Table 8.4

Estimated Equivalent Gamma Dose for Workers at Water-Treatment Plants or in Point-of-Entry Applications Using GACa

Water Flow (m3 d-1)

Estimated Gamma Dose at 1 m (µSv hr-1)

Hours until 1,000 µSv Dose

1 (point of entry)

0.14

7,143

11 (water plant)

0.75

1,333

981 (water plant, pressure driven)

7.0

143

981 (water plant, gravity driven)

6.4

156

a See appendix E for calculations.

exposure would need to be monitored to ensure worker safety. If it were necessary to work on a GAC unit for a substantial number of hours, the gamma emissions could be reduced by first taking the unit off line for about 20 to 30 d to allow the radon responsible for the short-lived progeny to decay.

The calculations also show that as radon loading increases, the dimensions of the tank increase, providing increased absorption of gamma radiation within the tank. Modeling gamma emissions from the tank as a point source will be satisfactory only for very small units (for example, point-of-entry applications). The equivalent gamma dose from a GAC system that removes radon from a public water supply should be modeled with an extended-source model that can be modified to the dimensions of the treatment units. It is clear that treating water that contains more radon (over 185,000 Bq m-3), where high removal efficiencies are required, or at high flow rates (high radon loading) will probably lead to unacceptable equivalent gamma doses to water-treatment plant personnel.

Rydell and Keene (1993) have developed a computer program (CARBDOSE 3.0) that calculates the probable gamma-exposure dose with distance from a typical point-of-entry unit. The program "approximates a 25.4 cm diameter, 12.7 cm high cylindrical volume of GAC as a cylindrically-corrected 24 cm × 24 cm × 13 cm array of 1 cm3 sources using the 72 gamma energies reported for 214Bi and 214Pb and allowing for self absorption and build-up." Rydell and others (1989) reported that the CARBDOSE models' estimated gamma dose rates and the measured values for 10 point-of-entry GAC units were in "reasonably good agreement." They suggest that CARBDOSE can be used as a design tool to estimate the potential gamma radiation exposure during operation of the GAC unit. The committee notes that CARBDOSE should only be applied to GAC units that have very small dimensions (that is, ones that treat very small flows) and are similar to those used in developing the model. Equivalent gamma doses for larger GAC units should be predicted with an extended-source model that can address more-complex geometries.

Disposal Issues.

A few weeks after a GAC unit ceases operation, the major radionuclide remaining sorbed to the carbon is 210Pb because of its relatively long



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement