REACTOR PHYSICS DESIGN OF SUPERCRITICAL CO2-COOLED FAST REACTORS by Michael A. Pope B.S. Nuclear Engineering Texas A&M University, 2002 SUBMITTEDTO THE DEPARTMENT OF NUCLEARENGINEERING IN PARTIAL FULFILLMENTOF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN NUCLEAR ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEPTEMBER 2004 The author hereby grants MIT permission to reproduce and to distribute publicly paper and electronic copies of this report document in whole or in part MASSACHUSETTSNSTTTE Copyright © Massachusetts Institute of Technology (MIT) OF TECHNOLOGY All rights reserved OCT I 112005 LIBRARIES Signature of Author: f Department of Nuclear Engineering August 6, 2004 Certified by: AT Dr. Michael J. Driscoll Professor Emeritus of Nuclear Engineering Thesis Supervisor Certified by: "/ -Dr. Pavel Hejzlar ___zPrincipal Research Scientist in Nuclear Engineering /4 A'.· Thesis Reader Accepted by: -r'x 7 IT A I - LJr.Jeiirey A. Coaerre Chairman, Department Committee on Graduate Students ARCHIVES REACTOR PHYSICS DESIGN OF SUPERCRITICAL CO2-COOLED FAST REACTORS By Michael A. Pope Submitted to the Department of Nuclear Engineering on August 6, 2004 in partial fulfillment of the requirements for the degree of Master of Science in Nuclear Engineering Abstract Gas-Cooled Fast Reactors (GFRs) are among the GEN-IV designs proposed for future deployment. Driven by anticipated plant cost reduction, the use of supercritical CO2 (S-CO 2) as a Brayton cycle working fluid in a direct cycle is evaluated. By using S- ° CO 2 at turbine inlet conditions of 20 MPa and 550 C - 700°C, efficiencies between 45% and 50% can be achieved with extremely compact components. Neutronic evaluation of candidate core materials was performed for potential use in block-type matrix fueled GFRs with particular concentration on lowering coolant void reactivity to less than $1. SiC cercer fuel was found to have relatively low coolant void worth (+22¢ upon complete depressurization of S-CO2 coolant) and tolerable reactivity- limited burnup at matrix volume fractions of 60% or less in a 600 MWth core having H/D of 0.85 and titanium reflectors. Pin-type cores were also evaluated and demonstrated higher kff versus burnup, and higher coolant void reactivity than the SiC cercer cores (+$2.00 in ODS MA956-clad case having H/D of 1). It was shown, however, that S-CO2 coolant void reactivity could be lowered significantly - to less than $1 - in pin cores by increasing neutron leakage (e.g. lowering the core H/D ratio to 0.625 in a pin core with ODS MA956 cladding), an effect not observed in cores using helium coolant at 8 MPa and 500°C. An innovative "block"-geometry tube-in-duct fuel consisting of canisters of vibrationally compacted (VIPAC) oxide fuel was introduced and some preliminary calculations were performed. A reference tube-in-duct core was shown to exhibit favorable neutron economy with a conversion ratio (CR) at beginning of life (BOL) of 1.37, but had a coolant void reactivity of +$ 1.4. The high CR should allow designers to lower coolant void worth by increasing leakage while preserving the ability of the core to reach high burnup. Titanium, vanadium and scandium were found to be excellent reflector materials from the standpoint of koff and coolant void reactivity due to their unique elastic scattering cross-section profiles: for example, the SiC cercer core having void reactivity of +$0.22 with titanium was shown to have +$0.57 with Zr3Si2. Overall, present work confirmed that the S-CO 2-cooled GFR concept has promising characteristics and a sufficiently broad opion space such that a safe and competitive design could be developed in future work with considerably less than $1 void reactivity and a controllable Ak due to burnup. Thesis Supervisor: Michael J. Driscoll Title: Professor Emeritus of Nuclear Engineering Thesis Reader: Pavel Hejzlar Title: Principal Research Scientist and Program Director of the Advanced Reactor Technology Program at the Center for Advanced Nuclear Energy Systems (CANES) 2 Acknowledgments I would first like to thank my research advisor, Professor Emeritus Michael Driscoll. This work would not have been possible without his patient guidance and continuous stream of ideas. I also want to thank my thesis reader, Dr. Pavel Hejzlar, for his scrutiny of the technical content of this thesis. His broad knowledge has been invaluable to this thesis. This financial support for this work was provided by Idaho National Engineering and Environmental Laboratory (INEEL) through their Lab Directed Research and Development (LDRD) program entitled "An Innovative Gas-Cooled Fast Reactor". Nathan Carstens assembled and maintains the Echelon Beowulf cluster for the Department of Nuclear Engineering at MIT. This machine allowed the many thousands of processor-hours represented by this thesis to be performed in a manageable amount of time. Peter Yarsky provided valuable feedback and comments on earlier drafts of this thesis. Jonathan Plaue gave me vials containing titanium and silicon carbide so that I could see what they look like. He also reminded me from time to time that I cannot simply make up chemicals to satisfy neutronic requirements. Jan Outcalt created some of the drawings used as figures in this thesis. I would finally like to thank my parents, Alex and Barbara Pope, for their financial and moral support throughout my education. You are wonderful parents and will soon be wonderful grandparents. Thank you and I love you both very much. 3 Table of Contents Abstract........... 2.................. Acknowledgments ............................................................................................................... 3 Table of Contents .................................... ................... .................. ..............................4 Table of Figures ..................................................................................................................6 List of T ables ......................................................................................................................9 Dedication ......................................................................................................................... 10 1 Introduction and Background ......................................... 11 1.1 Gas-Cooled Fast Reactors .......................................................... ...................... I 1.2 Supercritical CO2 Brayton Cycle ......................................... 11 1.3 Matrix Fuel........................................................................................................12 1.4 Coolant Void Reactivity ......................................... 12 1.5 Objectives and Organization of Thesis .............................................................15 2 Reactor Modeling Methodology ......................................... 17 2.1 Introduction .......................................................................................................17 2.2 MCNP Model ....................................................................................................17 2.3 Burnup Calculations..........................................................................................22 2.4 Summary ......................................... 24 3 Comparison of the Reactivity Effects of Reflector, Matrix and Shielding Materials25 3.1 Introduction .......................................................................................................25 3.2 Multiplication Factor and Coolant Void Reactivity. ............................. 25 3.2.1 Required TRU Enrichment ................................................................... 27 3.2.2 Results of Comparison ......................................... 27 3.2.3 Helium versus CO 2 Coolant ......................................................................28 3.2.4 Sensitivity to Cross-Section Sets ......................................... 29 3.3 Neutronic Advantage of Titanium, Vanadium and Scandium Reflectors ........ 30 3.4 Burnup Comparison ......................................... 35 3.5 Neutron Fluence in Reactor Vessel ...................................................................37 3.6 Summary ......................................... 38 4 Parametric Studies ....................................................................................................40 4.1 Introduction .......................................................................................................40 4.2 Effects of Matrix Volume Fraction...................................................................40 4.3 Effects of Variation of Core Size......................................................................42 4.4 Prospect of Pin-Type Cores ..............................................................................43 4.4.1 Validity of Single-Enrichment Calculations ........................ 47 4.4.2 Determination of Axial Reflector Height .. ..................... 48 4.4.3 Coolant Void Reactivity versus Core H/D .. ................. 49 4.4.4 Effect of Pressure on Coolant Void Reactivity .. ............... 54 4.4.5 Comparison of Cladding Material Effect on Burnup ...................... 56 4.4.6 Neutron Fluence in Cladding Material .. ................... 57 4.4.7 Burnup Comparison of S-CO2 and Helium Coolants ....................... 58 4.4.8 Use of Oxide Fuel in Pin Cores ......................................... 59 4.4.9 Power Shaping in Pin Cores ......................................... 61 4.5 Tube-In-Duct Fuel Assembly ........................................