African Mining Brief

Ventilation of Uranium Mines

The planning and operation of ventilation network systems for uranium mines require special consideration since ventilation is the primary technique of controlling ambient concentrations of radon progeny.

The ventilation of uranium mines is notably more complex due to the need to protect the workforce from radioactive occupational exposure. A number of strategies are employed by the ventilation engineer to effectively control exposure to radon progeny(decay products of radon gas) and other environmental hazards. Typically, the ventilation system is based on single pass ventilation and negative pressures are normally used. The primary goal is to exercise proper control and containment of radon sources.

By employing proper management strategies, sound operating procedures and ventilation practices, and a detailed instrumentation program, occupational exposures can be kept to a minimum.

An understanding of the behavior of radon gas and progeny is imperative to eliminate the potential for occupational exposures to radon sources. The radiation in underground mines results primarily from the presence of radon, a gaseous decay product of the uranium series.

On entering the mine atmosphere, radon continues to decay to form airborne radon progeny, positively charged atomic sized particles which tend to attach to respirable dust and to other free surfaces in the mine atmosphere. Fresh air volume flow rates through a mine, the distribution of airflow within the mine, and the radon emanation rate are the primary factors affecting the radon and progeny concentrations and working levels in ventilated areas. The total air volume flow rate through the mine determines the average time air takes to travel from the inlet to the production areas and to the outlet of the mine. During this residence time, radon progeny accumulate.

The main method of controlling radon and progeny concentrations in underground mines is ventilation. It is essential to maintain a low radon concentration through dilution with fresh air and to allow the radon a short residence time (10-15minutes) so that only 10-20 percent of the progeny are produced in the mine atmosphere.

Protecting Worker Exposure
The major challenge to the ventilation engineer is the requirement to dilute radon progeny to four working level months per year of worker exposure; this is the annual radon progeny limit for nuclear energy workers. Occupational exposure to radon progeny needs to be controlled so that no person will receive an exposure of more than two WLM (annual maximum permissible exposure) in any consecutive three month period and no more than four WLM in any twelve month period.

In high grade underground mines the above limits can be easily exceeded if effective management of ventilation and operating practices are not carefully exercised. In Congo, Shinkolobwe uranium mine, 25 km West of Likasi in Katanga has set its exposure limit for radon progeny to1 WL(working level) and normal targets are to maintain radon progeny levels below 0.10 WL in active working areas. The radon gas exposure limit is set to 60,000 Bq/s, and radiation work permits are required to work at any level above 3,000 Bq/s (Congo Shinkolobwe uranium mine 2009). By employing a work force management program, the cumulative radiation exposure of miners can be effectively controlled. A tracking system is normally used to determine the cumulative radiation exposure of each worker.

Air Transit Time
Air transit time is the period of time that radon gas resides in a producing mining area and contributes to the accumulation of radon progeny. If the radon which emanates into the mine atmosphere can be removed through rapid air change, then radon gas entering the mine air will have insufficient time to build up appreciable quantities of its progeny products. Ventilation engineers normally try to limit radon residence times to 10-15 minutes, to limit to 10-20 percent of the theoretical yield of progeny products in the mine atmosphere. When the average residence time of radon in mine air is 20 minutes, 30 percent of the equilibrium decay product working levels will be developed underground.

Ventilation Planning
Engineering design of the ventilation system is based on the total mine operation; on ventilation-air-transit time; on the radon emissions from wall rock, broken ore, material handling, tailings backfill and groundwater. Effective mine production/ventilation planning can reduce the radon progeny control problem by providing an organised system of mine openings of adequate dimensions that maximises the air distribution efficiency and minimises the residence time.

Proper planned relationship of the mining sequence to ventilation patterns are exercised so that most radon contamination resulting from mining is exhausted downwind from other active areas. Series ventilation systems, where air from upstream operations is used to ventilate downstream operations, is normally avoided. A parallel ventilation system is preferable because it reduces the hazards of cumulative air contamination. Contaminated air is never allowed to recirculate. Leakage of contaminated air into the fresh air stream is controlled through properly sealed doors and stoppings.

The Ventilation of Headings
Proper ventilation of headings in uranium mines is critical to control the cumulative radiation exposure of miners. Auxiliary ventilation must be used to ventilate development headings and dead-end stopes; an air change every three to four minutes should ideally be planned for working headings.

Ventilation Design Factors
Important factors to be considered when designing and managing a ventilation system are:

• - The ventilation system should be designed for flexibility to permit adaptation to changing mining conditions.
• - The design must consider mining and equipment systems, ventilation air-transit time, and total radon emissions from all sources.
• - Ideally, a retreat system should be used with a minimum possible number of working faces ventilated in series.
• - Mining areas should be separated into different ventilation blocks, affording better control capabilities.

Ideally one should utilise a split system of ventilation in which relatively uncontaminated intake air is proportioned between working blocks according to individual requirements. Air from each section is collected in an isolated, return airway and exhausted to the surface without contaminating other active mining areas.

• - Broken ore should be removed as soon as possible after blasting to eliminate radon progeny formation. The ore handling system should be isolated from the fresh air intake system as much as possible.
• - All sources of radon contamination must be adequately sealed or isolated.
• - Primary airways serving as fresh air inlets should not intersect uranium ore bodies and should be driven through waste rock. Mined-out areas should be kept on the return side of the ventilation system.
• - Maintaining an appropriate pressure profile in the mine is important. Employing a push-pull system, a positive pressure can be maintained in the working area, preventing air recirculation from worked-out areas which are maintained at a lower pressure.
• - Pressure differentials across sealed old workings should be in a direction such that any leakage will pass into return airways and not into intakes.
• - Because of the appreciable solubility of radon in water, entering groundwater should be isolated, collected in pipelines and pumped to surface as quickly as possible.
• - Main airways should be kept as free from mining activity as is practicable, so that relatively high air velocities can be readily maintained.
• - Ore passes, conveyor ways and crushing stations should be ventilated so that exhaust air can be directed to the return air system quickly.
• - Shops and repair garages should be positively ventilated by controlled air volume flow rate.
• - The number of personnel required to work or travel in return airways should be kept to a minimum.

Ventilation Management and Operating Practices at Kayelekera
Uranium Deposits
The Kayelekera Uranium Deposit is located in Northern Malawi in Southern Africa. It is 8km south of the road connecting the townships of Chitipa and Karonga and is accessible via dirt road from Karonga, 40km to the West at the main North/South road of Malawi. Planned management and operation of the ventilation system is vital to ensure the system meets all regulatory and safety requirements. Management plans are carried out on a regular basis to meet the objectives of its management program.

Operating practices at the mine generally consist of three levels of safety: ventilation, containment, and instrumentation. Single-pass ventilation is practised to allow the radon a short residence time so that only minimum concentrations of progeny are produced in the mine atmosphere.

Ventilation surveys and safety inspections are used to verify if the ventilation system is in compliance and if it meets all defined objectives of the program. Any deficiencies found during an inspection are acted upon promptly. Ventilation directives are issued for all changes in the ventilation of underground workings and verification programs are used to ensure that conditions are adequate and as expected.

The release of radon and its progeny in the mine atmosphere can be quite unpredictable depending on the hydro geological complexity of the ore body. The most effective way of minimising atmospheric radiation contamination and human exposure is to control radon emanation at the source.

Underground mining has yet to see an industry wide solution for control of their ventilation systems due to the complexity and sensitivity of the underground environment. Complexity arises from multi-level, multizone operations and sensitivity is associated with the impact that changes in equipment status and output have on air flows and quality throughout the mine.

References Ventilation of uranium mines, in a manual of mine ventilation design practices. Congo Shinkolobwe uranium mine 2007.
Radiation Code of Practice, Kayelekera Uranium Deposits.