Synthesis and Leachate Analysis of Dry Synthesized Substituted Fluorapatite Nuclear Surrogate Waste Forms for Molten Salt Reactors
Abstract
The development of commercialized molten salt reactors nuclear reactors (MSR), a nuclear reactor where the fuel is in a liquid state dissolved in carrier salt, is underway throughout the nuclear power community. With this renewed interest in MSRs multiple unresolved issues need to be addressed before commercialization is to occur. One area is waste management of spent fuel. However, this is a complex scenario for MSRs, as the fuel is dissolved within a carrier salt. Further complications arise once operation begins and fission product buildup begins to occur in the carrier salt. Various removal methods have been proposed depending on the needs of the fuel, carrier salt, and state of the fission product: whether they are a gas, soluble compound, or insoluble metal. The aforementioned waste streams will culminate in a final waste product that will require adequate and safe disposal methods.
A direct dry conversion process is presented for synthesizing fluorapatite minerals from solu-ble fission products, carrier salt, and the fuel for fluorine salt bearing MSRs. A lithium fluoride and beryllium fluoride salt (FLiBe) was combined with tricalcium phosphate (TCP) and sintered to produce substituted fluorapatites. Upon verification of apatite formation, additional fission product substituted apatites were fabricated using the same conversion process to incorporate cesium fluo-ride (CsF), strontium fluoride (SrF2), gadolinium fluoride (GdF3) and uranium tetrafluoride (UF4): these compounds are representative of the fuel and represent a gamut of fission products expected from this waste stream.
The apatites are characterized using Powder X-Ray Diffraction (XRD) methods to determine phases present in the sintered apatites. Additionally, scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) techniques were employed to verify XRD predictions and to determine the nature of any additional phases found during SEM and XRD analysis. Afterwards the XRD data were updated and Rietveld refinement techniques were employed to quantify the phases.
The synthesized surrogate waste form minerals were subjected to a common leach testing procedure devised by the American Society for Testing and Materials (ASTM), specifically ASTM’s C1285, commonly known as the Product Consistency Test (PCT). The leach testing is used to determine leaching rates of the constituents to quantify leaching behavior on directly fluorapatite surrogate waste forms. Leachate from the PCTs were subjected to analytical chemistry techniques to quantify the leached cations and anion concentrations. From the chemical concentration data normalized concentrations (NCi) and normalized elemental mass loss (NLi) were tabulated. The NCi of the elements of interest ranged from 5.27X10−4 to 1.08 g/L and NLi ranged from 7.76X10−9 to 20.3 g/m2. The calculated values were then compared to other waste form studies to show the efficacy of fluorapatite waste forms.
Citation
Livingston, Richard J. (2022). Synthesis and Leachate Analysis of Dry Synthesized Substituted Fluorapatite Nuclear Surrogate Waste Forms for Molten Salt Reactors. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /197422.