(193i) A Comprehensive Study of Dissolution Methods Utilized to Analyze Insoluble Solids Found in Nuclear Waste: The Cold Chemical Dissolution Method

This study focuses on the dissolution of insoluble solids that are a result of spent nuclear fuel and nuclear waste at the Defense Waste Processing Facility (DWPF). The dissolution of insoluble solids is of importance to the analytical qualification of macro batches of nuclear waste, which is necessary to inform and meet processing criteria. For this study, the insoluble solids of interest are that which are found in alkaline adjusted waste tanks and may also be referred to as sludge herein. Elemental analyses of sludge receipt and adjustment tank (SRAT) Receipt and SRAT Product process control samples are studied herein for sludge batch 10. These samples are acquired from a procedure that is performed at DWPF where insoluble solids in waste tanks are blended and processed into vitrified glass (which is identified as a safe long-term waste-form for nuclear waste).

The nuclear waste in DWPF waste tanks is unique in that it contains a wide range of both scarce and abundant elements and chemical species. In order to analyze the wide array of elements and chemical species, three digestion methods were studied. The three methods included a sodium peroxide fusion (PF), a hot aqua regia (AR), and a cold chemical (CC) digestion method. The CC digestion is similar to the DWPF CC method and involved digesting DWPF SRAT samples with high concentrations of HF-HNO₃ acid at room temperature. All samples were studied in quadruplicates within a shielded cell facility at SRNL. Furthermore, this study aids in demonstrating the robustness of various digestion methods by studying insoluble solids that contains a wide gambit of elements, a result of fission products, safety chemicals, corrosion control, cladding materials, etc.

This work provides a statistical analysis and comparison of elemental analysis data of SRAT Receipt and SRAT Product samples generated during an SB10 demonstration. Once digestion was performed, the methods were quantified primarily by inductively coupled plasma – optical emission spectroscopy (ICP-OES). Statistical determinations and equivalence results were based upon Levene, ANOVA, and Welch tests for a total of 19 elements based on elemental analyses. Elemental analyses performed after the CC method did not reveal any major biases for any of the primary elements (≥0.5 wt.% dry solids basis) in the sludge when compared with analyses obtained following dissolution of hot AR or PF methods (even with possible residual solids and fluoride-based precipitates present for the CC method). However, this study did reveal some minor biases and also indicated that no single method is perfect for determining all of the elements, where some elements cannot be considered for specific methods.

The CC elemental analyses agreed with the AR and PF methods well enough that it should be adequate for routine process control analyses in the DWPF. As a result, the CC method can be used to provide DWPF process control. Furthermore, the CC method is implemented as a faster and more convenient sample preparation method than that of the tandem sample preparation methods that employ heating (specifically, hot AR and PF methods performed at 115 and 675 °C, respectively). However, since no heating is utilized for the CC method, it is considered to be a less rigorous method than those based on heating and should be reviewed periodically for its effectiveness in the DWPF process.