Workplace information should be gathered in order to understand the exposure situations, e.g., radionuclides that may have been incorporated (including equilibrium assumptions for the natural series), chemical form, presumed particle size (typically 1 or 5 mm), likely time, pattern and pathway of any intake.

If no special information is available, the following default parameter values could be used (reference procedure):

  • Mode of intake: Single intake

  • Time of intake: Mid-point of the monitoring interval, i.e. the mid-point of the time range between the date of the measurement being considered and the date of either the previous measurement or the beginning of monitoring

     

    Inhalation:

     

  • Absorption Type and f1 value: defaults according to ICRP publications.

  • Particle size: 5 mm AMAD

     

    Ingestion:

     

  • f1 value: defaults according to ICRP.


In the case of exposures that lead to effective dose estimates higher than about 0.1 mSv (i.e. above Level 0 of the IDEAS guidelines), it is desirable to use parameter values in the calculation of tissue and organ equivalent dose that are more specific to the conditions of exposure and to the individual. By using such workplace specific parameters a more realistic dose assessment can be obtained.

For the interpretation of direct and indirect measurements in terms of the intake and resulting effective dose, data on the time pattern and pathway of intake, the chemical and physical form of the radionuclides and on previous intakes are needed. In many cases however the information may not be available.

The time pattern of intake is a main source of uncertainty in the interpretation of bioassay data. Assumptions about the time of intake and of whether the intake was acute, lasted for a short period of time or extended for a long time is a major point in the reliability of the interpretation of the bioassay data. For example, in some cases the retention and excretion functions diminish by orders of magnitude within a few days, therefore the choice of the time pattern of intake can influence the assessed dose within the same range.

Inhalation is the main pathway of intake in the workplace. The characterisation of the intake in terms of aerosol size and absorption type are needed for the application of the m(t) values to estimate the intake. The m(t) values are the calculated values of the measured quantities for unit intake at time t after the intake. The aerosol size will influence deposition in the HRTM and as a consequence the transfer of unabsorbed particles to the GI tract. In some working environments more than one particle size is detected. The rate of absorption of a radionuclide to blood is very important for interpreting bioassay data. It is a critical parameter in interpreting urine excretion data. The differences between the true absorption rates and the default parameters which have been assigned to the compound being inhaled are sources of errors that can be very large, especially when deriving intakes from urinary excretion bioassay data.

Further uncertainty is added when the activity of a radionuclide in the body could not be measured directly but is derived from progeny radionuclides. Contributions from intakes from natural sources, especially in the diet, may also contribute to the uncertainty of a bioassay result.

 

Knowledge of radionuclides handled

 

For many elements there are a number of radionuclides that could be present in the workplace with quite different physical characteristics. At the same time their behaviour after entry into the body can also be very variable depending upon the physical and chemical form present. Some examples are given below for uranium and plutonium to illustrate the potential for exposure to complex mixtures.

 

 

 

Tables 8-10 show the composition of natural and enriched (3.5% and 92.8%) uranium in terms of activity. Note that the composition in terms of mass is completely different. The Tables illustrate the diversity and complexity of uranium compounds found in the workplace. 

 

 

 

Table 8:                 Isotopic composition of natural uranium 

 

Isotope

% Isotopic compositiona

% Alpha activity

Alpha activityb Bq/g

U-238

99.2745

48.16

1.23E+04

U-236

0.0000

0.00

0.00E+00

U-235

0.7200

2.25

5.76E+02

U-234

0.0055

49.59

1.27E+04

Total alpha activity, Bq/g

2.564E+04

Alpha activity ratio U-234/U-238

1.030

Alpha activity ratio U-235/U-238

0.047

 

                                                                         a Composition is given as weight % of total U isotopes

 

                                                                         b Alpha activity per gram uranium

 

 

 

 

 

Table 9:                 Isotopic composition of enriched (3.5 %) uranium 

 

Isotope

% Isotopic compositiona

% Alpha activity

Alpha activityb Bq/g

U-238

96.471

14.73

1.20E+04

U-236

0.0000

0.00

0.00E+00

U-235

3.5000

3.44

2.7992E+03

U-234

0.02884

81.84

6.6679E+04

Total alpha activity, Bq/g

8.1478E+04

Alpha activity ratio U-234/U-238

5.556

Alpha activity ratio U-235/U-238

0.233

 

 

 

 

 

Table 10:               Isotopic composition of enriched (92.8 %) uranium 

 

Isotope

% Isotopic compositiona

% Alpha activity

Alpha activityb Bq/g

U-238

6.06

0.039

7.51E+02

U-236

0.34

0.428

8.16E+03

U-235

92.8

3.89

7.42E+04

U-234

0.79

95.64

1.82E+06

Total alpha activity, Bq/g

1.91E+06

Alpha activity ratio U-234/U-238

2452

Alpha activity ratio U-235/U-238

99.8

 

 

 

 

 

 

Table 11:              Isotopic composition of Pu and Am in spent nuclear fuel from reprocessing plant 

 

Isotope

% Isotopic composition, Pu+Am a

% Pu-Alpha activity

% Total-Alpha  activity

Pu-238

0.30

38.47

38.47

Pu-239

78.65

36.56

36.56

Pu-240

14.64

24.95

24.95

Pu-241

5.55

-

-

Pu-242

0.860

0.02

0.02

Pu-244

0

0

0

Am-241

0

-

0

 

 

 

 

 

Table 12:              Isotopic composition of Pu and Am in spent Light Water Reactor fuel 

 

Isotope

% Isotopic composition, Pu+Am b

% Pu-Alpha activity

% Total-Alpha activity

% Total activity

Pu-238

2.23 (0.69)

81.53

72.86

2.75

Pu-239

54.05 (3.08)

7.17

6.41

0.24

Pu-240

23.18 (0.67)

11.25

10.05

0.38

Pu-241

12.98 (1.42)

-

-

96.23

Pu-242

5.94 (1.32)

0.05

0.04

0.0

Am-241

1.62 (1.22)

-

10.64

0.40

Pu-241 activity/Total Pu a activity

 

 

 

29

Pu-241 /(Pu-239+Pu-240) a activity

 

 

 

155

Am-241 activity/Pu-241 activity

 

 

 

0.004

 

(a)        Light Water Reactors consitute >80% of all commercial reactors, the remaining 20% being evenly divided between Pressurised Heavy Water Reactors (PHWR) of the CANDU type and Gas Cooled (Magnox) Reactors. This data is compiled from six sets of data covering various LWR sub types and reactor power outputs. The composition is for spent Low Enriched Uranium fuel (LEU).

 

(b)        Composition is given as mean atom % of total Pu isotopes + Am-241. Sample Standard Deviations in parentheses.

 

 

 

 

 

The plutonium composition of some materials encountered in the nuclear industry are given in Tables 11 and 12 which show the composition of Pu and Am radionuclides in the reprocessing of spent fuel (Pu-nitrate, Pu oxides) and fuel from a light water reactor (LWR). Again there are widely different chemical characteristics and composition [55].

 

 

 

 

Time(s) and pattern of intake

 

A principal source of uncertainty in the interpretation of bioassay data is the determination of the time of intake. In general, the time will not be known beforehand, especially in the case of routine monitoring when it is required to estimate an intake from a measurement made at the end of a monitoring interval. If an unusual occurrence triggered special bioassay monitoring, then the time of that occurrence is usually taken as the time of intake.

 

 

 

Since the bioassay function that gives the predicted measurement depends on the time since the intake it follows that the estimate of intake will vary, depending on when it is assumed the intake took place. If the time of the intake is known then the assessment is straightforward. However, if the time of intake is unknown then a judgement has to be made when it occurred. In ICRP Publication 54 [47] and ICRP Publication 78 [38] it is argued that in the absence of any information, the time of intake is equally likely to have occurred before the end of the monitoring interval, and therefore suggests that in these situations, a value of t=T/2 should be used, i.e. the intake is assumed to have occurred at the mid pint of the monitoring interval.

 

 

 

If a significant intake and effective dose is calculated, using this assumption, then a more realistic determination may be required. Sometimes a review of workplace monitoring data, such as airborne or surface contamination levels can indicate a likely time for the intake to have occurred. Similarly, if other workers in the same workplace have exhibited positive routine bioassay samples, a review of the data and monitoring schedules for the individual workers may help determine the time of intake for all. Of course, an individual worker may be able to recall the incident that led to the intake. In addition, if several bioassay results are available, perhaps including different types of measurement, a comparison of these results with the m(t) tables may help in narrowing the choice of the time the intake occurred.

 

 

 

While the mid point assumption is a pragmatic and simple approach, it does not result in an unbiased estimate of the intake. If this assumption is applied regularly, to an individual worker, it is subject to bias and will tend to overestimate the worker’s real intake. It is always preferable to avoid bias, and it can be demonstrated that this can be avoided by assuming a constant chronic intake. If a software program is being used to estimate the intake, then this method can be simply applied by selecting the intake regime to be constant and chronic throughout the monitoring interval. Theoretical considerations aside, however, the choice of an appropriate method of estimating intakes is also dependent on broader considerations such as ease of implementation, and in many situations, the mid-point method is adequate, considering the other uncertainties involved in dose assessment.

 

 

 

 

 

Intake pathways

 

Although intakes by inhalation alone are the most frequent in the workplace, intakes by ingestion and uptake through wounds and intact skin cannot be excluded. Sometimes the worker touches the mouth with contaminated hands and ingestion occurs. If the pathway of intake is not known and several bioassay results are available, including different types of bioassay measurements, a comparison of these results with the m(t) tables may help in determining the pathway of intake. In some facilities simultaneous intakes by inhalation and ingestion can occur. In principle, results from the ingestion and inhalation tables can be combined to give predicted values of m(t). Alternatively, it can be modelled by assuming inhalation with a large AMAD (> 5 mm).

 

 

 

If the radionuclide activity can be assessed by direct measurements, lung counting can be used to differentiate between inhaled and ingested material. However, if this is not possible and the radionuclide is in an insoluble form, interpretation of activities excreted in faecal and urine samples in terms of intake is quite problematic. Both the ingested material and the inhaled material deposited in the upper respiratory tract will clear through the faeces in the first few days after intake. Consequently, it is important to initiate excreta sampling as soon as possible after the intake, continuing for an extended period. Material in the faeces after the second week will be exclusively from the respiratory tract, and can be used, together with the appropriate values of m(t), to correct the earlier faecal samples for this component. In the monitoring of workers chronically exposed to long–lived, insoluble radionuclides, activities in the faeces after a 15 days’ absence from work will mostly reflect the delayed clearance from inhaled material (IAEA, [14, 16]).

 

 

 

 

 

 

 

 

 

 

 

 

 

Particle size/chemical composition

 

Although recent reviews of reported measurements of AMAD in workplaces (eg. Dorrian and Bailey [56])  support the ICRP publication 66/68 default value of 5 μm for occupational exposure, they also show that a wide range (about 1–20 μm) has been observed. If the airborne contamination in the workplace has been well characterised, it may be possible to use a more realistic value based on measurements of the activity size distribution. Alternatively, if there are suitable early measurement data available, an “effective” AMAD can be inferred a posteriori from the measurements. The main effect of the aerosol AMAD is to determine the relative amounts deposited (i) in the upper respiratory tract (extrathoracic airways, ET, bronchi, BB, and bronchioles, bb, in the HRTM), which (if not absorbed into blood) is mainly cleared rapidly to the alimentary tract and hence to faeces within a few days, and (ii) in the lower respiratory tract (alveolar‑interstitial, AI, region in the HRTM), which is mainly cleared slowly from the lungs. For a relatively insoluble (Type M or S) material inhaled by a Reference Worker, the ratio of cumulative faecal excretion over the first 3 days to lung activity on day 3 increased almost linearly from about two at 1 μm AMAD to twelve at 10 μm AMAD. Hence the observed ratio could be used to infer the “effective” AMAD. It is referred to as “effective”, because the ratio will be determined not only by the aerosol size, but also by the subject’s breathing pattern (especially if it involves mouth‑breathing) and inter-subject variation in deposition under any given set of conditions. Because it takes account of these, it is preferable for dose assessment than a priori measurements of the AMAD.