IAEA-TECDOC-233 RESEARCH REACTOR CORE CONVERSION FROM THE USE OF HIGHLY ENRICHED URANIUM TO THE USE OF LOW ENRICHED URANIUM FUELS GUIDEBOOK PREPARED BY A CONSULTANTS' GROUP, COORDINATED AND EDITED BY THE PHYSICS SECTION INTERNATIONAL ATOMIC ENERGY AGENCY A TECHNICAL DOCUMENT ISSUED BY THE k?. INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1980 RESEARCH REACTOR CORE CONVERSION FROM THE USE OF HIGHLY ENRICHED URANIUM TO THE USE OF LOW ENRICHED URANIUM FUELS GUIDEBOOK IAEA, VIENNA, 1980 Printed by the IAEA in Austria August 1980 Please be aware that all the Missing Pages in this document were originally blank pages FOREWORD In view of the proliferation concerns caused by the use of highly enriched uranium (HEU) and in anticipation that the supply of HEU to research and test reactors will be more restricted in the future, this document has been prepared to assist reactor operators in determining whether conversion to the use of low enriched uranium (LEU) fuel designs is technically feasible for their specific reactor, and to assist in making a smooth transition to the use of LEU fuel designs where appropriate. This book has been prepared and coordinated by the International Atomic Energy Agency, with contributions from different organizations. The experts from these organizations have participated in the Consultants' Meeting on Preparation of a Program on Research Reactor Core Conversions to use Low Enriched Uranium Instead of Highly Enriched Uranium, and have assisted in preparing this text. CONTRIBUTING ORGANIZATIONS Argonne National Laboratory ANL United States of America Atomic Energy Research Establishment Harwell AERE Harwell United Kingdom Commisi6n Nacional de Energia At6mica CNEA Argentina Comissariat a l'Energie Atomique CEA France Compagnie Pour l'Etude et la Realisation CERCA France de Combustibles Atomiques Eidg. Institut fur Reaktorforschung EIR Switzerland General Atomic Company GA United States of America General Electric Company Reactor Equipment Ltd. GEC United Kingdom Internationale Atomreaktorbau GmbH INTERATOM Federal Republic of Germany Japan Atomic Energy Research Institute JAERI Japan Kernforschungszentrum Karlsruhe GmbH KFK Federal Renublic of Germany Kyoto University Research Reactor Institute KURRI Japan NUKEM GmbH NUKEM Federal Republic of Germany Osterreichische Studiengesellschaft OSGAE Austria fur Atomenergie The IAEA is grateful for the contributions volunteered by these organizations and thanks their experts for preparing the detailed investigations and for evaluating and summarizing the results presented in this Guidebook. SUMMARY This Guidebook has been prepared to assist reactor operators and physicists in determining both the feasibility of converting their specific reactors from HEU to LEU fuel and the options available for implementation. A wide variety of information is presented on the physics, thermal-hydraulics, and fuels of light water moderated and cooled research and test reactors. Most of the methods discussed in this Guidebook can also be directly applied to the analysis of research reactors containing heavy water as moderator and/or coolant. However, in consideration of the special features of heavy water reactors, an addendum to this Guidebook is planned to address the feasibility of converting these reactors to LEU fuel and the options available for implementation. The following is a brief outline of how the results were obtained, and how this Guidebook can be used most effectively. 1. Actions Needed For Conversion From HEU* Fuels to LEU* Fuels Section 1.5 gives a summary of the type of studies that are needed to prepare for core conversion. It is possible for these studies to be performed by the reactor operators/ physicists themselves, or with the aid of laboratories which have offered technical assistance. Appendix G lists the typical data needed for enrichment reduction conversion studies. Section 1.4.2, Chapter 3, and Appendix H contain information on the current status, development potential, and commercial availa- bility of fuels with high uranium densities. Appendix I analyses the main economic aspects of core conversions to LEU fuel. 2. Generic Studies Calculations have been performed by different laboratories for two generic MTR-type reactors with power levels of 2 MW and 10 MW to determine their potential for conversion. The results are summarized in Section 2 and include the uranium densities that would be required with different fuels and fuel element designs, the corresponding thermal-hydraulic safety margins, and the performance that would be expected from the converted core. Detailed information on the methods and procedures used and the results obtained for the various core conversion options are presented in Appendices A through D. 3. Specific Studies The methods and results of core conversion studies for two specific reactors with power levels of 3.5 MW and 50 MW, respectively, are provided in Appendix E. 4. Benchmark Calculations In order to compare the accuracy of calculation methods used in the different research centers, benchmark problems were defined and calculated with the different methods. The main core calculations using 93%, 45% and 20% enrichment are based on an idealized 6 x 5 element, plate-type core with a power of 10 MW reflected by single graphite rows on two sides, and surrounded by water. Results of the calculations, including cross section data, and descrip- tions of various burnup conditions are summarized in Section 2.4 and described in detail in Appendix F. As a first step in core conversion, it is recommended that reactor operators/physicists use their own methods and codes to calculate this benchmark problem, and to compare the results. 5. IAEA Assistance The IAEA can be contacted, through official channels, to provide assistance for the core conversion of specific reactors. The IAEA can offer coordinating assistance between reactor organizations and those laboratories in the USA, the FRG, and France which have offered technical assistance (Section 1.3). If necessary, the IAEA can also provide fellowships to visit those laboratories for joint studies on core conversions. The preparation of a second guidebook addressing safety and licensing issues related to core conversions is planned under the auspices of the IAEA. For simplicity, the following definitions have been adopted for this publication: 2 3 5 HEU - Highly Enriched Uranium (>70 wt% U) 2 3 5 MEU - Medium Enriched Uranium ( 45 wt% U) LEU - Low Enriched Uranium (<20 wt% 235U) REU - Reduced Enriched Uranium (includes MEU and LEU) CONTENTS 1. Major Considerations in Reactor Conversions .............................. 1 1.1 Introduction ...................................................... 1 1.2 Reasons for Reactor Conversions to LEU ............................... 1 1.3 Agencies and Laboratories Available to Provide Technical Assistance with the Conversions ..................................... 2 1.3.1 The Reduced Enrichment Program of France ...................... 2 1.3.2 The Reduced Enrichment Program of the Federal Republic of Germany 3 1.3.3 The Reduced Enrichment Program of Japan ....................... 4 1.3.4 The Reduced Enrichment Program of the United States ............ 7 1.4 Main Options Available for Conversions ............................... 9 1.4.1 General Technical Basis to Achieve Conversions Meeting Desired Criteria ...................................... 9 1.4.2 Status of Current Near-Term, and Long-Term Fuel Technologies .... 11 1.5 Main Activities Needed in Preparation for a Typical Conversion ...... 11 1.5.1 Characterization of Present Performance ....................... 11 1.5.2 Performance Calculations with MEU and LEU ...................... 13 1.5.3 Engineering Studies ........................................... 14 1.5.4 Safety Analysis Revisions and Licensing ........................ 14 1.5.5 Effects on Utilization ........................................ 14 1.5.6 Evaluation of Gradual Transition Feasibility .................. 14 1.5.7 Detailed Technical Specifications ............................. 15 1.5.8 Time and Cost Estimates .............. ..................... 15 2. Demonstration of Conversion Calculations .................................. 15 2.1 Overview ............................................................ 15 2.2 Studies of Generic 2 MW Reactor Conversions from HEU to LEU Fuel ....... 16 2.2.1 Conversion Studies Based on Classical Plate-Type Fuel ........... 18 2.2.2 Conversion Studies Based on TRIGA Fuel ......................... 34 2.2.3 Conversion Studies Based on Caramel Fuel ....................... 38 2.3 Studies of Generic 10 MW Reactor Conversion from HEU to MEU and LEU Fuel ................................................. 39 2.3.1 Conversion Studies Based on Classical Plate-Type Fuel .......... 40 2.3.2 Conversion Studies Based on TRIGA Fuel ......................... 52 2.3.3 Conversion Studies Based on Caramel Fuel ....................... 58 2.4 "Benchmark" Calculations ............................................. 60 3. Status and Development Potential of Research and Test Reactor Fuels ......... 71 3.1 Overview ............................................................ 71 3.2 Status of Plate-Type Fuel Technology ................................. 71 3.2.1 UAlx-Al Fuel ................................................. 74 3.2.2 U 308-Al Fuel ................................................. 75 3.2.3 U3Si Fuel .................................................... 75 3.2.4 Summary ...................................................... 75 3.3 Status of UZrH Rodded Fuel Technology ................................ 76 3 3.3.1 Up to 1.3 g U/cm (20 wt% U) ................................... 76 3 3.3.2 Up to 3.7 g U/cm (45 wt% U) ................................... 76 3.4 Status of U0 2 Plate-Type Fuel ........................................ 77 3.5 Status of U02 Rodded Fuel ............................................ 79 APPENDICES APPENDIX A Generic Enrichment Reduction Calculations for both Plate-Type and Rod-Type Reactors, ANL (USA) .............. 81 APPENDIX B Generic Enrichment Reduction Calculations for Rod-Type Reactors, GA (USA) .................................. 241 APPENDIX C Generic Enrichment Reduction Calculations, INTERATOM (FRG) ........ 287 APPENDIX D Generic Enrichment Reduction Calculations, CEA (France) .......... 359 APPENDIX E Methods and Specific Examples of Enrichment Reduction Calculations E-1. CNEA (Argentina) .......................................... 401 E-2. JAERI (Japan) ............................................. 411 APPENDIX F Benchmark Calculations F-0. Specifications ............................................ 443 F-1. ANL (USA) ................................................. 447 F-2. INTERATOM (FRG) ........................................... 469 F-3. EIR (Switzerland) ......................................... 485 F-4. OSGAE (Austria) ........................................... 531 F-5. CEA (France) .............................................. 553 F-6. JAERI (Japan) ............................................. 583 F-7. CNEA (Argentina) .......................................... 609 APPENDIX G Typical Research Reactor Data Needed for Enrichment Reduction Conversion Studies, OSGAE (Austria) .................... 629 APPENDIX H Notes on the Status and Development Potential of Research and Test Reactor Fuels, ANL (USA) ....................... 659 APPENDIX I Economic Aspects of Reactor Core Conversions I-1. Classical Plate-Type Fuel, OSGAE (Austria) ................. 679 1-2. TRIGA Fuel, GA (USA) ...................................... 699 APPENDIX J List of Participants in the Consultants' Meetings ................ 707 1. MAJOR CONSIDERATIONS IN REACTOR CONVERSIONS 1.1 INTRODUCTION In the 1950s and 1960s, low power research reactors were built around the world which utilized MTR-type fuel elements containing <20% enriched uranium (LEU). This value was chosen because it was considered to be a limit for weapon usable material. However, the demand for higher specific power created a need 2 3 5 for greater U concentrations and led to the substitution of highly enriched uranium (HEU) in place of the LEU fuel previously utilized. HEU also yielded other benefits including longer core residence time, higher specific reactivity, and somewhat lower cost. HEU then became readily available and was used for high power reactors as well as low power reactors where LEU would have sufficed. The trend toward higher and higher specific power also led to the development of the 3 dispersion type fuels which utilized HEU with a density of about 1.6 - 1.7 g/cm . In the 1970s, however, concerns were again raised about the proliferation- resistance of fuels and fuel cycles, and since enrichment reduction to less than 20% is internationally recognized to be a fully adequate isotopic barrier to weapons usability certain Member States have moved to minimize the international trade in highly enriched uranium and have established Reduced Enrichment Research and Test Reactor (RERTR) Programs. The goal of these programs is to develop the technical means, such as design modifications and development of new fuels, to assist in implementing reactor conversions to LEU fuels with minimum penalties. These programs have been established in the U.S., France, the Federal Republic of Germany, and Japan. It is anticipated that through the continued efforts of these programs, and with IAEA coordination, many reactors currently utilizing fuel element materials and designs less advanced than currently feasible may soon be converted to the use of LEU fuel. For other reactors, whose conversion to the use of LEU fuel may be feasible only after significant fuel development, a temporary decrease of the enrichment to an intermediate range of 45% (MEU) would be a worthwhile improvement in proliferation resistance. Concern has also been expressed about the presence of plutonium in spent fuel, especially when the fuel is irradiated in reactors utilizing very low enrichment and/or operating at high powers, and it is necessary to consider both the plutonium produced and the enriched uranium in the overall assessment of the proliferation potential of a particular reactor. 1.2 REASONS FOR REACTOR CONVERSIONS TO LEU Operators of research and test reactors that use highly enriched uranium may consider converting their reactors to the use of low enriched uranium fuels for several closely related reasons. One could be the desire to reduce the proliferation potential of research reactor fuels. A second reason could be a desire to increase the assurance of continued fuel availability in the face of probable restrictions on the supply of highly enriched uranium. A third reason could be the possible reduction in requirements for physical security measures during fabrication, transportation, storage, and use. All these reasons are connected with each other and cannot be considered individually. I