High concentrations of radon and its progeny can be significantly decreased by proper ventilation, which sweeps away these materials before the radon (whose half-life is 3.8 days) has time to decay into its progeny. Fresh air must be provided to each working station in sufficient quantity and velocity to guarantee that worker exposures remain below the regulatory limit. This will be accomplished by ensuring that the rate of air change maintains the radon progeny concentrations at an acceptably low level.
Chiefly as a result of improved ventilation systems, radon progeny concentrations in Canadian uranium mines have greatly decreased during the past decade. Ventilation for the proposed Cigar Lake mining methods will be based on a once-through system and will minimize the number of dead-end headings.
Water can carry radon gas, and therefore mine water control will be important at Cigar Lake in limiting the influx of radon into working areas and fresh air streams.
The radioactive atoms in the uranium decay chain also emit gamma radiation. Whereas radon progeny impart their dose mainly “internally” to lung tissue after inhalation, gamma doses irradiate the whole body usually from an external source, in amounts which increase with ore grade.
The maximum permissible whole body worker dose equivalent due to gamma radiation is 50 millisieverts (5 rem) per year. The Control Board has proposed a new dose limit which will be the weighted sum of doses from all exposure pathways, including radon and thoron progeny, gamma radiation and radioactive dust. The proposed annual limit for the dose equivalent is 50 millisieverts per year.
There is no direct correlation between gamma radiation doses and exposure to radon progeny in any mine. Moreover, the methods generally used to control radon progeny and gamma radiation are different. For example, airflow can reduce exposure from radon progeny in the mine atmosphere but has no effect on the gamma radiation originating from the ore. Gamma radiation is controlled by maximizing distance from the source and minimizing time of exposure, and by shielding.
In the low-grade uranium mines at Elliot Lake, Ont., gamma radiation levels (which give annual worker doses of about 6 millisieverts or 0.6 rem) are below those at higher-grade underground deposits and are exceeded as risks to miners by radon and thoron progeny exposures. However at the very high grade Cigar Lake deposit the health risk from gamma radiation is as great as that from radon progeny and is a significant factor in the design of the mining method.
The proposed Cigar Lake mining equipment will be designed for control outside the ore zone. In any case where human contact with the ore is necessary the equipment will be designed to maximize the shielding from radiation and to limit the time that personnel spend working near the ore zone. Stephen Salaff
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