expressed herein are those of the author, and are not necessarily represen- tative of official policies or positions of the Department of Defense, the United States Navy, or any other affiliate of the United States Government. The information contained herein is derived entirely from open source material. No aspect of this project was informed by access to classified information of any level. 5 6 Contents 1 Introduction and Background 15 1.1 History . 15 1.2 Navy HEU Stockpile Concerns . 17 1.2.1 Estimating HEU Consumption . 18 1.3 Potential Solutions to the Fuel Shortage . 23 1.4 RERTR and the Navy . 28 1.5 New Work: A Proof of Concept . 29 2 Requirements and Background / Fuel Selection 31 2.1 Margins and Requirements . 31 2.1.1 Power Requirement . 31 2.1.2 Temperatures and Mass Flow Rate . 33 2.1.3 Total Energy Requirement . 33 2.1.4 Size Limitations / Dimensions . 35 2.1.5 Reactivity . 35 2.1.6 Burnup . 41 2.1.7 Fuel Blistering and Melting . 41 2.1.8 Departure from Nucleate Boiling (DNB) . 42 2.1.9 Plate Thickness . 46 2.2 Materials Considerations . 46 2.2.1 High-Temperature Effects . 47 2.2.2 Corrosion Mechanisms and Consequences . 48 2.2.3 Radiation Effects . 49 2.2.4 Chemical and Nuclear Compatibility . 51 2.2.5 Fission-Gas Swelling . 51 2.3 Cladding . 51 2.4 Structural Materials . 54 2.5 HEU Fuel . 56 7 2.5.1 Open-Source Naval Reactor Estimates . 57 2.5.2 Alteration of the Open-Source Model . 58 2.6 LEU Fuel . 61 2.6.1 High Density Fuels . 61 3 Core Design Process 68 3.1 Iteration Requirements . 69 3.2 Thermal Analysis . 70 3.2.1 Plate Geometry . 70 3.2.2 Horizontal Temperature Variation . 70 3.2.3 Vertical Temperature Variation . 76 3.3 Moderator to Fuel Ratio . 79 3.3.1 Pitch and Moderator-to-Fuel Ratio . 79 3.3.2 Safety Significance . 79 3.3.3 Optimization Process . 80 3.3.4 Preparation for Full Core . 82 3.4 Pressure Drop . 83 3.4.1 Pressure Differential Goal . 85 3.4.2 Pressure Loss Theory and Equations . 85 3.4.3 Natural Circulation . 88 3.4.4 Conclusions . 89 3.5 Monte Carlo and Convergence . 90 3.6 Burnable Poisons . 92 3.6.1 Power Profile and Peaking . 92 3.6.2 Burnable Poison Theory . 92 3.6.3 Commercial Methods . 93 3.6.4 Simulation Application . 94 3.7 Full Core Serpent Simulation . 95 3.7.1 Maximum Required Burnup . 96 3.7.2 Burnup Steps . 96 3.7.3 Results Analysis and Requirements . 96 4 Highly Enriched Uranium Core 97 4.1 Fuel Temperature and Thermal Margins . 97 4.1.1 Fuel Melting and Blistering Margin . 98 4.1.2 DNB Margin . 100 4.1.3 Cladding Margins . 101 8 4.2 Moderator-to-Fuel Ratio . 102 4.3 Primary Loop Pressure Drop . 105 4.4 Shannon Entropy / Neutrons per Cycle . 105 4.5 Burnable Poison Loading . 108 4.5.1 Poison Zones . 110 4.5.2 Final Gadolinium Loading and BoL Power Distribution . 111 4.6 Full Core Run and Results . 113 4.6.1 Excess Reactivity . 113 4.6.2 Power Distribution . 114 4.7 Fuel Fabrication . 114 4.7.1 UO2 Grain Size . 114 4.7.2 Oxide Volume Fraction . 117 4.8 Flux Spectrum . 118 4.8.1 Thermal Spectrum Comparison . 118 4.8.2 Xenon Poisoning Magnitude . 119 4.9 LEU in UO2-Zr Fuel . 119 5 Low Enriched Uranium Core 121 5.1 Fuel Temperature and Thermal Margins . 121 5.1.1 Fuel Melting and Blistering Margin . 123 5.1.2 DNB Margin . 123 5.1.3 Cladding Margins . 125 5.2 Moderator-to-Fuel Ratio . 125 5.3 Primary Loop Pressure Drop . 127 5.4 Shannon Entropy / Neutrons per Cycle . 127 5.5 Burnable Poison Loading . 127 5.6 Full Core Run and Results . 129 5.6.1 Excess Reactivity . 129 5.6.2 Power.