Helium bubbling in a Molten Salt Fast Reactor A flotation process Dirkjan Journée Master of Science Thesis Delft, Januari 2014 Supervisors: dr. ir. J.L. Kloosterman TNW, TU Delft Prof. dr. R. Konings TNW, TU Delft Committee: prof. dr. R. Konings TNW, TU Delft dr. ir. J.R. van Ommen TNW, TU Delft dr. ir. M. Rohde TNW, TU Delft Delft University of Technology Faculty of Applied Sciences Dept. of Radiation Science & Technology Nuclear Energy and Radiation Applications 2 Abstract The Molten Salt Reactor (MSR) is one of the Generation IV nuclear reactor concepts. The molten salt reactor uses a liquid fuel, which gives the possibility to reprocess the fuel while operating the reactor. Reprocessing the fuel during operation strongly decreases the radiotoxicity of the waste and optimizes the use of natural resources. The first part of this thesis is to examine the reprocessing steps in a Molten Salt Reactor. There are five reprocessing steps, which can be divided in on-line and off-line reprocessing. Helium bubbling is used as the on-line reprocessing technique. Fluorination, protactinium removal, actinide extraction and lanthanide extraction are used as the off-line reprocessing techniques. It was found that the reprocessing steps show a promising molten salt reactor concept, but the individual techniques are still to be optimized. And the different steps in the reprocessing of the salt must still be connected together to form a continuous process. It was also found that the helium bubbling technique, which was initially used to extract the gaseous fission products, also extracted a part of the noble and semi-noble metals from the molten salt. These metals will not dissolve in the salt and plate out on metal surfaces in the reactor. If the plate-out is excessive, the decay heat from the noble and semi-noble metals may damage the reactor. The extraction of these metals with helium is proposed to be achieved by a flotation process. The factors that play a role in a flotation process are described by Schulze [Schulze, 1984] and Nguyen [Nguyen, 2004]. The second part and final goal of this thesis was to develop an experimental method to determine the influence of the bubble size and gas flow in a flotation process. A flotation process is described as an extraction of solids from a liquid with gas bubbles. The bubble size and gas flow are the variables that are adjustable in the helium bubbling technique in a Molten Salt Reactor. The development of this experimental method included the making of a new set-up with a newly fabricated piece of equipment and developing a method to work with this set-up. The result of the set-up is a modified version of a Hallimond tube. The set-up is built to use the methods to examine a flotation process, which are described by Nguyen [Nguyen, 2004]. The methods are a direct and an indirect. The indirect method is based on the extraction of a known amount of particles that are added to the set-up. The direct method is filming the direct interaction of solids with gas bubbles and is used to get a better understanding of how the process works. It was found that the developed set-up is able to get good results for the direct method and promising results for the indirect method. This new equipment and the new experimental method offer the possibility for a number of follow-up studies on the topic of a flotation process. 3 4 Contents Abstract ..................................................................................................................................... 3 Contents ..................................................................................................................................... 5 1 Introduction ........................................................................................................................... 7 1.1 Nuclear energy ................................................................................................................. 7 1.2 Generation IV reactors ................................................................................................... 10 1.3 Thorium fuel cycle ......................................................................................................... 11 1.4 Molten salt reactor .......................................................................................................... 12 2 Offline Reprocessing Steps in a Molten Salt Fast Reactor .............................................. 17 2.1 Fluorination .................................................................................................................... 17 2.2 Protactinium Removal .................................................................................................... 20 2.3 Actinide Extraction ........................................................................................................ 21 2.4 Lanthanide Extraction .................................................................................................... 25 2.5 Conclusion ...................................................................................................................... 27 3. Solid particles in a molten salt reactor ............................................................................. 29 3.1 Helium bubbling ............................................................................................................. 29 3.2 Materials in a flotation process ...................................................................................... 30 3.3 Flotation process theory ................................................................................................. 32 3.4 Conclusion ...................................................................................................................... 37 3.5 List of common symbols ................................................................................................ 38 4 Experimental simulation of the flotation process ............................................................. 41 4.1 Experimental .................................................................................................................. 42 4.2 Materials ......................................................................................................................... 48 4.3 Description of the experimental methods ....................................................................... 48 4.4 Results ............................................................................................................................ 50 4.5 Discussion ...................................................................................................................... 56 5 Conclusions .......................................................................................................................... 57 5.1 Literature study .............................................................................................................. 57 5.2 Set-up ............................................................................................................................. 57 5.3 Recommendations .......................................................................................................... 58 5 References ............................................................................................................................... 61 Appendix A ............................................................................................................................. 63 Reductive Extraction of Plutonium ...................................................................................... 63 Appendix B .............................................................................................................................. 65 Blueprint of the modified Hallimond tube ........................................................................... 65 Appendix C ............................................................................................................................. 67 Safety Assessment Sheet for the floatation set-up ............................................................... 67 Appendix D ............................................................................................................................. 71 Making a coordinate system in Paint.Net ............................................................................. 71 Appendix E .............................................................................................................................. 73 Determine the positions of the bubbles and particles on an image using Graph Grabber ... 73 Appendix F .............................................................................................................................. 75 Determine the volume of the particles and bubbles using Graph Grabber .......................... 75 Nomenclature .......................................................................................................................... 77 6 1 Introduction The Molten Salt Reactor (MSR) is one of the Generation IV nuclear reactor concepts. The molten salt reactor uses a liquid fuel, which gives the possibility to reprocess the fuel while operating the reactor. Reprocessing the fuel during operation strongly decreases the radiotoxicity of the waste and optimizes the use of natural resources. There are five reprocessing steps, which can be divided in on-line and off- line reprocessing. Helium bubbling is used as the on-line reprocessing technique. Fluorination, protactinium removal, actinide extraction and lanthanide extraction are used as the off-line reprocessing techniques. The first part of this study is to examine the reprocessing steps in a Molten Salt Reactor, where the focus will be on the helium bubbling technique. Helium bubbling was initially used to extract the gaseous fission products, but it was found that this technique also extracted a part of the noble and semi-noble metals from the molten salt. These metals will not dissolve in the salt and plate out on metal surfaces in the reactor. If the plate-out is excessive, the decay heat from the noble and semi- noble metals may damage the reactor. Therefore it is important to extract these metals. The extraction of these metals with helium is proposed to be achieved by a flotation process. The second part of this research is to develop an experimental method to determine the influence of the adjustable variables in a flotation process in the Molten Salt Reactor. The variables that can be adjusted are the bubble size and the gas flow. 1.1 Nuclear energy 1.1.1 Worldwide Energy demand According to the International Energy Outlook 2013 the worlds energy consumption will grow with an estimated 56% between 2010 and 2040. Figure 1.1 shows a graph of the estimated energy consumption growth in quadrillion BTU or Quad (1 quad = 293 TWh). Most of the increase in global energy demand from 2010 to 2040 occurs among the developing nations outside the OECD (Organization for Economic Cooperation and Development), particularly China. Figure 1.1 World total energy consumption, 1990-2040 (in quadrillion Btu) 7 There are five different fuel types for energy production: - Petroleum and liquid fuels1 - Coal - Natural gas - Renewables (e.g. hydropower) - Nuclear The two fastest growing energy sources are renewables and nuclear power [IEO2013, 2013]. The impact of fossil fuel emissions on the environment and the sustained high world oil prices support the expanded use of these sources. 1.1.2 Principle of a nuclear reactor Nuclear power is generated by a nuclear reactor and converted into electrical energy. A nuclear reactor is used to initiate and control a sustained nuclear fission reaction in order to generate electricity. A nuclear reactor is part of the nuclear fuel cycle. The nuclear fuel cycle exists of the progression of nuclear fuel through different steps. The steps can be divided into three different sections. The front end, the service period and the back end. The front end are the preparation steps of the fuel, the service period is the period where the fuel is used during reactor operation and the back end are the disposal or reprocessing steps of the spent nuclear fuel. Figure 1.2 shows an overview of the nuclear fuel cycle of uranium. Figure 1.2. Overview of the nuclear fuel cycle of uranium. The reactor converts the thermal energy released from nuclear fission reactions into electric energy. The conversion from thermal energy to electricity is done by steam turbines. There are a lot of different nuclear reactor designs, but the principle is: the energy from a nuclear fission reaction is used to heat a working fluid (in most cases the working fluid is water or a gas) to generate steam which runs through turbines to generate electricity. This principle design is shown in figure 1.3. 1 In IEO2013, the term petroleum and other liquid fuels includes a full array of liquid product supplies. Petroleum liquids include crude oil and lease condensate, natural gas plant liquids, bitumen, extra-heavy oil, and refinery gains. Other liquids include gas-to- liquids, coal-to-liquids, kerogen, and biofuels. 8 Figure 1.3. Principle of a nuclear reactor. The reactor core heats up a working fluid, in this case water, to evaporate to steam. This steam runs through an turbine to generate electricity. The steam is condensed and pumped back to the core. The fission reactions are based on fissile materials that can sustain a chain reaction with neutrons. When a fissile nucleus absorbs a neutron, it will fission into two lighter elements (fission products) while releasing neutrons and a large amount of energy. The energy is released mainly due to the kinetic energy of the fission products and the ionizing radiation. The free neutrons can be captured by another fissile element to create a nuclear chain reaction. Figure 1.4 shows a drawing of a nuclear chain reaction. Figure 1.4 Nuclear chain reaction. A fissile nucleus captures an neutron to undergo fission while releasing energy and free neutrons. The released neutrons have an energy of 0.7 Mev - 10 MeV, these neutrons are called fast neutrons. By means of a moderator the neutrons are slowed down till they have a velocity corresponding to the most probable energy at 20 degrees Celsius, which is about 2.2 km/s, these neutrons are called thermal neutrons and have an energy about 0.025 eV. Neutrons with different energies have different neutron absorption cross-sections, which means that the probability of an interaction with a nucleus differs. 9 A nuclear reactor either uses a fast neutron spectrum or a thermal neutron spectrum. The different fuels and moderators determine the neutron spectrum. 1.2 Generation IV reactors There are a lot of different types of nuclear reactor designs. The different designs can be divided into different groups, called ‘generations’. Generation I reactors are the early prototype reactors. Generation II are the first commercial reactors. Generation III reactors are the more developed Generation II reactors. The next generation reactors are the Generation IV reactors. The Generation IV International Forum (GIF) considered six different type of reactors. These reactors have to measure up to: - reduced capital cost - enhanced nuclear safety - minimal generation of nuclear waste - proliferation-resistance The six different reactor types are summed up in table 1.1. Table 1.1. Overview of Generation IV Reactors. [GIF, 2009] Neutron Outlet System Fuel type Size (MWe) Spectrum Temperature ⁰C Very-high-temperature reactor Thermal Solid 900 - 1000 250 - 300 (VHTR) Supercritical-water-cooled 300 - 700 Thermal/Fast Solid 510 - 625 reactor (SCWR) 1000 - 1500 Gas-cooled fast reactor (GFR) Fast Solid 850 1200 30 - 150 Sodium-cooled fast reactor Fast Solid 550 300 - 1500 (SFR) 1000 - 2000 20 - 180 Lead-cooled fast reactor (LFR) Fast Solid 480 - 800 300 - 1200 600 - 1000 Molten salt reactor (MSR) Thermal/Fast Liquid 700 - 800 1000 The molten salt reactor is the only design to use liquid fuel. Not only the design of a reactor can accomplish the criteria of the GIF, also the used fuel can have an influence. Uranium and plutonium are traditionally used as a fuel for nuclear reactors, but in the 1960’s it was found that the thorium/uranium-233 fuel cycle can also be used for a nuclear reactor. Despite the success, the MSR program closed down in the early 1970’s in favor of the liquid metal fast-breeder reactor. 10