222Rn gas (half-life: 3.8 days) is the daughter element of 226Ra (half-life: ~1600 years). Radon is a known carcinogen. It has been reported that for each increase of 100 Bq/m3 of radon concentration, the risk of lung cancer increases by 16% (WHO, 2005). Radon migrates through the soil by convection or diffusion. In order to prevent the migration of radon, a special cap of highly compacted clay is placed on the tailings. The clay layers are designed to give protection for about 1000 years.