The biokinetic models are used for the assessment of internal dose from the monitoring data. There are some broad guidelines for routine, special and task-related individual monitoring recommended by ICRP in Publication 54 [47] and Publication 78 [38]. These guidelines leave most of the assumptions open, this resulting in many different approaches for the interpretation of monitoring data as demonstrated by the 3rd European Intercomparison Exercise on Internal Dose Assessment [48].

 

The 3rd European Intercomparison Exercise gave special consideration to the effects of the new models and the choice of input parameters on the assessment of internal doses from monitoring results. It also took into account some aspects which had not been considered in previous exercises, such as air monitoring, natural radionuclides, exposure of the public, artificially created cases and artificially reduced information. Seven case scenarios were distributed, dealing with 3H, 90Sr, 125I, 137Cs, 210Po, 238U and 239Pu, and covering different intake scenarios and all monitoring techniques. Results were received from 50 participants, 43 representing 18 European countries and 7 from five countries outside Europe. Most participants attempted more than half of the cases. Thus on average there were 35 responses per case with a total of about 240 answers, giving a good overview of the state of the art of internal dosimetry. The results in terms of intake and committed effective dose appeared to be log-normally distributed with the geometric standard deviation ranging from 1.15 for the cases dealing with 3H and 137Cs, up to 2.8 for the cases dealing with α-emitters (Table 6).  These figures reflect to large differences in the individual results which varied in the worst case over a range of five orders of magnitude. A key feature of the exercise was a Workshop, involving most of the participants, at which each case and the various approaches taken to assessing it were discussed.  Several reasons for the differences in the results were identified, including different assumptions about the pattern of intake, and the choice of model.

Table 6:     Statistical evaluations of the E(50) results of the 3rd European Intercomparison Exercise on Internal Dose Assessment (excluding outliers) [48]

Case number

Radionuclide

Geometric mean (mSv)

Geometric standard dev.

Number of results(a)

1

3H

0.00529

1.16

41 (7)

2

90Sr/90Y

0.093

1.78

38 (4)

3

125I

0.441

1.53

38 (2)

4

137Cs

0.198

1.15

43 (6)

5

210Po

3.18

1.25

20 (1)

 

238U (+daughters)

0.355

2.31

20 (0)

 

232Th (+daughters)

0.157

2.8

20 (0)

6(A)

239Pu

240

2.4

32 (3)

7

239Pu

347

2.16

30 (2)

                                                                    (a) number of outliers in brackets

 

The most important conclusion of the 3rd European Intercomparison Exercise was the need to develop agreed guidelines for internal dose evaluation procedures in order to promote harmonisation of assessments between organisations and countries, which has special importance in the European Union, because of the mobility of workers between member states. This was the reason to launch the IDEAS project in the 5th EU Framework Programme (EU Contract No. FIKR-CT2001-00160). The IDEAS project reviewed the “Science and art of internal dose assessment”. Based on this review “General guidelines for the estimation of committed effective dose from incorporation monitoring data” were developed which cover all aspects of internal dosimetry [49] 

For testing of the IDEAS Guidelines a new intercomparison exercise was organised jointly by the International Atomic Energy Agency (IAEA) and the IDEAS project [50].  Full details of the intercomparison exercise are provided by Hurtgen [51] and by the IAEA [52]. Six cases were selected to cover a wide range of practices in the nuclear fuel cycle and medical applications. These cases were: (1) acute intake of HTO, (2) acute inhalation of the fission products 137Cs and 90Sr, (3) acute inhalation of 60Co, (4) repeated intakes of 131I, (5) intake of enriched uranium and (6) single intake of Pu isotopes and 241Am. Four of the cases (1, 2, 5 and 6) are real, and all except case 5 have been published. Cases 3 and 4 were artificially constructed.

 

Table 7:     Statistical evaluations of the E(50) results of the IDEAS/IAEA intercomparison exercise [50-52] (excluding outliers)

Case number

Radionuclide

Geometric mean (mSv)

Geometric standard dev.

Number of results(a)

1

3H

25.8

1.06

46 (12)

2

137Cs

0.66

1.16

52 (6)

 

90Sr

7.22

1.94

48 (10)

3

60Co

5.0

1.4

56 (6)

4

131I

2.57

1.07

50 (13)

5

Enriched Uranium

36.8

2.4

38 (3)

6

241Am

52

2.1

32 (3)

 

239Pu

140

1.58

31 (5)

                                                                    (a) number of outliers in brackets

 

 Because of the easy access to the cases via the Internet, and the worldwide promotion of the intercomparison exercise by the IDEAS group and the IAEA, there were a large number of participants from all over the world. Participants were free to undertake only those cases relevant to their work. Of the 74 participants who assessed at least one case, 36% provided an answer to all six cases. The highest participation (84%) was for the cobalt and iodine cases and the lowest (57%) was for the americium part of case 6. The statistical procedure used in the previous exercise [46] was applied to identify outliers in each set of results. Table 7 summarises results (committed effective doses, E(50)) excluding outliers. The results were discussed with the participants during a workshop held by the Agency in April 2005.

As can be seen from Tables 6 and 7, the geometric standard deviation of the results tends to be smaller for the IDEAS/IAEA intercomparison exercise than for the 3rd European Intercomparison Exercise. The cases of the two intercomparisons cannot be compared with each other, but there seems to be an improvement. Actually, the IDEAS/IAEA intercomparison exercise showed that the IDEAS Guidelines have a positive influence on the harmonisation of reported intakes and doses. An important finding was the lower occurrence of outlying values among those who applied the Guidelines than among those who did not. However, even very detailed guidelines cannot help if unrealistic assumptions or simple mistakes are made. There is still a need for adequate training, experience and quality control.

Some 20% of participants used the IDEAS Guidelines correctly and reached results that can be considered accurate. In view of this, more effort should be put into the promotion and correct application of such guidelines in the international internal dosimetry community, together with dedicated training. So the IDEAS guidelines should be described in more detail also in this refresher course.

The disillusioning experience with the past intercomparison exercises was also recognised by the International Standardization Organization (ISO). Because of the substantial differences between national regulations, concepts, and dose assessment procedures the ISO recently initiated projects to standardize the monitoring of workers, the requirements for measuring laboratories and the processes for the quantitative evaluation of monitoring data [53]. The anticipated approaches correspond widely to the IDEAS Guidelines.