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For the relatively low concentrations of ²³⁸U and ²³²Th decay series radionuclides in phosphate ore and phosphate rock, the dose rate depends primarily on the activity concentration of the material rather than its total quantity and geometry. Using a theoretical modelling technique described in Ref. [50], gamma dose rates per unit activity concentration have been calculated at a TABLE 8. RADIOACTIVITY IN TAILINGS FROM THE BENEFICIATION OF PHOSPHATE ROCK

Source Activity concentration (Bq/g)^a

U-238 Th-230 Ra-226 Th-232 Th-228

Tailings from sedimentary ore^b USA, central Florida (clay)

[16, 47]

0.6–1.8 2.3 0.5–1.9 0.04 —

USA, central Florida (sand)

[16, 47]

0.055–0.4 0.25 0.06–0.45 0.004 —

USA, northern Florida [47] 0.01–0.03 — 0.07–0.1 — —

Tailings from igneous ore

South Africa [48] 0.26

(0.22–0.30)

— 0.25

(0.19–0.28)

0.33 (0.26–0.44)

0.34 (0.22–0.46)

a Some values have been rounded.

b Higher values than those listed in the table have been encountered, but these are not representative of current commercial operations. In tailings from former sedimentary phosphate mining in the United Republic of Tanzania, the activity concentrations were 4.3 Bq/g for ²²⁶Ra and 0.6 Bq/g for ²²⁸Ra and ²²⁸Th [49].

distance of 1 m from a 1000 m³ volume of material in which radionuclides in each of the ²³⁸U and ²³²Th decay series were assumed to be in equilibrium. The predicted dose rate associated with uranium series radionuclides is 0.39 μSv/h per unit activity concentration of ²³⁸U (in becquerels per gram). For thorium series radionuclides, the corresponding figure is 0.58 μSv/h per unit activity concentration of ²³²Th. The predicted dose rate from ⁴⁰K is 0.04 μSv/h per Bq/g.

These dose factors have been applied to phosphate ore and phosphate rock using radionuclide activity concentrations selected from Appendix II and Appendix IV, respectively.¹⁴ The results are given in Table 9. In Ref. [51] a mean dose rate of 0.54 µSv/h was calculated for 14 worldwide rock sources. Excluding an unrepresentative value for Tanzanian rock, the mean dose rate was 0.39 µSv/h.

These values are consistent with those given in Table 9.

Gamma dose rates measured at two mines and beneficiation plants in the Syrian Arab Republic [52] are shown in Table 10. These results are very similar to the theoretical values reported in Table 9.

Extensive area monitoring using thermoluminescent dosimeters (TLDs) in rock tunnels used for transporting rock to a phosphoric acid plant in central Florida, USA, was carried out over a period of four years. The absorbed dose rates were in the range 0.08–3.4 μGy/h and rarely exceeded 1 μGy/h. The average absorbed dose rate was 0.18 μGy/h [53].

Gamma dose rates from phosphate ore can be summarized as follows:

(a) Dose rates associated with sedimentary material are generally a few tenths of a microsievert per hour, rarely exceeding 1 μSv/h, with the ²³⁸U series radionuclides contributing almost the entire dose.

(b) Dose rates associated with igneous material are 0.1–0.3 μSv/h, significantly lower than those associated with sedimentary material. Material from Kola, Russian Federation, gives significantly lower dose rates than material from South Africa. About 60–80% of the dose is attributable to the ²³²Th series radionuclides, with the ²³⁸U series radionuclides contributing most of the balance.

14 In order to demonstrate the relative contribution of all radionuclides, only data sets that included activity concentrations of ²³⁸U and ²³²Th series radionuclides as well as ⁴⁰K were selected from Appendices II and IV. Some ⁴⁰K concentrations were determined from the K₂O content given in Appendix IV.

TABLE 9. GAMMA DOSE RATES NEAR LARGE VOLUMES OF PHOSPHATE ORE AND PHOSPHATE ROCK, PREDICTED FROM RADIONUCLIDE ACTIVITY CONCENTRATIONS

Source

Activity concentration (Bq/g) Dose rate

U-238 Th-232 K-40 Total

(μSv/h)

Contribution to total U-238 Th-232 K-40 Material of sedimentary origin

Phosphate ore:

Sudan (Uro) 4.0

(Ra-226) 0.01 0.06 1.6 >99% <1% <1%

Sudan (Kurun) 0.4 (Ra-226)

0.01 0.14 0.2 95% 2% 3%

USA

(central Florida)

0.8–3.1 0.02 0.003–0.019 0.3–1.2 98% 2% <1%

Egypt 0.4 0.02 0.05 0.2 91% 8% <1%

Phosphate rock:

Morocco 0.9–1.9 0.01–0.03 0.03^a 0.4–0.8 98% 2% <1%

Togo 1.0–1.5 0.03–0.1 0.01–0.03^a 0.4–0.6 93% 7% <1%

USA(central Florida)

(pebble)

1.3–2.5 0.03 0.03^a 0.5–1.0 97% 3% <1%

USA(central Florida)

(concentrate)

0.7–1.8 0.02 0.03^a 0.3–0.7 97% 3% <1%

Egypt 0.4–0.5 0.02–0.04 0.02 0.2 92% 7% <1%

Algeria 0.6 0.06 0.02 0.3 87% 12% <1%

Jordan 0.8–1.0 0.002–0.02 0.01–0.15 0.3–0.4 98% 1% <1–2%

Tanzania (Arusha) 4.1–4.6 0.61–0.63 0.29 2.0–2.2 83% 17% <1%

Tunisia 0.8 0.03 0.03 0.3 95% 5% <1%

Material of igneous origin Phosphate ore:

Russian Federation

(Kola)

0.07 0.1 0.1 0.1 31% 65% 4%

South Africa 0.2 0.4 0.003–0.02^a 0.3 25% 75% <1%

Phosphate rock:

Russian Federation

(Kola)

0.03–0.1 0.06–0.09 0.14 0.1 31% 60% 8%

South Africa 0.1–0.2 0.3–0.6 0.003–0.02^a 0.2–0.4 18% 82% <1%

a Estimated from the potassium content of the phosphate rock (see Table 51, Appendix IV).

TABLE 10. GAMMA DOSE RATES MEASURED AT TWO MINING AND BENEFICIATION FACILITIES IN THE SYRIAN ARAB REPUBLIC

Personnel

Al-Sharkeia mine Khnefees mine

Occupancy (%)

Dose rate (μSv/h)

Occupancy (%)

Dose rate (μSv/h)

Mine workers 37 0.4 37 0.5

3 0.3 3 0.3

4 0.2 4 0.1

Plant workers 30 0.2 30 0.1

15 0.3 10 0.3

5 0.5

Administrative workers 46 0.1 41 0.1

Maintenance workers

in workshops

30 0.1 30 0.1

TABLE 9. GAMMA DOSE RATES NEAR LARGE VOLUMES OF

PHOSPHATE ORE AND PHOSPHATE ROCK, PREDICTED FROM RADIONUCLIDE ACTIVITY CONCENTRATIONS (cont.)

Source

Activity concentration (Bq/g) Dose rate

U-238 Th-232 K-40 Total

(μSv/h)

Contribution to total U-238 Th-232 K-40

4.3.2.2. Inhalation of airborne dust

Measurements of airborne dust activity concentrations in sedimentary phosphate mining and beneficiation facilities in Florida, USA, are reported in Ref. [53]. Dust samples were collected on filters using either a high volume stationary air sampler or low volume personal air samplers. Alpha counting of the dust samples suggested an airborne ²³⁸U activity concentration of 5–10 mBq/m³ in the mining area. Higher dust levels, up to 100 mBq/m³, were measured in the crushing, grinding and screening areas of the beneficiation plant. Although some of the dust levels were high, the occupancy periods of workers were reported to be only a small fraction of the total working period. In addition, workers were often required to use respiratory protective equipment as part of normal OHS procedures.

4.3.2.3. Inhalation of radon

Radon exposures were measured at two mining and beneficiation facilities in the Syrian Arab Republic [52], at mining and beneficiation plants in central Florida, USA [53], and in underground mines in Egypt [54, 55]. The results are summarized in Table 11. Assuming an equilibrium factor of 0.4 and an annual working period of 2000 h, the potential alpha energy exposures at Syrian facilities (0.3–1.1 mJ·h·m⁻³) imply an annual average radon concentration of about 65–245 Bq/m³. Elevated exposures of administrative workers at the Al-Sharkeia mine were reportedly caused by the offices being built on phosphate sands and being poorly ventilated, especially in winter. Remedial action in these particular workplaces was proposed. At benefication plants in Florida, USA, the radon concentrations in the flotation plant, pit cars and railcar loading area were consistent with indoor radon concentrations. Measurements were also made in mining and rock handling areas. With the exception of rock tunnels, background levels were not exceeded. The high radon and radon progeny concentrations measured in Egyptian underground mines were the result of poor ventilation.

4.3.3. Effective dose