Effects of aging of radioactive fallout on timely decontamination of concrete using low and mild pressure washing

Abstract

Timely decontamination will reduce the consequences of a radiological contamination event. For this purpose, pressure washing can be rapidly deployed, but its effectiveness will change if the interactions between the surface and radionuclides changes as the contamination "ages" under the influence of time and precipitation. While effects of this aging have been reported for dissolved cesium, they have not been studied for radionuclides present as particulate, e.g., fallout. This work studied the effects of aging on decontamination with low (<280 kPa/40 psi) and mild (14,000 kPa/2000 psi) pressure washing, on concrete contaminated with surrogate fallout consisting of soluble Cs-137, 0.5 μm silica particles, and 2 μm silica particles. The samples were aged up to 59 days (time between contamination and decontamination) with and without simulated precipitation. The percent removal following decontamination of the soluble cesium decreased over the first ten days of aging until the removals were less than 10 % for both low and mild pressure washing. The particle decontamination was independent of aging time but decontaminating via mild pressure washing (>80 % particle removal) significantly outperformed decontaminating by low pressure washing by flowing solution across (parallel to) the contaminated surface (<25 % particle removal). The observed changes in decontamination efficacy are explained via measurements of the penetration depth of contaminants. For soluble cesium, the results compared favorably with prior studies and theoretical treatment of cesium penetration, and they yielded additional insight into the effect of washing pressures on decontamination. There are no comparable studies for particulate contamination, so this study resulted in several novel observations which are operationally important for timely decontamination of surfaces following a radiological incident. It also suggests an evidence-based pressure washing procedure for timely decontamination of soluble and insoluble radionuclides which can be used throughout the emergency phase and into the early recovery phase.

Keywords: Contaminant aging; Decontamination; Radioactive cesium; Radioactive fallout; Radiological recovery.

Fig. 1

A: Line (red) roughness profile of the sample in “B” from lower left to upper right quantifying the depth of some ridges. B: The light-grey spots in the bottom half of the coupon face are pitting of the coupon surface. C: The faint beige-tinted lines running from the bottom left to the upper right of the coupon face are ridges in the coupon surface. D: Line (red) roughness profile of the sample in “C” from lower right to upper left quantifying the depth of some ridges. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

Mild pressure wash (circles) and low-pressure flow (triangle) decontamination results using 0.1 M KCl solution for aging with no precipitation (left column) and aging with precipitation (right column) for 2 μm (top), 0.5 μm (middle) particles and dissolved Cs-137 (bottom) contaminants. (The error bars on each data point show the standard deviation for the five samples for each test condition. The counting error was propagated through the average calculation, but the resulting errors were less than the standard deviations. The large error bars for some data points were a result of relatively low total counts in the original photopeak.)

Fig. 3

Effect of surface roughness effects on activity remaining as a function of depth removed. The orange and grey data are from relatively flat surfaces, and the curves are more exponentially shaped, reflecting that the grinding method removes surface layers more easily, compared to the blue and yellow data which are from relatively rough and cupped coupon surface. The more rough/cupped surface requires deeper layer removal to remove the particles (left) and dissolved Cs-137 (right). The effect appears independent of applied precipitation, as blue and grey received no precipitation applications, whereas yellow and orange received 12 or 16 applications, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

Effect of precipitation versus no precipitation on activity remaining versus depth of surface removed. (Left) Duplicate coupons of samples with 2 μm particles (left) and soluble Cs-137 (right) with precipitation (orange and yellow data, eight precipitation applications) and without applied precipitation (blue and grey). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5

Effect of number of precipitation applications on activity remaining versus depth of surface removed. The orange and yellow correspond to 3 and 20 precipitation applications, respectively. The blue and grey data were collected from samples without precipitation application but held for the same amount of time as the orange and yellow, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)