|ICRP - Supporting Guidance Document Interpretation of Bioassay Data
From: Dunstana Melo
The guidance will be very useful for dose assessment, but there are some issues that need be addressed:
1) According to the guidance if the dose is into the range from 1 to 6 mSv a more detailed evaluation is needed, requiring more urinary and fecal samples to be analyzed and interpreted. For example, if the assessed dose is 1.5mSv due to intake of Pu-239, 3 urinary and fecal samples should be collected for each worker during an interval of 30 days. For alpha emitting radionuclides, radiochemical analysis is time consuming. For a big facility with large numbers of urine and fecal samples to be analysed every month for the routine monitoring program, it will be very difficult to implement those recommendations. Is it justifiable to refine the dose assessment for such a low dose as 1.5mSv?
2) The aim of the document is to provide guidance on the interpretation of bioassay data to allow for a better harmonization of approaches by individuals and laboratories. In case of intakes that result in doses higher than 6 mSv it is recommended to perform a more sophisticated dose assessment, including changing the lung retention parameters. This in fact may result in lack of harmonization again, see results of Case 6 of the IDEAS/IAEA Intercomparison Exercise on Internal Dose Assessment: intercomparison for Pu-239. The Case 6 was a single intake of Pu radioisotopes and 241Am (below): two dosimetrists assessed different intake and effective dose values (8kBq and 100mSv; 24kBq and 400mSv). The harmonization will be reached until the step 5.15; from this step, differences will occur depending on the assumption made in the dose assessment.
IDEAS / IAEA Intercomparison Exercise on Internal Dose Assessment
VI. Case 6: Single intake of Pu radionuclides and 241Am .
Conclusion: Pu-239 dose can be changed substantially by changing ss value from 3E-5 d-1 (large lung dose, large intake) to 2E-4 d-1 (much smaller lung dose, smaller intake) with little evidence in urine and fecal data.
Item 7.6.5 (page 65) and Table 7.12 (page 8) - Number and type of data required for assessment of dose:
In Table 7.12 it is recommended to assess dose due to intake of all types of alpha emitters by fecal analysis for radionuclides, for expected doses below 1 mSv. The main problem is the detection in excreta of alpha emitting radionuclides in insoluble forms. It should be the responsibility of individual laboratories to decide the type of sample that should be analyzed according to the measurement systems available. If a sensitive measurement system is available it is better and easier to analyze urine samples instead of feces samples. Fecal samples are very difficult to obtain, especially for three consecutive days. In general the workers do not feel comfortable collecting feces. And also there is a large daily variation on fecal excretion; the uncertainty can be very high for routine monitoring.
According to the Table 27.6, the typical detection limit for uranium in urine is 1mBq, but 0.1mBq is achievable; the typical detection limit for uranium in feces is 10 mBq, but 1 mBq is achievable. The simulation for inhalation of uranium compound Type S, AMAD=5um (see Table 1), and also for ingestion of insoluble uranium (see Table 2) show that for all intervals of monitoring uranium can be detectable in urine samples for both pathway of intakes, assuming that the MDA is 0.1mBq.
Table 1- Predicted uranium excretion due to inhalation of uranium compound Type S AMAD=5um - routine monitoring.
Expected U excretion for dose of 1mSv
Interval of monitoring Days after intake IRF (24 hrs)
Urine Feces Urine Feces
(Bq/Bq) (Bq/Bq) (Bq/d) (Bq/d)
8 4 2.32E-05 3.53E-02 3.41E-03 5.19E+00
14 7 1.89E-05 2.47E-03 2.78E-03 3.63E-01
30 15 1.23E-05 4.91E-04 1.81E-03 7.22E-02
60 30 7.68E-06 3.49E-04 1.13E-03 5.13E-02
90 45 6.01E-06 2.52E-04 8.83E-04 3.70E-02
120 60 5.18E-06 1.85E-04 7.61E-04 2.72E-02
180 90 4.28E-06 1.07E-04 6.29E-04 1.57E-02
360 180 3.29E-06 3.72E-05 4.84E-04 5.47E-03
Table 2- Predicted uranium excretion due to ingestion of insoluble uranium compound - routine monitoring.
Interval of monitoring
Days after intake IRF (24 hrs) Expected U excretion for dose of 1mSv
Urine Feces Urine Feces
(Bq/Bq) (Bq/Bq) 24 hrs (Bq) 24hrs(Bq)
8 4 3.24E-05 8.12E-02 3.90E+00 9.78E+03
14 7 2.44E-05 4.37E-03 2.94E+00 5.27E+02
30 15 1.25E-05 1.53E-06 1.63E+00 4.90E-01
60 30 4.77E-06 2.24E-08 5.75E-01 2.70E-03
90 45 2.49E-06 1.41E-08 3.00E-01 1.70E-03
120 60 1.60E-06 9.93E-09 1.93E-01 1.20E-03
180 90 8.44E-07 5.51E-09 1.02E-01 6.64E-04
360 180 2.19E-07 1.45E-09 2.64E-02 1.75E-04
Uranium annex page 20 Table 27.10
The data for urine and feces appear incorrect.