Application of IDEAS Guidelines: the IDEAS/IAEA Intercomparison Exercise on Internal Dose Assessment.

Transcription

1 Application of IDEAS Guidelines: the IDEAS/IAEA Intercomparison Exercise on Internal Dose Assessment C Hurtgen 1), A Andrasi 2), MR Bailey 3), A Birchall 3), E Blanchardon 4), V Berkovski 5), CM Castellani 6), R Cruz-Suarez 7), K Davis 3), H Doerfel 8), B LeGuen 9), I Malatova 10), J Marsh 3), J Zeger 7) 1) SCK CEN, Belgian Nuclear Research Centre, Mol, Belgium 2) KFKI-AERI, Atomic Energy Research Institute, Budapest, Hungary 3) HPA, Health Protection Agency, Chilton, Didcot, United Kingdom 4) IRSN, Institute de Radioprotection et de Sécurite Nucléaire Fontenay-aux-Roses, France 5) RPI, Radiation Protection Institute, Kiev, Ukraine 6) ENEA Institute for Radiation Protection, Bologna, Italy 7) IAEA, International Atomic Energy Agency, Vienna, Austria 8) FZK, Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany 9) EDF-GDF, Electricité de France Saint-Denis, France 10) NRPI, National Radiation Protection Institute Praha, Czech Republic Corresponding author Christian Hurtgen Head of Department Low Level Radioactivity Measurements Radioprotection Unit SCK CEN Boeretang 200, B-2400 Mol, Belgium Tel: Fax: <mailto:churtgen@sckcenbe> - 1 -

2 ABSTRACT As part of the EU 5 th Framework Programme IDEAS project General Guidelines for the Evaluation of Incorporation Monitoring Data, and in collaboration with the IAEA, a new intercomparison exercise for the assessment of doses from intakes of radionuclides was organised Several cases were selected, to cover a wide range of practices in the nuclear fuel cycle and medical applications The cases were: 1 - Acute intake of HTO, 2 - Acute inhalation of the fission products 137 Cs and 90 Sr, 3 Acute inhalation of 60 Co, 4 - Repeated intakes of 131 I, 5 - Intake of enriched uranium, 6 - Single intake of Pu isotopes and 241 Am This intercomparison exercise especially focused on the effect of the guidelines proposed by the IDEAS project for harmonisation of internal dosimetry - 2 -

3 Application of IDEAS Guidelines: the IDEAS/IAEA Intercomparison Exercise on Internal Dose Assessment C Hurtgen 1), A Andrasi 2), MR Bailey 3), A Birchall 3), E Blanchardon 4), V Berkovski 5), CM Castellani 6), R Cruz-Suarez 7), K Davis 3), H Doerfel 8), B LeGuen 9), I Malatova 10), J Marsh 3), J Zeger 7) 1) SCK CEN, Belgian Nuclear Research Centre, Mol, Belgium 2) KFKI-AERI, Atomic Energy Research Institute, Budapest, Hungary 3) HPA, Health Protection Agency, Chilton Didcot, United Kingdom 4) IRSN, Institute de Radioprotection et de Sécurite Nucléaire Fontenay-aux-Roses, France 5) RPI, Radiation Protection Institute, Kiev, Ukraine 6) ENEA Institute for Radiation Protection, Bologna, Italy 7) IAEA, Vienna, Austria 8) FZK, Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany 9) EDF-GDF, Electricité de France Saint-Denis, France 10) NRPI, National Radiation Protection Institute Praha, Czech Republic Abstract: As part of the EU 5 th Framework Programme IDEAS project General Guidelines for the Evaluation of Incorporation Monitoring Data, and in collaboration with the IAEA, a new intercomparison exercise for the assessment of doses from intakes of radionuclides was organised Several cases were selected, to cover a wide range of practices in the nuclear fuel cycle and medical applications The cases were: 1 - Acute intake of HTO, 2 - Acute inhalation of the fission products 137 Cs and 90 Sr, 3 Acute inhalation of 60 Co, 4 - Repeated intakes of 131 I, 5 - Intake of enriched uranium, 6 - Single intake of Pu isotopes and 241 Am This intercomparison exercise especially focused on the effect of the guidelines proposed by the IDEAS project for harmonisation of internal dosimetry - 3 -

4 INTRODUCTION Several intercomparison exercises have been organised at national and international levels for the assessment of radionuclide intakes and doses from individual monitoring data (1,2,3) These exercises revealed significant differences in the approaches, methods and assumptions used, and consequently in the results obtained by participating laboratories Within the EU 5 th Framework Programme, the IDEAS project drafted General Guidelines for the Evaluation of Incorporation Monitoring Data (4) to promote harmonisation of internal dose assessment The draft Guidelines were tested by means of an intercomparison exercise open to participants from all over the world IDEAS / IAEA INTERCOMPARISON This exercise was performed jointly by the International Atomic Energy Agency (IAEA, the Agency) and the IDEAS project Full details are available in a report (5) on the Internet Six cases were selected to cover a wide range of practices in the nuclear fuel cycle and medical applications: (1) Acute intake of HTO (2) Acute inhalation of the fission products 137 Cs and 90 Sr (3) Acute inhalation of 60 Co (4) Repeated intakes of 131 I (5) Intake of enriched uranium (6) Single intake of Pu isotopes and 241 Am 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 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 Agency, there was 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 This paper focuses on the results of assessed doses The statistical procedure used in a previous exercise( 3 ) was applied to identify outliers in each set of results Table 1 summarises results (committed effective doses, E(50)) excluding outliers The results were discussed - 4 -

5 with the participants during a workshop held by the Agency in April 2005 Based on these discussions the IDEAS Guidelines were finalised, and are presented at this conference (4) CASE 1: ACUTE INTAKE OF HTO This case is related to a real event (6) Effective doses from intakes of HTO can be assessed using the direct dose assessment method (area under the urine activity concentration data) to determine the number of nuclear transformations This method is not covered by the structured approach in the IDEAS Guidelines but by a separate section Most participants (51) applied the direct method using the trapezoid integration procedure while seven applied it using a multi-exponential fitting procedure to calculate the cumulative activity The frequency distribution of the results is very narrow: its geometric standard deviation (GSD) is only 106 (Table 1) However, the number of outliers is relative high because the small GSD caused the statistical criteria for outliers to become very strict In some cases it is not clear if the participant actually followed the Guidelines It is thus difficult to assess the benefit of the guidelines for direct internal dosimetry However, the percentage of outliers was significantly lower among those who followed the Guidelines CASE 2: ACUTE INHALATION OF FISSION PRODUCTS 137 Cs and 90 Sr This is a real case used in a previous exercise (1) Fifty-eight participants assessed it For 137 Cs the spread of results is small: several outliers were identified, mainly due to the exclusion of the first whole body measurement at time zero or application of an erroneous dose coefficient The distribution of E(50) results without outliers is narrow (GSD = 116) For 90 Sr the spread of results without outliers is wider (GSD = 194) There are more outliers, reasons being: the assumption of lung absorption Type F or M; the use of a single data set (urine) instead of both urine and faeces (only 43% of the participants used both datasets); and the exclusion of some data in the assessment Use of the IDEAS Guidelines was indicated by 48% ( 137 Cs) and 39% ( 90 Sr) of participants The main effect of using the Guidelines for both 137 Cs and 90 Sr was a reduction in the occurrence of outliers - 5 -

6 However, a deeper analysis showed that a lower percentage (20%) of participants used the Guidelines correctly CASE 3: ACUTE INHALATION OF 60 Co This is an artificially generated case designed to illustrate the special evaluation for inhalation (Stages 5A and 5B) of the IDEAS Guidelines In particular, it addresses Step 513 assessment of dose by fitting a mixture of absorption types The data were generated by assuming an acute inhalation of aerosol of 60 Co, with a mixture of absorption types (50% Type M (f 1 = 01) + 50% Type S (f 1 = 005)) and ICRP default particle size Uncertainty on each of the whole body and urine data points was also simulated Both urine and whole body data were given so that the assessor could assess the mixture of absorption types Sixty-two participants assessed this case For the assessed doses there were only 6 outliers Excluding the outliers the distribution of the E(50) results had a GSD of 14 Twenty-four participants (39%) assumed a mixture of absorption types or specific absorption parameter values For this group the distribution of values is narrow (GSD = 114) Almost 50% of the participants stated that they followed the IDEAS guidelines Of these, 14 participants (23%) fitted a mixture of absorption types and reached the expected final step For this group the GSD is even smaller (109) Therefore, the correct application of the guidelines further reduced the spread of the accurate results CASE 4: REPEATED INTAKE OF 131 I The case was designed to test the application of the Guidelines in a simple but general case of contamination during the preparation and handling of 131 I for therapeutic purposes The intake was supposed to occur on 3 consecutive days of one week and 3 thyroid measurements the following week were provided The E(50) results have a narrow distribution (GSD =107) Out of 63 submitted results 13 (21%) were identified as outliers As for case 1, this large proportion arises because of the small GSD The most common cause was the application of an incorrect dose coefficient - 6 -

7 Twenty-seven participants (43%) stated that they followed the IDEAS guidelines The others either indicated that they did not follow the Guidelines (25) or did not comment (11) The application of the Guidelines gave greater consistency: a lower GSD for those who followed (105), than for those who did not (109) CASE 5: INTAKE OF ENRICHED URANIUM This is a real, but unpublished, case of enriched uranium inhalation with few available data: only four lung measurements (two of which are below the minimum detectable activity, MDA) and four urine measurements in a time interval of 5 months For the purpose of the exercise, participants were asked to assess E(50) due to an accident and the previous 13 years occupational exposure Hence an acute and/or a chronic intake had to be evaluated In this case, the time pattern of intake by inhalation and the data used for the assessment have large influences on the doses assessed The results submitted fall into four groups: Acute intake associated with the reported incident: mostly influenced by the lung measurements Chronic intake: mostly influenced by the urine data Acute intake using only the urine data gives results similar to the chronic intake Acute + chronic intakes with the lung data representing the acute intake and the urine data the chronic intake; this is the correct assessment The acute and chronic intakes have associated assessed doses based on the lung and urine data of about 10 and 70 msv The overall distribution of the results is thus bimodal with a GM of about 30 msv The distribution of results with the chronic assumption is broad (GSD 28) because of the different assumptions made in the absence of comprehensive information The scarcity of data in this case raises the important point that all available data have to be taken into account to estimate the intake and the dose It is surprising that one third of the participants used only half of the monitoring data provided - 7 -

8 Forty-one percent of participants stated that they followed the IDEAS Guidelines However for this case it is difficult to evaluate the impact of the application of the guidelines because of the scarcity of data, and the lack of other information, which allowed different interpretations The IDEAS Guidelines are an important tool, but judgment is still needed for the correct assessment of a contamination case CASE 6: SINGLE INTAKE OF Pu ISOTOPES AND 241 Am This case is one of the best-documented cases of a single intake of trans-uranium elements worldwide and has been used in several previous intercomparison exercises Because there are comprehensive excretion and organ content data over a 10-year period it is possible to determine specific parameter values for the ICRP Human Respiratory Tract Model (HRTM) The case aimed to illustrate the special evaluation for inhalation (Stages 5B and 5C) of the IDEAS Guidelines, in particular the adoption of specific HRTM parameter values for absorption and particle transport Two radionuclides were considered for this exercise ( 239 Pu and 241 Am) and their intake and dose evaluations can be connected as their isotopic ratio in the material involved is known Thirty-five participants assessed the intake of 241 Am and the resulting E(50) There are three outliers, and the reasons are unrealistic assumptions such as ingestion and absorption Type F Excluding outliers, the GM of the assessed E(50) is 52 msv with a GSD of 213 The assessment of the intake of 241 Am and E(50) can be carried out by following the Guidelines, and in particular reducing the particle transport rates from the AI region (step 519 of the Guidelines) Seven participants (20%) derived specific particle transport rates For this group (excluding one participant who used unrealistically low rates) the GSD is small (12) Hence for those participants that followed the guidelines correctly, and therefore reached step 519 of the guidelines, the results for 241 Am have a relatively narrow distribution Thirty-six participants assessed the intake of 239 Pu and calculated the resulting E(50) There are 5 outliers and the reasons are erroneous assumptions and applying an incorrect dose coefficient Excluding outliers the GM of the assessed E(50) is 140 msv with a GSD of

9 The 239 Pu assessment should not be carried out in isolation from the 241 Am assessment Using the 241 Am data it is possible to determine specific particle transport parameter values, which should be applied to the 239 Pu assessment Seven participants did so, and for this group (excluding one who used unrealistically low rates) the GSD is narrow (138) Hence the correct application of the IDEAS Guidelines, in this very sophisticated evaluation, gives consistent values However, the Guidelines do not give detailed advice on how to use all the available information when bioassay data from more than one radionuclide are available and the initial activity ratios are known CONCLUSIONS This intercomparison exercise showed that the Guidelines for evaluation of incorporation monitoring data developed in the IDEAS project could have a positive influence on the harmonization of reported intakes and doses Considering the limited time provided for the solution of the cases (2 months), the need to be acquainted with the Guidelines and their use, the number of participants that have correctly applied the Guidelines is encouraging 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 can not 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 will be put into the promotion and correct application of such guidelines in the international internal dosimetry community, together with dedicated training It is considered that their greatest effect will be on new professionals in the field The individual results of participants in this intercomparison exercise should also help their laboratories to demonstrate (eg to customers and regulators) their effectiveness in assessing occupational doses to individuals from intakes of radionuclides It is important that participation in such exercises is available on a regular basis in future ACKNOWLEDGMENT - 9 -

10 The project was partly funded by the European Commission under contract No FIKR-CT , and was partly supported by the IAEA REFERENCES 1 JA Gibson et al A European Intercomparison of Methods Used for the Assessment of Intakes of Internally Deposited Radionuclides, Radiation Protection Dosimetry Vol 40 N 4, pp , International Atomic Energy Agency: Intercomparison and Biokinetic Model Validation of Radionuclide Intake Assessment (Report of a Co-ordinated Research Project ), IAEA- TECDOC-1071 (1999) and in 3 Doerfel, H, Andrasi, A, Bailey, MR, Birchall, A, Castellani, C-M, Hurtgen, C, Jarvis N, Johansson, L, LeGuen, B, Tarroni, G, Third European Intercomparison Exercise on Internal Dose Assessment, Forschungszentrum Karlsruhe, Report FZKA 6457, April 2000and in 4 Doerfel, H, Andrasi, A, Bailey, MR, Blanchardon, E, Cruz-Suarez, R, Berkovski, V, Castellani, C-M, Hurtgen, C, LeGuen, B, Malatova, I, Marsh, J, Stather, J, Zeger, J, General guidelines for the assessment of internal dose from monitoring data : progress of the IDEAS project (These proceedings) and in or 5 Hurtgen, C, Andrasi, A, Bailey, MR, Birchall, A, Blanchardon, E, Berkovski, V, Castellani, CM, Cruz-Suarez, R, Davis, K, Doerfel, H, LeGuen, B, Malatova, I, Marsh, J, Zeger, J IDEAS / IAEA Intercomparison Exercise on Internal Dose Assessment Scientific Report SCK CEN BLG SCK CEN, Mol, Belgium, October 2005 and in 6 Henry P, Study of accidental contamination by tritiated water Rayonnements Ionis Tech Mes Prot No 2, pp ,

11 Table 1 Statistical evaluations of the E(50) results excluding outliers Case Radionuclide GM GSD AM ASD Coefficient of Number of N (msv) (msv) (msv) variation results* 1 3 H % 48 (12) Cs % 52 (6) Sr % 48 (10) 60 Co % 56 (6) I % 50 (13) 234 U % 38 (3) U Chronic % 22 (0) 241 Am % 32 (3) Pu % 31 (5) * number of outliers in brackets; GM = geometric mean; GSD = geometric standard deviation; AM = arithmetic mean; ASD = arithmetic standard deviation