Inhalation of Co-60


This case study  was part of the intercomparison exercise organized in 2004 by IDEAS in cooperation with IAEA. The case refers to an incident which happened in a laboratory for production of Co-60 sources by irradiation of Cobalt capsules in a reactor. Such a capsule containing 900 TBq Co-60 was opened in a hot cell. About 10 min after opening the capsule there was an alarm due to increased dose rate outside the cell. The operator closed the capsule again, put on protection clothes and breathing mask, closed the leakage and decontaminated the working place. Ten days after the incident a long term in vivo monitoring program was started and whole body measurement were performed over 3 years. In parallel urine samples were taken and analysed.  



The case study is available in the standard database of IDEA System and can be called via the Person ID "IAEA-IDEAS-3". When selecting this Person ID you will see the respective person data together with the case description (see first screenshot on the left).


















In the next screenshot you see all measuring data of the case. On the right hand side you see a table with a listing of all data. On the left hand side you see the detailed information for the measuring value which is marked with the black triangle in the list on the right hand side. You can move the triangle to any measured value by clicking in the respective line of the table.


The values for the lower limit of detection (LLD) have been allocated automatically by the IDEA software because there is no other information available in the case description. The uncertainty has been calculated according to the IDEAS General Guidelines.





























For evaluating the measured values you open the "Evaluation" window. Here you go with the curser to "Select radionuclide" and click on "Co-60" in the pull-down menue in order to select the radionuklide under investigation (in this case there is only one radionuclide available).


Then you go to "Select mixture" and click on "0 Single radionuclide" in the pull-down menue (in this case Co-60 has been incorporated as a single radionuclide; note that in practice Co-60 is likely to be incorporated in mixtures of various activation and/or fission products; for this reason there is the option to define Co-60 as part of one of the nuclide mixtures given in the pull-down menue). 


For the evaluation all data should be used. So you click on "Mark all" to identify all available data and then on "Select data" to select the marked data.


The system checks automatically the earlest time of intake. This is done automatically and of minor importance in this case because the time of intake is known.



















After confirmation of the message concerning the earliest time of intake the system checks automatically if there are records of previous intakes of Co-60 for the person in the database. In this case there are no previous intakes.







































In the next step you can select the evaluation procedure. In this case most of the intake parameters (time and pathway of intake) are known and so you should start the evaluation with "Standard (Level 2; selected parameters)". Now a frame opens where you can select the evaluation parameters. The defaults suggested for the time pattern and pathway of intake are correct. The date of the intake, however, has to be changed to the actual date of the incident, which was 10 days before the first measurement, i.e. 01.01.2000.


In addition the absorption type should be set to the default (S).






























Now you can start the evaluation. The system checks automatically the significance of the data according to the IDEAS General Guidelines and comes to the conclusion "There is a new intake".








































After confirmation of the message a window opens with the results of this evaluation. The left part of the window gives an overview over the evaluation procedure, the evaluation parameters and the most important results in terms of intake and dose (intake 603 KBq Co-60 and committed effective dose 10.3 mSv). In the right part of the window you see a table with more detailed information about the committed effective dose and all organ doses (column D(QD)) as well as the doses in terms of dose limit fractions (column DR(QD)). In this case the organ with the maximum dose limit fraction (critical organ) is the lung (organ dose 57.9 mSv, corresponding to 38.6 % of the limit.  The dose values marked in yellow are above 6 mSv or 30 % of the dose limits, respectively. So the case is allocated to "Level 3" according to the IDEAS General Guidelines (this conclusion is displayed in the upper left part of the window).


The potential error (emprical scattering factor SF) of the evaluation is 2.24. This value is much higher than the typical values according to the IDEAS Guidelines(SF = 1.18 for the whole body measurements and SF = 1.8 for the urine measurements). Thus, the fit is not acceptable.










This is also evident from the plots which show the fitted retention and excretion functions together with the respective measured values (the x-axis represents the time since the begin of monitoring).


Obviously the default absorption type (S) does not apply to this case. The measured whole body retention is lower than the model prediction whereas the measured urinary excretion is higher than the model prediction. Thus, the solubilty of the involved material seems to be higher than the solubility of type S material.
































So the evaluation should be repeated with adjusting the absorption type. For doing so you should select the evaluation procedure Special (Level 2 or 3; fitting of AT or f1).
































































As can be seen in the next message, the best fit results with absorption type M. The expert system provides the option to interpolate the absorption type in order to improve the fit. You should make use of this option.














The interpolation results in an intake of 207 kBq Co-60 with absorption type M and 207 kBq Co-60 with absorption type S. The effective dose is now below 6 mSv and also the organ doses are below 30 % of the respective limits. The potential error (empirical scattering factor SF) is 1.56. This is a typical value for an acceptable fit according to the IDEAS Guidelines.





























The plots confirm the quality of the fit.


Thus, according to the IDEAS General Guidelines, the evaluation procedure can be finished at this point because (i) the fit is acceptable and (ii) the committed effective dose is less than 6 mSv.


P.S. This is an artificial case where the "real" value of the intake is known: The "real" intake is 200 kBq Co-60 with absorption type M and 200 kBq Co-60 with absorption type S, this being almost identical with the result obtained with IDEA (207 kBq Co-60 with absorption type M and 207 kBq Co-60 with absorption type S).





Prof. Dr.-Ing. Hans Richard Doerfel

IDEA System GmbH, Am Burgweg 4, D-76227 Karlsruhe, Germany.