INL/EXT-12-26758 Tritium Formation and Mitigation in High Temperature Reactors Hans A. Schmutz Piyush Sabharwall Carl Stoots August 2012 The INL is a U.S. Department of Energy National Laboratory operated by Battelle Energy Alliance DISCLAIMER This information was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness, of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trade mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof. INL/EXT-12-26758 Tritium Formation and Mitigation in High Temperature Reactors Hans A. Schmutz Piyush Sabharwall Carl Stoots August 2012 Idaho National Laboratory Idaho Falls, Idaho 83415 http://www.inl.gov Prepared for the U.S. Department of Energy Office of Nuclear Energy Under DOE Idaho Operations Office Contract DE-AC07-05ID14517 ABSTRACT Tritium is a radiologically active isotope of hydrogen. It is formed in nuclear reactors by neutron absorption and ternary fission events and can subsequently escape into the environment. In order to prevent the tritium contamination of proposed reactor buildings and surrounding sites, this paper examines the root causes and potential solutions for the production of this radionuclide, including materials selection and inert gas sparging. A model is presented that can be used to predict permeation rates of hydrogen through metallic alloys at temperatures from 450–7505C. Results of the diffusion model are presented for one steady- state value of tritium production in the reactor. v vi CONTENTS ABSTRACT .................................................................................................................................................. v 1. INTRODUCTION .............................................................................................................................. 1 2. TRITIUM FORMATION ................................................................................................................... 1 3. TRITIUM IN THE ENVIRONMENT ............................................................................................... 2 3.1 Tritium Detection ..................................................................................................................... 2 4. HIGH TEMPERATURE REACTOR TYPES ................................................................................... 3 5. TRITIUM FORMATION IN HIGH-TEMPERATURE MOLTEN SALT (FLIBE) REACTORS ....................................................................................................................................... 3 5.1 Coolant Salt Composition ........................................................................................................ 4 6. TRITIUM FORMATION IN HTGRs ................................................................................................ 4 7. TRITIUM PERMEATION ................................................................................................................. 5 7.1 Tritium Permeation in Coolant Salts ........................................................................................ 5 7.2 Tritium Permeation Containment Metal .................................................................................. 6 8. MITIGATION .................................................................................................................................... 6 8.1 Materials Resistant to Permeation ............................................................................................ 7 8.2 Inert Gas Sparging ................................................................................................................... 7 8.3 Additional Coolant Lines ......................................................................................................... 7 8.4 Chemical Removal and Metal Hydrides .................................................................................. 8 8.5 Heat Exchanger Geometric Considerations ............................................................................. 8 9. MODELING COMPARISON OF HYDROGEN AND TRITIUM PERMEATION IN SELECTED ALLOYS ....................................................................................................................... 8 10. RESULTS AND DISCUSSION ....................................................................................................... 10 11. CONCLUSIONS .............................................................................................................................. 14 12. FUTURE WORK ............................................................................................................................. 15 13. REFERENCES ................................................................................................................................. 15 Appendix A,Visual Basic Model Code ....................................................................................................... 18 FIGURES Figure 1. Solubility of hydrogen in coolant salts. ....................................................................................... 10 Figure 2. Selected alloys' permeation rate at 5505C. .................................................................................. 11 Figure 3. Selected alloys' permeation rate at 6005C. .................................................................................. 12 vii Figure 4. Permeation in Incoloy 800H at 4505C. ........................................................................................ 14 Figure 5. Permeation in Incoloy 800H at 5505C. ........................................................................................ 14 TABLES Table 1. Sources and rates of production of tritium 1000 MW(e) reactor.4,5,6 ............................................. 2 Table 2. Sources and rates of production of tritium in a 1000 MW(e) (2250 MW(t)) MSBR.5 .................. 3 Table 3. Sources and rates of production in a 2,250 MW(t) HTGR. ........................................................... 5 Table 4. Permeabilities of hydrogen in selected alloys.37 ............................................................................. 6 Table 5. Arrhenius parameters for permeability. .......................................................................................... 9 Table 6. Arrhenius parameters for solubility. ............................................................................................... 9 Table 7. Literature values of permeability and activation energy for hydrogen permeation in selected alloys.2 .......................................................................................................................... 12 viii ix Tritium Formation and Mitigation in High Temperature Reactors 1. INTRODUCTION Hydrogen is the most common element in the universe, representing about three-quarters of the measurable mass. It is very light, often just one proton weighing approximately one atomic mass unit (amu) and propagates readily through most media, especially at elevated temperatures. It is also highly reactive, reducing other compounds such as atmospheric oxygen: ͂ ƍ ͥ ͉ Ŵ͉͂ ;͂ͤ Ɣ ƎTZW & ͦ ͦ ͦ ͦ ; (*' (1) The large negative value for the standard enthalpy of formation, +H0 indicates that the reaction is very exothermic, so the reverse reaction takes a significant amount of energy to complete, and elemental hydrogen (diatomic form) is rarely observed in nature. ͧ Tritium ʚͥ͂ʛ is a radioactive isotope of hydrogen with a nucleus composed of one proton and two neutrons. It is formed by ternary fission events (rare emissions of three nuclides rather than two during a fission) and neutron absorption (and subsequent decay) of predecessor radionuclides, particularly 6Li and 7Li.1 It is used in low-temperature chemical lighting applications, such as exit signs, guns sights, and wristwatches; however, its permeability has not been extensively investigated in high temperature systems (>450°C), possibly because of the costs of mitigating the thermal and radiological hazards present when working with tritium at these temperatures. Tritium is of special interest among the fission products created in next-generation nuclear power plants because of the large quantities produced. Substantial amounts of experimental data suggest that hydrogen propagates readily in metals at elevated temperatures, and some data may be found suggesting that tritium behaves similarly in these high temperature applications.2 The anticipated difficulty in containing tritium justifies special care in system design. A sound understanding of tritium’s generation pathways, as well as its properties and possible ways to prevent its escape are an integral part of any containment plan for advanced high temperature reactors (AHTRs). 2. TRITIUM FORMATION In general, tritium is formed in the fission event that follows neutron absorption by parent nuclides. Differing reactor types form tritium in different ways and in different amounts: