Design Strategies for Optimizing High Burnup Fuel in Pressurized Water Reactors

Design Strategies for Optimizing High Burnup Fuel in Pressurized Water Reactors by Zhiwen Xu B.E., Engineering Physics, Tsinghua University, P.R.China (1998) B.E., Computer Science, Tsinghua Univerisity, P.R.China (1998) Submitted to the Department of Nuclear Engineering in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY January 2003 Copyright © 2003 Massachusetts Institute of Technology All rights reserved Signature of Author Department of Nuclear Engineering January 10, 2003 Certified by Michael J. Driscoll (thesis supervisor) Professor Emeritus of Nuclear Engineering Mujid S. Kazimi (thesis supervisor) TEPCO Professor of Nuclear Engineering Accepted by Jeffrey A. Coderre Chairman, Department Committee on Graduate Students Design Strategies for Optimizing High Burnup Fuel in Pressurized Water Reactors by Zhiwen Xu Submitted to the Department of Nuclear Engineering on January 23, 2003, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract This work is focused on the strategy for utilizing high-burnup fuel in pressurized water reactors (PWR) with special emphasis on the full array of neutronic considerations. The historical increase in batch-averaged discharge fuel burnup, from ~30 MWd/kg in the 1970s to ~50 MWd/kg today, was achieved mainly by increasing the reload fuel enrichment to allow longer fuel cycles: from an average of 12 months to about 18 months. This also reduced operating costs by improving the plant capacity factor. Recently, because of limited spent fuel storage capacity, increased core power output and the search for increased proliferation resistance, achieving burnup in the 70 to 100 MWd/kg range has attracted considerable attention. However the implications of this initiative have not been fully explored; hence this work defines the practical issues for high-burnup PWR fuels based on neutronic, thermal hydraulic and economic considerations as well as spent fuel characteristics. In order to evaluate the various high burnup fuel design options, an improved MCNP-ORIGEN depletion program called MCODE was developed. A standard burnup predictor-corrector algorithm is implemented, which distinguishes MCODE from other MCNP-ORIGEN linkage codes. Using MCODE, the effect of lattice design (moderation effect) on core design and spent fuel characteristics is explored. Characterized by the hydrogen-to-heavy-metal ratio (H/HM), the neutron spectrum effect in UO2/H2O lattices is investigated for a wide range of moderation, from fast spectra to over-thermalized spectra. It is shown that either wetter or very dry lattices are preferable in terms of achievable burnup potential to those having an epithermal spectrum. Wet lattices are the preferred high burnup approach due to improved proliferation resistance. The constraint of negative moderator temperature coeffi- cient (MTC) requires that H/HM values (now at 3.4) remain below ~6.0 for PWR lattices. Alternative fuel choices, including the conventional solid pellets, central-voided annular pellets, Internally- & eXternally-cooled Annular Fuel (IXAF), and different fuel forms are analyzed to achieve a wetter lattice. Although a wetter lattice has higher burnup potential than the reference PWR lattice, the requirement of a fixed target cycle energy production necessitates higher initial fuel enrichments to compensate for the loss of fuel mass in a wetter lattice. Practical issues and constraints for the high burnup fuel include i neutronic reactivity control, heat transfer margin, and fission gas release. Overall the IXAF design appears to be the most promising approach to realization of high burnup fuel. High-burnup spent fuel characteristics are compared to the reference spent fuel of 33 MWd/kg, representative of most of the spent fuel inventory. Although an increase of decay power and radioactivity per unit mass of initial heavy metal is immediately ob- served, the heat load (integration of decay power over time) per unit electricity generation decreases as the fuel discharge burnup increases. The magnitude of changes depends on the time after discharge. For the same electricity production, not only the mass and volume of the spent fuel are reduced, but also, to a lesser extent, the total heat load of the spent fuel. Since the heat load in the first several hundred years roughly determines the capital cost of the repository, a high burnup strategy coupled with adequate cooling time, may provide a cost-reduction approach to the repository. High burnup is beneficial to enhancing the proliferation resistance. The plutonium vector in the high-burnup spent fuel is degraded, hence less attractive for weapons. For example, the ratio of Pu-238 to Pu-239 increases with burnup to the 2.5 power. However, the economic benefits are uncertain. Under the current economic conditions, the PWR fuel burnup appears to have a shallow optimum discharge burnup between 50 and 80 MWd/kg. The actual minimum is influenced by the financing costs as well as the cost of refueling shutdowns. Since the fuel cycle back-end benefits will accrue to the federal government, the current economic framework, such as the waste fee based on the electricity produced rather than volume or actinide content, does not create an incentive for utilities to increase burnup. Different schemes exist for fuel management of high burnup PWR cores. For the conventional core design, a generalized enrichment-burnup correlation (applicable between 3 w/o and 20 w/o) was produced based on CASMO/SIMULATE PWR core calcu- lations. Among retrofit cores, increasing the number of fuel batches is preferred over increasing the cycle length due to nuclear fuel cycle economic imperatives. For future core designs, a higher power-density core is a very attractive option to cut down the busbar cost. The IXAF concept possesses key design characteristics that provide the necessary thermal margins at high core power densities. In this regard, the IXAF fuel deserves fur- ther investigation to fully exploit its high burnup capability. Thesis Supervisor: Michael J. Driscoll Title: Professor Emeritus of Nuclear Engineering Thesis Supervisor: Mujid S. Kazimi Title: TEPCO Professor of Nuclear Engineering Director, Center for Advanced Nuclear Energy Systems ii Acknowledgements The keen help, patient guidance and generous support of Professor Michael J. Driscoll and Professor Mujid S. Kazimi, my thesis supervisors, are greatly appreciated. Lots of inspiration throughout this work comes directly from them. I am grateful to two members of the Advanced Fuel Cycle Group at the MIT Cen- ter for Advanced Nuclear Energy Systems (CANES): Dr. Edward E. Pilat for his contri- bution to conventional PWR core review and invaluable advice on burnable poison us- age, Dr. Pavel Hejzlar for numerous discussions on MCODE development and the internally and externally cooled annular fuel concept. Special thanks are due to my colleagues Dr. Xianfeng Zhao, Dr. Yun Long, Mr. Dean Wang, Mr. Eugene Shwageraus, and Mr. Dandong Feng who provided help on various aspects of the work. In terms of the MCODE benchmark problem (9.75 w/o UO2 with a conventional PWR lattice under cold conditions), I wish to express my gratitude to Dr. Hyung-Kook Joo from KAERI for providing results of HELIOS and to Dr. Tamer Bahadir from Studs- vik of America, Inc., for providing results of CASMO-4 (internal version). The corrected version of ORIGEN2.1 supplied by Dr. Allan G. Croff from Oak Ridge National Labora- tory is also acknowledged. This thesis could not have been completed without the endless love and support from my family. I would like to give special thanks to my wife, Kun Yu, for wading through past five years of difficulties and achievements with me at MIT. My dear son, Chen Y. Xu, three years old, is now able to speak words clearly and likes to jump and run. I thank my mother-in-law, Ying Ma, for helping us taking care of my son. The thesis is dedicated to them. This work has been partially funded by the Idaho National Engineering & Envi- ronmental Laboratory (INEEL) through the Advanced Fuel Project, and by the DOE’s support of the annular fuel project through a NERI grant. iii Table of Contents Abstract............................................................................................................................... i Acknowledgement............................................................................................................ iii Table of Contents ............................................................................................................. iv List of Figures.....................................................................................................................x List of Tables ....................................................................................................................xv Abbreviations for Organizations................................................................................. xvii Nomenclature ............................................................................................................... xviii 1. Introduction....................................................................................................................1 1.1 Background............................................................................................................1 1.2 High Burnup Review .............................................................................................3

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