Effect of Junctional Shape (Smooth vs Round) in T-junctions

suspension of dust. Dust is also likely to be generated during the landfill disposal of digester residue.

In feedstock storage areas, dust levels are likely to be low except when the material is disturbed, such as during loading and unloading. Milling and drying of feedstocks encourage the formation of dust, but are carried out using enclosed equipment. Industry data indicate that the maximum airborne dust concentrations that can be expected are 1 mg/m³ in feedstock handling areas and 1.7 mg/m³ in feedstock milling and drying areas. In a plant using ilmenite sand as a feedstock, the average airborne dust concentration was found to be 0.14 mg/m³, about an order of magnitude below these maximum values [30]. Using the average radionuclide activity concentrations measured in the feedstock at that plant (which were close to the mid-points of the ranges given in Table 7), this dust concentration corresponded to average airborne activity concentrations of 0.05 and 0.01 mBq/m³ for ²³²Th and ²³⁸U, respectively. For titanium slag feedstocks, the corresponding airborne activity concentrations would typically be an order of magnitude lower because of the lower activity concentrations in the feedstock material.

For subsequent processing steps, such as digestion and hydrolysis, no airborne dust concentration data are available, but such operations are inherently non-dusty because they are carried out with wet or damp materials.

No dust concentration data are available for maintenance operations.

However, the conditions inside vessels, such as weak acid stock tanks, are likely to be non-dusty and the use of high pressure water jets to remove scale from equipment helps to minimize any dust generation.

(c) Doses from the inhalation of dust are calculated assuming that protective respiratory equipment is not used. In practice, such equipment is used during dusty operations.

5.3.2.1. Feedstock preparation

Maximum gamma exposure periods are reported to be 500 h/a in feedstock storage and conveying areas, 200 h/a in belt weighing areas and 1700 h/a in feedstock milling areas. It can be deduced from the dose rates reported in Section 5.3.1.1 that the maximum external dose likely to be received by a worker in a year is about 0.16 mSv in the storage and handling areas and about 0.29 mSv in the milling area, with average doses being perhaps half these values.

For a worker in a plant using ilmenite feedstock, reference to the airborne dust activity concentrations reported in Section 5.3.1.2 and the inhalation dose coefficients reported in Table 5 (assuming an AMAD of 5 μm and a breathing rate of 1.2 m³/h) suggests that the inhalation dose received over a 2000 h annual exposure period is about 0.007 mSv. In a worst case situation, the radionuclide activity concentrations in the ilmenite could be about five times higher than those reported in Ref. [30], and the dust mass concentrations in air could be about ten times higher. This would give a maximum dose from dust inhalation of about 0.35 mSv in a year. For a worker in a plant using titanium slag feedstock, the corresponding doses would be an order of magnitude lower because of the lower radioactivity content of the feedstock.

The total effective dose received by a feedstock preparation worker is the sum of the external and internal doses. In a plant using ilmenite feedstock, the average total effective dose received in a year would therefore be about 0.1 mSv, with a maximum of about 0.5 mSv. In plants using titanium slag as a feedstock, the doses would be an order of magnitude lower.

5.3.2.2. Acid digestion

At one location in an ilmenite plant, dose rates in the range of 1–6 µSv/h are reported (see Table 19). Although the occupancy period is not stated, the annual effective dose at this location is reported to be less than 1 mSv, implying that the occupancy period is a few hundred hours per year or less. Elsewhere, the dose rates are much lower (see Section 5.3.1.1) and maximum annual gamma exposure periods are reported to be 2000 h for the general digester area, 600 h for the digester residue filters, 200–250 h for digester residue storage and 250 h for the polishing filter, ilmenite residue thickener and inlet to the liquor settlers. It can be deduced from these data that the maximum external dose received by a worker in a year is in the range 0.02–0.3 mSv.

5.3.2.3. Copperas separation

The annual exposure period at all gamma dose rate measurement locations is reported to be, at most, 250 h. In one plant, the annual exposure period in the area with the highest gamma dose rates (the crystallization area) is reported to be less than 50 h [30]. It can be deduced from the dose rates reported in Table 19 that the maximum external dose received by a worker in a year is in the range of 0.02–0.6 mSv.

5.3.2.4. Hydrolysis

Reference to Table 19 shows that, in ilmenite plants, this area is associated with particularly high gamma dose rates and, by implication, the potential for high annual effective doses. In an area with a maximum occupancy period of 500 h/a, the reported gamma dose rates of 1–12 µSv/h imply a maximum external dose of 0.5–6 mSv received in a year by a worker. At the Moore feed tank, where the maximum annual exposure period is 2000 h, the reported dose rate of 3 µSv/h implies a maximum external dose of 6 mSv in a year. For pipes near a walkway, where the dose rate is reported to be 50 µSv/h, the occupancy period is not specified but is evidently very short because the annual dose is reported to be less than 1 mSv. For a plant using a feedstock containing 90% titanium slag, the dose rates are lower (see Section 5.3.1.1) and measurements in an area with a maximum annual exposure period of 1700 h correspond to a maximum external dose of 1.2 mSv.

5.3.2.5. Gypsum production and acid recovery

The maximum annual exposure periods are reported to be 1000 h for ferrous sulphate storage areas and 2000 h elsewhere. For the dose rates reported in Table 19, these correspond to annual effective doses of less than 0.1 mSv.

5.3.2.6. Calcination and finishing

Dose rates are very low in these areas, and even with annual exposure periods of up to 2000 h, the annual effective dose received by a worker is less than 0.2 mSv.

5.3.2.7. Landfill disposal of digester residue

Landfill workers are assumed to be exposed to gamma radiation and airborne digester residue particles for periods of up to 2000 h in a year. The

incremental gamma dose rate from unshielded bulk digester residue is reported to be in the range of 0.06–0.5 µSv/h (see Table 19). This corresponds to an annual effective dose of 0.1–1 mSv. However, once the digester residue is covered by 1 m of chalk, the dose rate is only 0.06 µSv/h (see Section 5.3.1.1), giving an annual effective dose of 0.1 mSv. For inhalation of airborne digester residue particles, the annual effective inhalation dose derived from the results of personal exposure measurements is reported to be less than 0.11 mSv. Over the course of a year, therefore, the maximum effective dose from gamma radiation and airborne dust particles is estimated to be in the range of 0.2–1.1 mSv, with values at the lower end of this range being more likely.

5.3.2.8. Maintenance work

For maintenance work involving entry into vessels, the exposure periods are limited by the nature and frequency of the work. In those instances where dose rates exceeding 1 µSv/h are found, the maximum annual exposure periods are reported to be 10–50 h. A similar situation is reported for exposure to equipment and scale inside a waste disposal store. It can be deduced from the dose rates reported in Table 21 that the maximum external dose received by a maintenance worker in a year is in the range of 0.05–0.5 mSv.

Maintenance of filtration systems in the acid digestion and hydrolysis areas involves gamma exposure to the filters for maximum annual periods of 250 h (digester residue filters) or 1200 h (Moore filters) at instantaneous dose rates ranging from 0 to1.9 µSv/h (see Table 21). Annual effective doses are reported to be less than 1 mSv, although there is clearly the potential for higher doses should the exposure periods be longer.

5.3.3. Measures to reduce doses