MSR Salt Spill Accident Testing Using Eutectic NaCl-UCl<sub>3</sub>

Abstract

Assessing the potential consequences of identified accident scenarios is an essential part of the licensing process for a new nuclear reactor and is achieved by using accident progression models. A likely accident scenario that will be evaluated by developers of molten salt reactors (MSRs) is a spill of radionuclide-bearing fuel salt onto the reactor containment floor (i.e., a salt spill accident). Models to determine the consequences of a molten salt spill accident need to be parameterized and validated using experimental data, but little experimental effort has been dedicated to fill these data gaps to date. Specifically, data is needed to enable model development for individual processes (e.g., spreading, heat transfer, corrosion, radionuclide vaporization, and aerosol generation) that quantifies the sensitivities towards the initial conditions of the spill, the ambient environment, and the features of the containment. In addition, integrated experiments that simulate salt spill accidents will need to be conducted to provide insight into coupled processes and data for model validation, but these experiments will require the use of proven methods to quantify the processes under evaluation. The overarching objectives of this work are to develop universal methods to simulate processes that are important to the consequences of a salt spill accident for a variety of salt compositions, to determine the effectiveness of the methods and measurement techniques in producing the data required for model development, to generate data that can be used to parameterize individual process models, and to provide key insights into the behavior of spilled molten salt that should be considered in models and future integrated salt spill tests. The tests described herein were conducted using a chloride salt composition representative of fast spectrum MSRs (eutectic NaCl UCl3) to highlight individual processes expected to affect the fate of spilled molten fuel salt and the radionuclides within during a salt spill accident. The processes addressed in this report include 1) molten salt spreading on stainless steel, 2) molten salt heat transfer (as a static pool and during spreading), and 3) molten salt splashing and aerosol generation. The test methods to assess molten salt spreading and molten salt splashing and aerosol generation were developed previously and successfully applied to eutectic NaCl-UCl3 as part of this work. New methods to assess molten salt heat transfer from a static pool were developed as part of this work and applied to both FLiNaK and NaCl-UCl3. The primary variables that were evaluated include initial salt temperature, the initial amount of salt being spilled, and the presence of volatile surrogate fission products (e.g., cesium and iodine) in the salt. The major accomplishments of this study include demonstrating the effectiveness of the developed methods and measurement techniques in quantifying desired processes for a range of salt compositions and applying the methods to a uranium-bearing salt. These methods can be applied to study the complex salt compositions of interest to MSR developers, the wide range of environmental conditions of interest to modelers, and the effects of variables relevant to salt spill accident scenarios. The developed methods and insights gained from these laboratory tests that are focused on individual processes will be incorporated into scaled-up, integrated process tests that simulate molten salt spill accidents.