AN IMPROVED RN-222 CANISTER USING A 2-STAGE CHARCOAL SYSTEM

A prototype for an improved passive Rn-222 canister (R-Canister) was designed and compared to conventional charcoal canisters for its adsorptive and desorptive characteristics following exposures to Rn-222 at 23-degrees-C in the presence of water vapor. The R-Canister, containing a two-stage charcoal system, minimizes the adverse effects of water vapor by maintaining the amount of adsorbed water vapor in the primary Rn adsorbent below the "break-point" of the charcoal. This is achieved by the placement of a desiccant charcoal cartridge 6 cm above the primary Rn adsorbent. The optimal bed depth of the primary adsorbent, determined from a diffusion study, was found to be 2.3 cm. The measured value for the effective diffusion coefficient of Rn in a peat-based charcoal at 15% humidity and 25-degrees-C is 7.97 x 10-10 m2s-1. Exposures to 70% humidity for 7 d increased the buildup time-constant of Rn in the R-Canisters by 33% as compared to R-Canisters exposed to 15% humidity. At relative humidities ranging from 15-70%, the Rn-222 buildup time-constant of the R-Canister ranged from 43-94 h, whereas the desorption time-constant ranged from 46-64 h. Typical buildup time-constants and desorption time-constants for conventional fully-opened charcoal canisters currently in field use ranged from 30-43 h and 17-29 h, respectively, over the same range of humidities.