Underground mining is generally used for deposits deeper than approximately 200 metres. It is only economically viable for high-grade deposits due to the higher cost of extraction.
There are two main methods:
- Stope mining: Larger areas of high uranium concentration (stopes) are mined and connected by tunnels and shafts. Stopes are typically 2 m or more in height.
- Room and pillar mining: Used for flatter, more extensive deposits.
Radiation safety: Underground mining carries the greatest radiological hazard of all mining methods. The main exposure routes are:
- External exposure to gamma-emitting nuclides, especially Bi-214 (a uranium daughter product)
- Internal exposure from inhalation of radon gas (Rn-222) and its short-lived daughter products, which attach to airborne dust particles
Early uranium mining (particularly in the USA and Eastern Europe) gave rise to significantly increased rates of lung cancer among miners, before the hazards were properly understood and controlled.
Modern controls include:
- Forced ventilation to maintain airborne radon concentrations below 10 Bq/m (per ICRP recommendations)
- Dust masks and respiratory protective equipment
- Personal dosimetry for all workers
- Typical annual doses: 3 mSv average, 10 mSv maximum