COMPARISON OF EXPOSURE SCENARIOS USED BY CONTRACTORS WITH THOSE DESCRIBED IN RP107

3.1 Applied scenarios defined in RP107

RP107 reported on the establishment of reference levels for regulatory control of workplaces where materials containing enhanced levels of naturally occurring radionuclides are processed. The exposure pathways according to RP107 are the same as those described above. In addition, skin contamination is considered. The reference levels in RP107 corresponding to specified dose levels were derived from ‘normal assumptions’ and ‘unlikely assumptions’, defined for two different exposure scenarios for each exposure pathway. In contrast, the dose calculations carried out by the contractors of the TENORMHARM project are mostly based upon measurements of dose rate and radionuclide concentration at a working place or another location of interest. As far as possible the following scenarios defined in Ref.

RP107 are taken into account for comparison:

(i) Exposure from stockpiles

This scenario considers the exposure of a worker to a large pile of material in a warehouse.

(ii) Exposure from scales and residues

This scenario considers the exposure of a worker to chemically or physically concentrated radionuclides, where the following two basic assumptions are met:

• volatilization of Po (and sometimes Pb) in high temperature furnaces,

• enrichment of radium in pipe scales in certain oil and gas fields.

(iii) Exposure from process vessels and pipes

In this scenario, workers are exposed only to external radiation from a shielded source.

Deviating from these scenarios, the national reports include exposure situations at outdoor working places.

Because RP107 is directed towards establishing reference levels for the regulatory control of workplaces, the dose calculation procedures are sometimes different from that applied by the contractors. Therefore, the calculation of the dose for each pathway in RP107 is based on the ratio between the annual effective dose (Sv/a) and the radionuclide activity concentration (Bq/g). Subsequently, the dose coefficient is expressed as Sv/h per Bq/g, instead of the dose per activity unit (Sv/Bq). The dose calculation for skin contamination is based on the ratio between dose rate (Sv/h) and surface activity concentration (Bq/cm²).

3.2 Data and assumptions used for comparison

According to RP107, the classification of a workplace would be carried out using the reference levels given in Table 7b in RP107 for ‘normal assumptions’ corresponding to

≤ 1 mSv/a and ‘unlikely assumptions’ corresponding to ≤ 6 mSv/a. Afterwards, the measured individual radionuclide concentrations would be compared with the reference levels for a particular reference point (in this case the reference point of 1 mSv/a for normal assumptions) given by the formula:

∑

= ^N

n ni

i reflevel n A conc

1

where conc_n is the concentration of radionuclide n; n

reflevel ⁱ is the reference level for radionuclide n and reference point i.

Only if A_i ≤ 1 does the material contain concentrations below those corresponding to the specified reference point. The reference levels correspond to a dose of 1 mSv/a if the material-specific scenarios and parameters are used as described in Chapter C4 of RP107.

Thus, if a parameter used by a contractor for dose calculation did not correspond with the relevant value specified in RP107, corrections were necessary. Differences between exposure times were eliminated by dividing the applied exposure time by the corresponding exposure time given in RP107.

The comparison of external exposure was more complicated because in RP107 the ratio between dose and specific activity (Sv/h per Bq/g) is calculated for a distance of 1 m from the source. For some scenarios described in the national reports, other distances to the source were used. The relationship between the gamma dose rate (nSv/h) and different distances to the source is given in a German national report [5].

Where a reported scenario had no counterpart in RP107, the applied dose calculation procedure was compared with the methods of calculating dose according to section 3.5 in RP107 using the same exposure time for both calculations.

4. RESULTS AND DISCUSSION

Table I gives an overview of the results of the comparison for selected scenarios and workplaces described in Ref. [4]. Table I illustrates that the dose calculated by the contractors sometimes deviated from that calculated according to RP107. The main reasons for these deviations are summarized in Table II.

The applicability of the RP107 reference levels for regulatory control of workplaces seems to be limited because the scenarios used for deriving the reference levels do not reflect sufficiently the current work conditions. Moreover, many scenarios described by the contractors have no applicable counterparts in RP107.

Except for the Czech contractor no party calculated the dose via ingestion of dust and by contamination of skin. According to modern work conditions and measures required to prevent the ingestion of contaminated dust, this exposure pathway can be neglected in most cases. The same concerns the contamination of skin.

TABLE I. COMPARISON OF THE RESULTS OF DOSE CALCULATIONS BETWEEN CONTRACTORS AND RP107

Reported by contractors According to RP107 Exposure pathway Effective dose

(mSv/a) Scales in pipes (at drilling platform), GRS

External exposure

20.88 External dose coefficient:

Table C1 (pipe scale)

× 3.33 (direct contact) texp: 400 h/a

Removal of scales (by sandblasting), GRS

External exposure 0.51 ²²⁶ Ra: 200 Bq/g

228 Ra: 68 Bq/g

228Th: 80 Bq/g texp1, texp2, texp3 as in Ref. [4]

0.14 External dose coefficient:

Table C1 (pipe scale)

Disposal of ash and slag (from coal fired power plant), ZVD External exposure 0.0012 ²²⁶ Ra: 0.25 Bq/g

228 Ra: 0.03 Bq/g

228 Th: 0.035 Bq/g

0.047 External dose coefficient:

Table C1 (pyrochlore feedstock)

Radon inhalation 0.16 ²²² Rn: 50 Bq/m³ 0.11 Rn dose coefficient:

Section F3.3 Inhalation of dust 0.00062 ²²⁶Ra: 0.001 Bq/m³

210Pb: 0.06 Bq/m³

210Po: 0.06 Bq/m³

0.15 Inhalation dose coefficient: Table D9 AMAD = 5 µm Disposal of red sludge, ICPMRR

External exposure 0.27 Background included Comparable to ‘exposure from stockpiles’

Radon inhalation ²²²Rn: 0.56

220Rn: 4.70

0.088 Inh. dose coefficient:

Table D9. AMAD: 5 µm Flood plain soils affected by pit water discharge from coal mines, UESSEN

External exposure 0.72 texp: 200 h/a 0.11 ‘Exposure from stockpiles’, ext. dose coefficient: Table C1

TABLE II: REASONS FOR DIFFERENCES IN THE RESULTS OF DOSE CALCULATIONS BETWEEN CONTRACTORS AND RP 107

Scenario and exposure

pathway Reasons for deviation

Scales in pipes

External exposure Dose coefficients derived in RP 107, Table C1 do not sufficiently consider real conditions (e.g. self-absorption of gamma radiation by BaSO4 and its non-linear relationship to scale thickness, or the consideration of a pipe as a shielded linear source) and lead to a dose 10 times higher

Removal of scales

External exposure Smaller deviation compared with ‘scales in pipes’ results from better fitting of theoretical assumptions to natural conditions for unshielded volumetric sources

Inhalation of dust Significantly higher inhalation dose coefficient used in RP 107 Disposal of ash and slag

External exposure Possibly the use of external dose coefficients for pyrochlore feedstock (Table C1)

Radon inhalation No significant deviation

Inhalation of dust Possibly significant differences in dose coefficients Disposal of red sludge

External exposure See Table I

Radon inhalation Use of Rn concentration measured 0.1 m above ground by Romanian contractor

Inhalation of dust No significant deviation

Flood plain soils affected by pit water discharge from coal mines

External exposure Smaller deviation compared with ‘scales in pipes’ results from better fitting of theoretical assumptions to natural conditions for unshielded volumetric sources. Use of external dose coefficients (Table C1) for Ra sludge removal could also influence result In contrast to RP 107, the TENORMHARM contractors excluded ⁴⁰K from the project work. This approach is justified because the dose resulting from ⁴⁰ K is unavoidable and the ratio between stable potassium and ⁴⁰ K remains constant all over the world.

Table C1 of RP107, containing external dose coefficients for different industries and materials, is highly complicated and its practical application is limited. In practice, the use of results from site-specific gamma dose rate measurements is much more suitable for external dose calculations because the theoretical approaches in RP107 do not reflect the natural conditions sufficiently. Furthermore, these data are not really hard to reconstruct. Table C1 presents external dose coefficients for all long-lived naturally occurring radionuclides and for a lot of different materials. Most of the listed radionuclides can be neglected for external dose estimation purposes. Only certain decay products of ²²⁶Ra and ²²⁸Ra/²²⁸Th (and ⁴⁰K) contribute to the external dose through gamma radiation.

The same is true for the reference levels in Tables 7a–7d of RP107. In the case of those raw materials in which equilibrium of the natural decay chains is preserved, it is always the radionuclide with the highest contribution to the dose that defines the activity level of interest, and the ratio between ²³⁸U and ²³⁵U series radionuclides is unchanged. This scheme is also questionable for non-equilibrium conditions. Consider, for example, the reference levels of

228Ra and ²²⁸Th in radium scales where the value of ²²⁸Th is one order of magnitude lower than that for ²²⁸Ra. If ²²⁸Ra is acting as the starting point of the segment of the thorium decay chain, the activity concentration of ²²⁸Th will rise, in theory, to 1.46 times that of ²²⁸Ra after a certain time and, in parallel, will decay with the half-life of ²²⁸Ra.