Eliminating a Flammability Risk in High-Level Radioactive Waste Processing

While working as an intern at the Savannah River National Laboratory, I completed laboratory-scale experiments in order to find processing conditions that minimized the production of hexamethyldisiloxane (HMDSO) during the processing of the high-level radioactive waste contained in forty-three 1 million gallon tanks at the Savannah River Site (SRS). HMDSO, a flammable gas, is a degradation product of antifoam 747, which is used to combat foaming during waste treatment at the Defense Waste Processing Facility (DWPF) at SRS. If unmitigated, the foam generated during processing could cause a foam-over and lead to contamination, a complete facility shutdown, and the loss of millions of dollars. The amount of HMDSO produced in the new Nitric-Glycolic processing flowsheet is greater than the amount seen in the current Nitric-Formic flowsheet. The Nitric-Glycolic flowsheet is planning on being implemented in DWPF in 2017, due to this flowsheet improving processing qualities such as reducing the amount of catalytic hydrogen produced. Being able to reduce the peak amount of HMDSO produced in the new Nitric-Glycolic flowsheet is important for the flammability strategy of the flowsheet. Working alongside chemists, statisticians, and engineers, the production rates of HMDSO in simulated, non-radioactive waste processing experiments were analyzed using a Fourier Transform Infrared Spectrometer (FTIR). The goal of these experiments was to reduce the maximum rate of HMDSO production to below the current safety basis. In order to both minimize flammability hazards and assist in HMDSO quantification, an air purge with a helium tracer was employed during experimentation. Process safety and interdisciplinary communication were key components in the success of the experiments. Four short-duration experiments were completed with varying operating conditions. In addition, a full-length, multi-day experiment was conducted in order to demonstrate that the revised antifoam strategy was effective throughout DWPF processing. The reduced antifoam strategy was deemed successful after demonstrating a significant reduction in HMDSO generation rates to within processing limits while simultaneously controlling foam. This antifoam strategy will be implemented alongside the new flammability safety basis for the Nitric-Glycolic flowsheet at DWPF in 2017.