Persistence and Decontamination of Radioactive Cesium in a Model Drinking Water System
Szabo, Jeff. Persistence and Decontamination of Radioactive Cesium in a Model Drinking Water System. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-18/017, 2018.
During this study, conditions within operational drinking water pipes were simulated using annular reactors (ARs) (i.e., ring-shaped reactors). The ARs consist of a glass outer cylinder and a rotating polycarbonate inner cylinder with 20 flush mounted rectangular coupons that are made of materials that simulate drinking water pipe surfaces. Concrete-lined, copper, and PVC coupons (sample surfaces installed in the ARs) with biofilm were used. Shear stress was applied to the coupon surfaces by setting the reactors’ inner cylinder rotation to 100 revolutions per minute, which produces shear forces similar to 1ft/sec flow velocity in a 6-inch pipe. The study was initiated with a persistence evaluation. Pipe material coupons with biofilm were contaminated with cesium-137 for 24 hours and then exposed to fresh tap water in annular reactors operating at 100 rpm for five days. The results of the persistence evaluation showed that exposure to clean drinking water reduced the contamination level by 75% for the copper pipe materials, and 91% and 93% for the PVC and concrete pipe materials, respectively. In the next phase of the study, a decontamination evaluation with two chemical cleaning agents was performed. The pipe materials were exposed to a solution containing 1 molar (M) ammonium chloride (NH4Cl) or 1 M potassium chloride (KCl) in separate experiments as cleaning agents in reactors operating at 100 rpm for up to six experimental days. The cleaning agents were used in separate experiments, not together. After the persistence evaluation (without re-contaminating), the NH4Cl CCAE took place for four days. decontamination with NH4Cl removed 89% of the remaining cesium activity from the copper coupons. However, there was more variability in the CCAE results on the copper coupons compared to the PE results, possible due to a precipitate that formed during decontamination. The precipitate was likely corrosion of the copper piping material and could cause physical damage to a water system. The NH4Cl CCA treatment on PVC and concrete piping materials removed 91% and 85% of the remaining adhered cesium, respectively. At the conclusion of the NH4Cl decontamination experiment, the ARs were cleaned, re-contaminated and the KCl CCAE was conducted. After contamination, one day of tap water exposure was used to remove adhered cesium. The one day tap water exposure results show a similar initial decrease in Cs-137 of approximately 80% for all coupon types. The KCl solution was applied during the following 5 days (6 days for the concrete coupons). The remaining cesium adhered to copper coupons was persistent with 50% remaining after five days of KCL exposure. KCl treatment removed 90% of the adhered cesium from both the PVC and concrete coupons. It should be noted that when cesium activity removal from both exposure to clean water and decontamination with the cleaning agents are added together, 99% of the activity was removed from PVC and concrete, regardless of which agent was applied or the duration of clean water exposure after contamination. Total cesium activity removal from copper ranged from 97% removal with 5 days of clean water exposure and NH4Cl treatment, to 89% removal with one day of clean water exposure and KCl treatment.
The objective of this report was to evaluate the persistence of radioactive cesium-137 (Cs-137), on concrete-lined, copper, and polyvinyl chloride (PVC) pipe, and possible decontamination approaches. During this study, conditions within operational drinking water pipes were simulated using annular reactors (ARs) (i.e., ring-shaped reactors). These results suggest that KCl would be a better decontamination agent compared to NH4Cl since the decontamination performance is comparable, and KCl does not corrode the copper pipe.