Diffusion of Tritiated Water Vapor into Concrete

Depth profiles of tritiated water in concrete walls measured in a heavy water reactor are analyzed using a diffusion model. The apparent diffusion coefficient of tritiated water in concrete made with a standard mixing proportion is 3.3 × 10⁻¹¹ m²/s. In addition to the primary diffusion mechanism, there is evidence of a second mechanism, possibly a fast diffusion process. The diffusion model can be applied to tritiated water penetration into concrete when the concrete walls of fusion reactors are exposed to air containing tritiated water vapor. In the heavy water reactor, the average concentration of tritiated water in the air over 20 yr is estimated to be ∼2.0 × 10⁻² Bq/cm³. The tritium inventory in concrete is ∼1.0 × 10⁷ Bq/m³ in the region <0.65 m deep. A 0.2-m-thick concrete wall is sufficient to prevent tritium release into the environment from exceeding the regulatory limit.