A Simple Model for Establishing Exemption Limits

Transcription

1 A Simple Model for Establishing Exemption Limits S. Baechler 1, A. Aroua 1, J.F. Valley 1, W. Zeller 2 1 University Institute of Applied Radiation Physics, Grand-Pré 1, CH-1007 Lausanne, Switzerland sebastien.baechler@hospvd.ch 2 Swiss Federal Office of Public Health, CH-3003 Bern, Switzerland Abstract. The concept of exemption is significant in radiation protection. It is meant to alleviate and optimize the system of regulatory control by setting levels of radiation for which this system is not concerned. The exemption levels (s) are based on a set of radiological principals and dose criteria. The main dose criterion for deriving s is the 10 µsv per year, but additional limits on the collective dose (1 person.sv) or on the skin dose (50 msv per year) to prevent deterministic effects are usually considered. The derivation of s from the dose criteria requires the use of a model, which may be based on generic scenarios or on scenarios specific to practices, involving one or more exposure pathways. The ideal model is conservative in the dose calculation and covers all the possible exposure situations. In this work a simple model is developed based on the different exposure pathways (external exposure, surface exposure, inhalation and ingestion). A modified version of this model was used to establish the s recommended by the Swiss Radiation Protection Authority. The design of this model was driven by three principles: (a) generality, to allow a wide application, (b) simplicity, to provide an easy and fast evaluation of s, and (c) transparency, to be used not as a black-box but with full understanding of all the physics involved, the assumptions made and the parameters adopted. The model will be described and the results obtained with it will be presented and compared to the values established with the EU-IAEA model. 1. Introduction In the field of radiation, a system of regulatory control is set by the Regulatory Authority to assure the best level of radiation protection associated with the radiation source or practice. For instance, the basis of regulatory control in the IAEA Basic Safety Standards (BSS) is a system of notification, registration and licensing, which makes it possible for the Regulatory Authority to impose appropriate requirements for radiation protection. To make the regulatory control efficient, the concept of exemption has been introduced. This concept is sometimes confused with other ways of exonerating radiation sources and practices, namely exclusion and clearance. Although these three terms specify radiation sources and practices which do not need to be regulated, the reason is different in. each case. "Exclusion refers to sources [and practices] which are not amenable to control; exemption and clearance apply to sources [and practices] which would normally be expected to be under control but which present such a low risk that it would be a waste of resources to exercise control by regulatory means. Exemption applies to sources [and practices], which never enter the regulatory control regime, while clearance applies to sources [and practices], which are released from regulatory control [1]. The present work compares two different models to ca1culate the exemption limits from given radiological principles and dose criteria. The first model was developed in Switzerland and is used in the Swiss Radiological Protection Ordinance, RPO [2]. The second has been developed by the European Union [3], adopted by the IAEA and used in a BSS report [4]. The basic principles of each model as well as the comparison of the results obtained by both models are presented in this paper. 1

2 2. Description of models 2.1 Radiological principles and dose criteria The ICRP states clearly that the derivation of exemption levels must comply with the principles of radiation protection and specifies two grounds for exempting a source or an environmental situation from regulatory control: "One is that the source gives rise to small individual doses and small collective doses in both normal and accident conditions. The other is that no reasonable control procedure can achieve significant reductions in individual and collective doses [5]. The IAEA is in full agreement with this approach [6]. The dose criteria for deriving the exemption are established considering an acceptable level of risk generally associated with the annual attributable risk of death. An excess risk of 10-6 to 10-5 is considered to be acceptable, and an annual individual dose of a few tens of µsv is thought to provide a good basis for exemption (a nominal probability of incidence of fatal cancer of 5% Sv -1 for the whole population is used). In order to take account of the situation where an individual is exposed from more than one exempt practice, both the Swiss model and the EU-IAEA model set the annual dose limit from a given exempt practice to 10 µsv. However, the EU-IAEA model adds a limit on the collective dose: "the collective effective dose committed by one year of performance of the practice is no more than about 1 man.sv or an assessment for the optimization of protection shows that exemption is the optimum option" [4]. In order to prevent deterministic effects, a further dose constraint of 50 msv yr -1 to the skin is set. This criterion relates to selective localized exposure of the skin and is therefore applied to the area of skin in contact with the source. To implement the dose criteria, the user may be asked to evaluate himself the dose generated by the source or practice he intends to employ, and to make sure it stands below the dose criteria. The approach has the advantage of making the evaluation on realistic grounds, but assumes a certain expertise of the user to perform the dose calculations. The other way, easier for the user, consists in converting the dose criteria into activity limits (specific and absolute) for each radionuclide. Ideally both approaches should be combined. The derivation of exemption limits from the dose criteria requires the use of a model, which may be based on generic scenarios (covering all situations of exposure that can be met with the different practices), or on scenarios specific to practices, involving one or more exposure pathways. The model may also be based simply on the different exposure pathways. The ideal model is conservative in the dose calculation, and covers all the possible exposure situations, with a reasonable degree of realism. 2.2 The Swiss Model The model for deriving the exemption limits presented in the Swiss RPO allows the conversion of the dose criteria into activity limits (specific and absolute) for a given radionuclide. It performs a conservative dose calculation. The model is based on three key concepts: Generality, to allow a wide application, covering as many situations as possible; Simplicity, to provide an easy and fast evaluation by the user with a low probability of error; Transparency, to be used with full understanding of all the elements (physics involved, assumptions made, parameters adopted, etc.), and not simply as black box. The Swiss model uses four exposure pathways in the dose calculation: external irradiation, surface irradiation, inhalation and ingestion. For all these pathways, the first assumption is to consider the Universe to be fully contaminated at the exemption limit. To be more realistic, a dilution of a factor 1000 has been introduced. The exemption limit in terms of absolute activity is deduced from the value associated with the specific activity. It is considered to be the activity of 1 kg of the substance 2

3 contaminated at the exemption limit of specific activity. Unlike the IAEA model that covers all substances, the Swiss model concerns non-gaseous substances only. Four radionuc1ide-dependent parameters are used to determine the dose from the activity through a simple relation: h& (msv h 1 GBq m2) : ambient equivalent dose rate in msv h -1, due to a 1 GBq source at a 10 distance of 1 m; h& (msv h 1 kbq cm2) : directional equivalent dose rate in msv h -1, due to a skin 0,07 contamination of 1 kbq cm -2 activity; (Sv Bq) : committed effective dose in Sv, due to the inhalation of 1 Bq activity; e inh (Sv Bq) : committed effective dose in Sv, due to the ingestion of 1 Bq activity; e ing The method to derive the exemption limit is analogous for the four pathways. For instance, in the case of external exposure, the dose equivalent rate is given by the following expression: & = A h& π (1) µ ρ H 10 For H & lim set by the dose criteria, the specific activity A corresponds to the exemption limit ext given in expression (2). The different pathways (or generic scenarios), the assumption made and the definition of the exemption limits used in the Swiss model are described in Table I. Table I. Exemptions limit for the four different pathways considered in the Swiss model. Pathway Assumptions Exemption Limits (Els) External exposure Surface exposure The individual is surrounded permanently by an infinite environment of density ρ, simulated by a sphere contaminated at a specific activity A. The individual is in permanent contact with a contaminated surface. The surface activity is assumed to be equivalent to the activity contained in a layer with a mass thickness ξ of 1 g cm -2 contaminated at specific activity A. ext = 1000 h& lim 10 µ ρ 4π µ : linear attenuation coefficient surf = 1000 h& lim 0,07 1 ξ 1 w skin w skin : skin risk factor (w skin =0.01) (2) (3) Inhalation Ingestion The individual breathes permanently an air loaded with a contaminated material in the form of dust at specific activity A. A dust concentration in air, C dust, for workers is assumed to be equal to the MAC value of kg m -3. The individual absorbs a quantity M & of 1000 kg y -1 of a substance at specific activity A. inh = 1000 e lim inh 1 V& MAC V & : breathing rate ( V & = 10 4 m 3 y -1 ) ing (4) = 1000 lim 1 e M& (5) ing The most restrictive of the four exposure pathways is selected to set the exemption limit. In other words, the minimum value is considered after evaluating the exemption limits of the different pathways: = min ( ext, surf, inh, ing ) (6) 3

4 2.3 The EU-IAEA model The model used for deriving the exemption limits adopted by the IAEA and published in a BSS report [4] is the same that has been developed jointly by IPSN, CEPN (France) and NRPB (UK), in the framework of the revised European Directive [3]. Exemption limits are derived in a conservative manner, using a series of selected limiting use and disposal scenarios. These scenarios, based on realistic assumptions agreed by the European experts, cover a wide range of situations: normal use and accidental irradiation, implying the workers and the population. For each scenario, a single or multiple exposure pathways are considered. Unlike the Swiss RPO approach where the same exposure pathways are considered for specific and absolute activities, the EU-IAEA model uses different scenarios for each of the two situations, and a total of twenty-four scenarios, shown in Table 2, nine are used to determine the exemption limit in terms of specifics activity (A.1 A.9) and fifteen are associated with absolute activities (B.1. B.15). The most restrictive scenario is then selected to set the exemption limit like in the Swiss model. The minimum value for abs or is considered. More details concerning the scenarios used in the EU-IAEA model to determine the exemption limits are available elsewhere [3, 4]. 3. Model comparison and discussion Ratios of the exemption limits recommended by the IAEA report and by the Swiss model are illustrated in Figure 1 for specific and absolute activities. For most radionuclides, the Swiss value is lower than the IAEA value, suggesting a higher conservatism with the Swiss model. (a) (b) FIG.1. Ratio of the exemption limit for (a) specific activities and (b) absolute activities, according to the Swiss model, considering the value given by the most restrictive pathway, and to the IAEA model. The major factor behind these large deviations between the results given by both models is the differences inherent to the scenarios considered. In order to analyze these results, a systematic comparison between each of the twenty-four IAEA scenarios and the corresponding Swiss scenarios (pathways) has been performed. The comparison focus on both the formalisms and the parameters used in dose calculation. Table II gives the overall ratios of the exemption limits between the IAEA and the Swiss models. The ratios of exemption limits are spread over a wide range of values. The exemption values established by the Swiss model are in general much lower than those determined by the IAEA. Among the twenty-four situations considered, only one case registered a 30% higher exemption limit of the Swiss model compared to the IAEA one. This concerns the inhalation of dust in the work place (IAEA scenario A.4.), in comparison with the Swiss inhalation pathway. For all the other situations, the Swiss exemption limits are well below the IAEA values. The extreme situation is the case of inhalation of dust in a landfill (IAEA scenario A.8.) compared to the Swiss inhalation pathway, which leads to an exemption limit twelve order of magnitude lower that the IAEA value. Both situations were not met for all the radionuclides studied in this work since the inhalation was never limiting in the Swiss model. 4

5 Table II. Comparison between the twenty-four EU-IAEA scenarios and the corresponding Swiss pathways used for the determination of exemption limits. EU-IAEA Scenario Corresponding pathway in the Swiss model Swiss / EU-IAEA or abs ratio A.1. External exposure from handling a source Surface exposure 10-2 A.2. External exposure from 1 m 3 source External exposure A.3. External exposure from a gas bottle External exposure (2-4) 10-2 A.4. Inhalation of dust Inhalation 1.33 A.5. Ingestion from contaminated hands Ingestion A.6. External exposure from wearing a ring External exposure 10-4 A.7. External exposure from a landfill site External exposure A.8. Inhalation of dust from a landfill site Inhalation A.9. Ingestion of an object from a landfill site Ingestion 10-3 B.1. External exposure from a point source External exposure ( ) 10-3 B.2. External exposure from handling a source Surface exposure 0.1 B.3. External exposure from contaminated hands (spillage) Surface exposure B.4. External exposure from contaminated face (spillage) Surface exposure B.5. External exposure from contaminated surface (spillage) External exposure B.6. Ingestion from hands (spillage) Ingestion 10-7 B.7. Inhalation of resuspended activity (spillage) Inhalation B.8. External dose from an aerosol or dust cloud (spillage) External + surface exposure / B.9. Contamination of skin (fire) Surface exposure B.10. Inhalation of dust or volatiles Inhalation 0.83 B.11. External dose from combustion products (fire) External + surface exposure / B.12. External exposure from a landfill site External exposure (0.25-1) 10-8 B.13. Inhalation of dust from a landfill site Inhalation 0.17 B.14. External exposure to skin from handling an object from a landfill site Surface exposure 10-3 B.15. Ingestion of an object from a landfill site Ingestion 10-3 However, the results in Table II are not sufficient to understand the comparisons in Figure 1. In fact, the limiting scenarios in the IAEA and the Swiss models do not necessarily respect the correspondence shown in Table II. For a number of radionuclides, the exemption limit is determined by a given pathway in the IAEA model, but set by a different one in the Swiss model. Table III illustrates for a few widely used radionuclides the situation where the limiting pathways for both models are different. The Swiss model is highly restrictive because of the parameters used and the assumptions considered in the scenarios. For instance, unlike the IAEA model that often introduces occurrence probabilities of an exposure (in the case of an accidental situation), the Swiss approach is more deterministic. Moreover, the Swiss model is generally extremist, even with a dilution factor of This extremist approach is particularly significant in the ingestion pathway where the ingestion quantities considered are enormous. In the case of radionuclides considered, the inhalation pathway never gave the most restrictive result and thus never determined the exemption limit. The first apparent reason is that the study covered exclusively non-gaseous substances. Secondly, the parameters used in the ingestion pathway lead to more restriction than the inhalation does. The main pathways of the Swiss model are external exposure and ingestion. 5

6 Table III. Examples of radionuclides for which the limiting pathways in the Swiss and IAEA models are different. Radionuclides or abs Swiss model IAEA P-32 Ingestion External abs Ingestion Skin Cr-51 abs External Skin Co-60 abs External Skin Sr-90 Ingestion External I-125 Ingestion External I-131 Ingestion External U-235 Ingestion Inhalation Pu-239 abs Ingestion Inhalation 4. Conclusion The IAEA and Swiss models used for the determination of the exemption limits were studied and compared. Both models adopt the same concept of exemption which designates the activity level of no interest for the Regulatory Authority and which is out of the scope of the Regulatory provisions in radiation protection. The Swiss model is characterized by three main properties, generality, simplicity and transparency, and is based on four basic exposure pathways, external exposure, surface exposure, inhalation and ingestion. The exemption limit is given by the most restrictive pathway. The IAEA model is based on twenty-four detail imaginary specific scenarios describing with high accuracy realistic situations that can lead to an exposure. Unlike the Swiss approach, the IAEA model studies separately exemption limits associated with specific activity and absolute activity using different scenarios. Only a small number of the IAEA scenarios proved to be of importance in determining the exemption limits: four in the case of specific activity and six in the case of absolute activity. The comparison of the four Swiss general scenarios with the twenty-four IAEA detailed scenarios revealed that the Swiss model is more conservative. In the case of specific activity, an overall factor of 5 difference was found for radionuclides for which external exposure was the limiting pathway in both models, a factor of 1000 when ingestion was the limiting pathway in both models, and a factor between 5 and 1000 in the cross-cases. For absolute activity, an overall factor of difference was found in the case of radionuclides for which external exposure was the limiting pathway in both models, a factor of 1000 for radionuclides for which ingestion was the limiting pathway in both models, and a factor between 10 and 1000 in the cross-cases. 6

7 References [1] A. Gonzales, G. Linsley, in Proceeding of the 1996 International Congress on Radiation Protection, IRPA9, Vienna, 1996, p [2] Swiss Radiological Protection Ordinance (RPO), edited by the Federal Chancellery, Bern, (1994). [3] Commission of the European Communities, Principles and Methods for Establishing Concentrations and Quantities (Exemption Values) below which Reporting is not Required in the European Directive, Radiation Protection 65, Doc. XI-028/93, CEC, Luxembourg (1993). [4] International Atomic Energy Agency, International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Source, Safety Series No. 115, IAEA, Vienna (1996). [5] International Commission on Radiological Protection, 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, Pergamon Press, Oxford and New York (1990). [6] International Atomic Energy Agency, Principles for the Exemption of Radioactive Sources and Practices from Regulatory Control, Safety Series No. 89, IAEA, Vienna (1988). 7