Over the past several years as interest in cleaner energy sources has grown nuclear power has come to the forefront. However, as interest in nuclear power grows so does the concern over the amount of high-level radioactive waste produced. Currently, the most popular way to deal with spent nuclear fuel is interim storage until a viable treatment option becomes available. Simply waiting for spent fuel to become safe to handle will take thousands of years and is not a reasonable long-term solution. We will soon run out of space in our spent fuel pools and while more dry storage space can be found it is not an ideal solution. One answer to this problem is the reprocessing of spent nuclear fuel. This could be done with either the plutonium uranium reduction extraction (PUREX) method or the pyroprocessing method. Since PUREX does not have the same level of built-in proliferation resistance as pyroprocessing, pyroprocessing is starting to be seen as a good alternative method. Pyroprocessing would take the spent nuclear fuel from a light water reactor and make it into a metal-based fuel that could be used in certain advanced reactors. Molten salt reactors are of particular interest when it comes to reprocessing spent nuclear fuel because of their unique property of using a liquid fuel. Molten salt reactors and spent fuel reprocessors could be directly connected which would save both time and money as little storage and transportation would need to be considered.
Regardless of how and where the used nuclear fuel is being recycled it is important to be able to keep track of the major actinides and fission products in the fuel as it moves through the process. Electrochemical concentration measurements are straightforward and well understood in static cases when there is only a single element to consider. When additional elements are added, or the system is flowing rather than static, things get slightly more complicated but are still decently well understood. However, in the case of spent fuel reprocessing the system is both be flowing and contains much more than a single element. This case is not well understood and is what this study attempts to understand.
Two different rotating electrodes were designed to simulate flowing conditions in an electrochemical cell. The first was a tungsten rotating disk electrode (RDE) and the second was a graphite RDE. We were not able to fully insulate the tungsten RDE and were therefore unable to achieve reliable results. Because of this the tungsten design was put aside in favor of the graphite design, which did prove to be sufficiently insulated. The graphite RDE was tested in two different salt systems: LiCl-KCl-NiCl2-CrCl2 and LiCl-KCl-EuCl3-SmCl3. In the nickel-chromium system the graphite RDE produced the expected results. The calculated nickel concentration was found to be within 10% of the measured concentration. Calculations of the chromium concentration, however, were not possible due to the deposition of nickel on the graphite surface, which increased the surface area of the working electrode. When the graphite RDE was tested in the second system it was first tested in the ternary salt LiCl-KCl-EuCl3 and was able to produce decent results. The concentration of europium calculated from the scan was within 10% of the measured value. When the RDE was tested in the LiCl-KCl-EuCl3-SmCl3 salt the results did not come out as expected. Several rather noisy CV curves were obtained and no alterations to the cell seemed to affect them. At this point it was determined that the reason for the confused scans was a connection problem that could not be remedied within the time frame of this study. While this study does not accomplish the task it set out to do, it is a good step in the direction toward understanding flowing systems containing more than a single element of interest and has successfully designed a reliable graphite RDE. / Master of Science / As interest in nuclear power continues to grow, so does the concern over the amount of high-level nuclear waste produced. More nuclear power means more nuclear reactors and thus more spent nuclear fuel to be dealt with. Currently most used nuclear fuel ends up in interim storage facilities where it is meant to wait until it is safe to handle, which could take several thousand years, or until a reliable disposal method is determined. On this path the amount of spent fuel that requires storage will quickly overrun the amount of storage space safely available. One way to reduce the amount of nuclear waste is to reprocess it to be used as fuel for different types of reactors. The pyroprocessing method takes the spent nuclear fuel from a typical light water reactor and recycles it into fuel that can be used in certain types of advanced reactors, such as molten salt reactors (MSR) and sodium-cooled fast reactors (SFR). The reprocessing system works to separate the usable actinide elements, such as uranium and plutonium, from any fission products or other contaminants. During these processes it is important to be able to keep track of the concentrations of each of these different elements to ensure proper separation.
This study examines the use of two rotating disk electrode (RDE) designs that are meant to simulate the flowing conditions found in many reprocessing systems. These RDEs were to be used to measure the concentrations of different elements in molten salt systems. The first design, a tungsten RDE, could not be properly insulated and thus was unable to produce reliable results when tested in the electrochemical cell. The second design was a graphite RDE. This design did prove to be properly insulated and was able to produce good results when tested in the cell. The graphite RDE was tested in both LiCl-KCl-NiCl2-CrCl2 and LiCl-KCl-EuCl3-SmCl3. In the first system the concentration of nickel was correctly calculated using the data collected with the graphite RDE, while the chromium concentration could not be due to the nickel deposition on the graphite. In the second system, good results were obtained before the SmCl3 was added to the salt. At this point a connection error became apparent and reliable results were no longer possible. Further study is needed to understand the LiCl-KCl-EuCl3-SmCl3 system using the graphite RDE.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111353 |
| Date | 25 July 2022 |
| Creators | Sullivan, Kelly Marie |
| Contributors | Mechanical Engineering, Zhang, Jinsuo, Link, Jonathan M., Haghighat, Alireza |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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