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Advances in Accident Tolerant Fuel Research by Doping of Uranium Nitride

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Advances in Accident Tolerant Fuel Research by Doping of Uranium Nitride THESIS FOR THE DEGREE OF LICENTIATE OF ENGINEERING Advances in Accident Tolerant Fuel Research by Doping of Uranium Nitride Luis G. Gonzalez F. Nuclear Chemistry Department of Chemistry and Chemical Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2020 Advances in Accident Tolerant Fuel Research by Doping of Uranium Nitride Luis G. Gonzalez F. © Luis G. Gonzalez F., 2020. Technical report no 2020:12 Department of Chemistry and Chemical Engineering Chalmers University of Technology SE-412 96 Gothenburg Sweden Telephone + 46 (0)31-772 1000 Cover: A chromium-doped uranium nitride microsphere showing Cr precipitation. Chalmers Reproservice Gothenburg, Sweden 2020 i Advances in Accident Tolerant Fuel Research by Doping of Uranium Nitride Luis G. Gonzalez F. Nuclear Chemistry Department of Chemistry and Chemical Engineering Chalmers University of Technology Abstract Nuclear energy is a carbon-free energy source alternative often considered less harmful to the environment than fossil fuels. However, accidents have shown that there are some safety concerns regarding nuclear energy that need to be assessed before it can be considered completely safe. The loss of cooling systems during the nuclear accident at Fukushima nuclear accident exposed the flaws of the fuel used today, UO2 encapsulated in a Zr alloy. Research into new types of improved fuels, also known as Accident Tolerant Fuels (ATF), has therefore became of great importance. Different alternative claddings and fuel materials have been explored in recent years. Amongst these, uranium nitride (UN) has shown to have very attractive thermomechanical properties. Nonetheless, UN reacts readily in oxidizing environments, making it undesirable for water-cooled reactors. In this study, UN microspheres were manufactured through a sol-gel method, followed by carbothermic reduction and nitridation. The as-produced microspheres were pressed and sintered into pellets using spark plasma sintering (SPS). It was seen that the spherical shape was lost during sintering and densities between 77 and 98% of theoretical density were obtained, depending on the sintering parameters. For example, sintering at 1650 °C and 75 MPa for at least 5 minutes proved to produce pellets with densities close to 95% of theoretical densities, which are similar to densities used today in nuclear reactors. Thorium and chromium were introduced as additives to form a protective oxide scale and improve the oxidation resistance of UN. It was seen that Th produced a homogeneous solid solution with uranium nitride between 0 and 20 % thorium molar metal ratio. Chromium, on the other hand, showed that there was a solubility limit in UN, and precipitation of a Cr-rich phase was observed. During exposure in air, the doped materials seem to reduce the oxidation kinetics, increasing onset temperatures and decreasing the reaction rates. Pellet exposure to water at high temperatures showed that pellets can survive at 100 °C and 1 bar pressure with zero mass change. However, at higher temperatures and pressures, 200 °C and 15 bar or 300 °C and 85 bar, pellet disintegration into a UO2 powder was observed. An incomplete reaction was also observed for the Th-doped pellet in the exposure test at 200 °C, indicating that no improvement in the corrosion resistance of UN in water was achieved by doping with thorium. Keywords: Uranium nitride, microspheres, doping, sintering, density, internal gelation, ATF, waterproofing. ii iii List of Publications and Manuscripts This thesis is based on the following paper and manuscript: Paper I L.G. Gonzalez Fonseca, M. Hedberg, L. Huan, P. Olsson, T. Retegan Vollmer, Application of SPS in the fabrication of UN and (U,Th)N pellets from microspheres, J. Nucl. Mater. 536 (2020) 152181. doi:10.1016/j.jnucmat.2020.152181 Contribution: Main author and most experimental work. Paper II L.G. Gonzalez Fonseca, M. Hedberg, T. Retegan Vollmer, Oxidation and hydrolysis of UN and (U,Th)N fuels for use as ATF. Manuscript. To be submitted by the end of September. Contribution: Main author and all experimental work. iv Abbreviations. ATF – Accident Tolerant Fuel BET – Brunauer-Emmett-Teller BWR – Boiling Water Reactors EDX – Energy Dispersive X-ray spectroscopy. FCC – Face Centered Cubic HMTA – HexaMethyleneTetraAmine ICP-MS – Inductively Coupled Plasma-Mass Spectrometry IGP – Internal Gelation Process LOCA – Loss of Coolant Accident LWR – Light Water Reactor ORNL – Oak Ridge National Laboratory PWR – Pressurized Water Reactor SEM – Scanning Electron Microscopy SPS – Spark Plasma Sintering TD – Theoretical Density TGA – Thermo-Gravimetric Analysis UNH – Uranyl Nitrate Hexahydrate XRD – X-Ray Diffraction v Table of contents 1. Introduction ........................................................................................................................ 1 2. Background ......................................................................................................................... 3 2.1. Nuclear accidents ........................................................................................................ 3 2.2. Accident Tolerant Fuels .............................................................................................. 3 2.3. Uranium Nitride .......................................................................................................... 5 3. Theory ................................................................................................................................. 7 3.1. Synthesis of UN .......................................................................................................... 7 3.2. Sol-Gel process chemistry ........................................................................................... 9 3.3. Pressing and Sintering ............................................................................................... 11 3.3.1. Spark Plasma Sintering ...................................................................................... 11 3.4. Oxidation and corrosion of UN chemistry ................................................................ 12 3.5. Corrosion improvement............................................................................................. 13 3.5.1. Solid solutions .................................................................................................... 15 4. Materials and Methods ..................................................................................................... 16 4.1. Chemicals .................................................................................................................. 16 4.2. Instrumentation.......................................................................................................... 16 5. Experimental ..................................................................................................................... 19 5.1. Microspheres production ........................................................................................... 19 5.2. Reduction and Nitridation process ............................................................................ 20 5.3. Pelletization and sintering ......................................................................................... 21 5.4. Density measurements............................................................................................... 22 5.5. Air/Water/Steam interaction tests ............................................................................. 23 5.5.1. Thermo-gravimetric Analysis ............................................................................ 23 5.5.2. Autoclave tests ................................................................................................... 23 6. Results and discussion ...................................................................................................... 24 6.1. Microspheres production and surface analysis.......................................................... 24 6.2. Elemental composition and XRD .............................................................................. 26 6.2.1. Th-doped microspheres ...................................................................................... 26 6.2.2. Chromium doped microspheres ......................................................................... 28 6.3. Pellet Production and Analysis ................................................................................. 30 6.3.1. Pelletization and SPS ......................................................................................... 30 6.3.2. Density measurements. ...................................................................................... 33 6.3.3. SPS parameters .................................................................................................. 35 vi 6.3.4. Dopant effect in the sintering ............................................................................. 36 6.4. Corrosion testing ....................................................................................................... 37 6.4.1. Air oxidation analysis ........................................................................................ 37 6.4.2. Water/steam corrosion experiments ................................................................... 42 7. Summary and Conclusions ............................................................................................... 45 Future Work ............................................................................................................................
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