Efficient Wattle Design for High-Level Nuclear Waste Tanks

At the Hanford Site in Washington state and the Savannah River Site in South Carolina, there are underground steel tanks that hold high-level nuclear waste (HLW) and nearly 450 million curies of radioactivity. At these sites, some leaks have been detected in HLW tanks. While detecting these leaks allows for identification of the issue, it does not solve the problem or prevent the leakage of HLW contaminants into collection pans and secondary containment. Repairing these leaks poses potential threats to workers for exposure risk. To address this issue, the present research seeks to create and utilize an active barrier in the form of wattles made of heat-resistant and high-strength woven fabric enclosing mixed sorbents to capture the HLW contaminants, specifically, cesium and strontium, that escape through leaks around the HLW tanks by using a prototype model tank to simulate leaks. Pressure was used as a surrogate for locations in leaks of tanks, higher pressures correlated to leaks in the bottom of tanks and vice versa for smaller pressures. The effect of contact time on contaminant removal efficiency was investigated to determine best suited design conditions for max removal efficiency.Results showed that removal efficiencies for cesium at 5, 10, and 15 psi was 52.43%, 55.71% and 60.94% with contact times of 0.942 min, 0.674 min, and 0.55 min respectively. Removal efficiencies for strontium at 5, 10, and 15 psi was 46.40%, 36.88%, and 33.17% with contact times of 1.00 min, 0.741 min, and 0.592 min respectively. The results of increasing the contact time yielded removal efficiencies for cesium and strontium respectively to be 49.6%-55.9% and 67.5%-73.2% for a contact time of 34.4 min at a flow rate of 5 mL/min. A flow rate of 2 mL/min and a contact time of 86 minutes yielded removal efficiencies of 52.4% and 41.39% for cesium and strontium respectively. A contact time of 344 min and 516 min at a flow rate of 1 mL/min yielded removal efficiency for cesium and strontium to be 98.46% and 96.07% respectively for 344 min and 96.07% and 95.3% respectively. These results suggest that higher contact times generated larger removal efficiencies, therefore it is recommended that high-level radioactive waste containing >100 mg/L cesium and strontium should be treated with contact times around 103.2 and 516 minutes for 200 g of natural zeolite.