The principal effluents carrying radioactive material from a uranium mill are the following:

1. Airborne effluents, carrying radon gas (222Rn) and radioactive dust particles

2. Liquid effluents, carrying water-soluble radionuclides

3. Solid effluents, in mill tailings

In present mills, radioactive liquid effluents are held in storage ponds with mill tailings and eventually evaporate to a solid.

The amounts of these radioactive effluents have been estimated [S2] for two model uranium mills, each with a capacity of 2000 short tons of ore containing 0.2 percent U3Oe/day. One mill uses a carbonate leach, sodium hydroxide precipitation flow sheet such as that described in Sec. 8.4. The other mill uses an acid-leach, amine extraction flow sheet such

as described in Secs. 8.5 and 8.6. Each mill is assumed to be associated with a storage pond and tailings pile in which 20 years of mill effluents have accumulated. Two alternative sites were studied for each mill type, one in New Mexico in an arid region with average wind speed of 7 mi/h and the other in Wyoming in a region with more vegetation and an average wind speed of 10 mi/h.

For each model mill a number of cases were examined, with progressively better retention of airborne radioactive effluents. The results to be summarized here are for case 1, at the New Mexico site. Case 1 represents 1975 practice, with least complete removal of airborne dust and no holdup of gaseous effluents to permit 3.8-day 222 Rn to decay.

Airborne effluents. Table 5.24 lists the annual emission rate of airborne radionuclides from the two types of model uranium mill, each after 20 years’ accumulation of tailings.

Yellow cake. Table 5.25 gives the percent of the uranium, thorium, and radium in the ore assumed [S2] to be recovered in the yellow cake uranium mill product, and the activity of the yellow cake due to the thorium and radium.

Tailings. The first two columns of Table 5.26 give the percent of the uranium, 230 Th, and 226 Ra and its daughters assumed [S2] to be recovered in the tailings sand and the tailings slime and liquid effluents for the acid-leach and carbonate-leach processes, and the calculated [S2] concentration of the principal radionuclides in the two classes of tailings. The third column gives the resulting nuclide concentration in the composite tailings. The fourth column gives the calculated total curies of each radionuclide in the tailings after 20 years of mill operation. The potential hazard from insecurely impounded tailings is suggested by the large amount of radioactivity.

Differences among uranium mills and the ores they process will cause the amounts of radioactivity in individual mills to vary considerably from the estimates given above.