Radiological Emergencies from the Malevolent Use of Radiation Sources. A. Rogani and P. Zeppa ^

Transcription

1 Radiological Emergencies from the Malevolent Use of Radiation Sources A. Rogani and P. Zeppa ^ National Institute of Health, Viale Regina Elena 299, Rome, Italy rogani@iss.it ^Agency for the Environmental Protection and the Technical Services, Via V. Brancati, 48, Roma, Italy zeppa@apat.it Abstract. A National Emergency Plan was set up in Italy after the Chernobyl accident, to cope with the following nuclear risk sources: accidents in foreign nuclear power plants or aboard nuclear- propelled ships, falling of satellites using nuclear power units, transportation of radioactive materials. Moreover, this plan gives the general requirements of the emergency preparedness and response to be implemented into the local planning (Provincial Emergency Plan), on the basis of reference accidents for research reactors, nuclear power and reprocessing plants (shut down) in Italy. However in the National Emergency Plan are not considered, neither for local scale nor for the national one, the accidents with radiation sources, used for a wide variety of purposes in industry, medicine and research. Besides, after the events of 11 September 2001, it is raised the awareness of the risks created by radiation sources, because of their potential use for malevolent purposes (exposure of public to radiation or use of a source in a radiological dispersion device, commonly referred to as dirty bomb ). Moreover, for a terrorist action, the personal danger from handling powerful radiation sources can not be seen as an effective deterrent. It is clear that a re-evaluation of risks involved and a planning of the interventions to protect the public is needed as also stated by the requirements set up in the new standard proposed by the IAEA. In this paper the radiological consequences of radioactivity dispersion by using conventional explosive have been evaluated for the main applications of radioactive sources. 1. Introduction After the attack on September 11, 2001, there is justified concern that a malevolent action may involve radioactive material to cause an environmental contamination, harm to human health and psychological effects. The hypothetical scenarios to achieved these objectives can be: 1) Attack on a nuclear facility (e.g. by using a fully fuelled large aircraft) or during a transport of radioactive material. 2) Nuclear weapons 3) Dispersion of radioactive material into the environment by using conventional explosive (dirty bomb) or other mechanisms (e.g. aerosol generator). Regarding the scenarios 1) and 2), an attack on a nuclear facilities requires a terrorist organization with a certain degree of sophistication, and a nuclear explosion is considered less likely than the dirty bomb scenario, in view of the many difficulties involved in acquiring or building a nuclear weapon. The scenario 3) is considered the most probable, because there are a variety of locations world-wide where radioactive sources are in use, such as medical institutions, research centres and industries, for which there is the concrete probability for a terrorist to obtain access. The use of small sources of low activity could involve a limited area and primarily result in a psychosocial impact, whereas a larger impact could be induced by combining explosives with radioactive material, which can be related to orphaned sources, to illicit trafficking or to malevolent actions. This paper doesn t deal on how to prevent the risks connected with this type of events, but provides an evaluation of risks related to radioactive material dispersion for a forthcoming revision of Italian National Emergency Plan. Radiological consequences for dirty bomb scenario involving commonly used radioactive sources are evaluated as function of distance from the source. Moreover values of source activity, corresponding to the intervention levels of dose for the implementation of protective measures recommended by International Commission on Radiological Protection (ICRP) [1], have been calculated. 1

2 2. Categorization of radioactive sources After 11 September attack the International Agency of Atomic Energy (IAEA) increased its efforts to prevent radioactive sources from becoming tools of malevolent actions. Because the sources used worldwide are several millio ns, the security measures should be directed at those sources that pose the greatest risks. For this purpose, in October 2003, the IAEA has developed a new categorization system of radioactive sources [2], to ensure that the sources are maintained under a control commensurate with the radiological risks. This categorization system is based on the potential for radioactive sources to cause deterministic effects. The category of each source has been determined by dividing the activity of the radionuclide by a numerical factor D, which corresponds to the activity that could cause severe deterministic effects for given scenarios, including malevolent actions. Taking into account the values of ratio A/D, a five category system of practices has been defined, from A/D = Exemption level/d to A/D = The values A/D = 1 correspond to dangerous sources, that is to sources that could, if not under control, give to exposure sufficient to cause severe deterministic effects [3] (see figure 1). The category assigned to each practice also takes into consideration other factors, such as the mobility of source. In the IAEA categorization system the stochastic effects are excluded, because for dangerous sources these effects are likely overshadowed by deterministic effects. 1.E+06 1.E+05 Activity (TBq) 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01 Sr-90 Pu -238 Ir-192 Se-75 Yb-169 Tm-170 Ir-192 Cf-252 Cf-252 Am-241/Be 1.E-02 1.E-03 Irradiators Teletherapy RTGs Industrial Radiography Brachitherapy HDR/MDR Industrial Gauges Well Logging CATEGORY 1 (A/D 10 3 ) CATEGORY 2 (10 3 > A/D 10) CATEGORY 3 (10 > A/D 1) FIG.1. Activity ranges of dangerous source applications and their categorization [2] 3. Applications of concern of the radioactive sources The main applications using radioactive sources that could cause health impact are: industrial irradiators, industrial radiography, well logging, industrial gauges, moisture detectors and radiotherapy (teletherapy units and brachytherapy). Due their widespread use, the following radionuclides result of particular concern. sources (from some GBq to few hundreds of PBq) are used for gauging, radiography, irradiation, calibration and radiotherapy. 2

3 sources (from few hundreds of MBq to a few hundreds of PBq) are used for gauging, irradiation, moisture detecting, well logging, calibration and radiotherapy. Am-241 or Am-241/Be sources (from some tens of MBq to 1.0 TBq) are used predominantly for industrial applications. Ir-192 sources (from 1.0 GBq to some TBq) are used for industrial radiography and radiotherapy. Sr-90 sources (from 0.1 GBq to 7.0 GBq) are used for gauging and brachitherapy. Pm-147, Po-210 and Cf-252 (from 1.0 to 4.0 GBq) are used predominantly for industrial applications. Pd-103, I-125, I-131, Gd-153, Au-198, Ra-226 (from 0.1 GBq to 60 GBq) are used in medicine (brachitherapy and bone densitometry). Sources with high risk of being diverted to make and use a dirty bomb are primarily the sources accessible, transportable and safely handy. The facilities where the irradiators and the teletheraphy units are located, have thick walls and are generally provided with security systems. Then an illicit acquisition of radioactive sources used in these applications to make a dirty bomb, appear to be unlikely. Moreover in some industrial irradiators (sterilization and food preservation) the activity of radioactive sources is too high (typically 10 2 PBq) to be considered for criminal diversion, because the handling of these sources would cause almost instantaneously the death. Therefore only a small fraction of all sources could be used to disperse radioactive material. The sources used for industrial radiography, well logging, moisture/density detector and brachytherapy are portable and generally small in term of physical size, so they appear as candidate for their malevolent use. 4. Radiological risk assessment The radiological risk has been assessed for a scenario of a terrorist attack involving conventional explosive to disperse radioactive material. The size of the contaminated area and the health impact depend primarily on the amount of explosive, the type of radioactive material used, the meteorological conditions at the time of the detonation and the characteristics of the environment where the radioactivity dispersion occurs. The principal exposure pathways will be the external irradiation and the inhalation. The radiological consequences of the reference scenario have been evaluated by means of a Gaussian model with the following assumptions (inter alia). The release is assumed to be impulsive and at ground level. The F Pasquill-Gifford diffusion category with a wind speed of 2 m/s is considered. Dry deposition is assumed to occur along the cloud path. Following the blast a 100% of airborne release fraction of source activity as respirable fraction is assumed. The relevant exposure paths in the early emergency phase - external irradiation from the cloud and inhalation - are taken into account. The most exposed age classes (adult and children) are considered. For each application, excluding the irradiators and the teletheraphy, and for each radionuclide used within the selected applications, the maximum activity, A M [2, 4], was divided by a numerical factor. For emergency planning of interventions to reduce the stochastic effects, this factor (referred to as A IL ) is given in terms of the activity which corresponds to lower value of the ranges of dose intervention levels recommended by ICRP for countermeasures implementation. The values of A IL, corresponding to an effective dose equal to 5 msv by inhalation at various distances from release point (from 10 m to 1000 m), have been assessed. The external irradiation from the plume resulted insignificant in comparison with the inhalation. In table I the values of A IL at 50 m from the explosion for the radionuclides here considered are reported. The distance of 50 m has been selected because it is assumed that in the short distances the explosion effects are prevailing. In table I some characteristics of the radioactive sources, such as their portability, and the most exposed age class are also reported. 3

4 Table I. Activity, A IL, corresponding to an effective dose by inhalation equal to 5 msv at a distance of 50 m from the explosion Radionuclide A IL (GBq) Critical group Application 8.7 Adult Industrial gauge Industrial radiography HDR / MDR brachytherapy * Sr Adult Industrial gauge LDR brachytherapy ** Pd Child LDR brachytherapy** I Child Bone densitometry LDR brachytherapy** I Child LDR brachytherapy** 6.9 Adult Industrial detector Well logging Industrial gauge LDR brachytherapy** HDR/MDR brachytherapy* Pm Adult Industrial gauge Gd Child Bone densitometry Ir Adult Industrial radiography LDR brachytherapy** HDR/MDR brachytherapy* Au Child LDR brachytherapy** Po Adult Static electricity eliminator Ra Adult LDR brachytherapy** Am-241/Be Adult Industrial detector Well logging Am Adult Bone densitometry Static electricity eliminator Lightning preventers Industrial gauge Cf Child LDR Brachytherapy** Well logging * HDR High Dose Rate, MDR Medium Dose Rate, ** LDR Low Dose Rate 4

5 5. Ranking of the sources related to protective measures implementation The A M /A IL ratios give a numerical ranking of the radioactive sources; indeed, on the basis of values of R= A M /A IL it is possible to divide the source applications into four classes (figure 2). 1.E+06 1.E+05 1.E+04 R = E+03 1.E+02 R = AM / AIL 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 1= R < 10 R < 1 10 = R < E-04 FIG. 2. Ranking of radioactive sources related to protective measures According to ICRP dose intervention levels (table II) the values of activity ratio R < 1 correspond to sources that, in the event here considered, should not require protective measures. For 1= R < 10 the sheltering could be adopted, while for 10 = R < 100 the sheltering would be warranted and the evacuation could be adopted. For R = 100 the evacuation would be warranted to avoid deterministic effects. Table II. Intervention Levels of dose for implementation of protective measures [1]. Protective measure Effective dose Intevention level / msv Equivalent dose Evacuation 50*- 500** Sheltering 5*-50** Iodine prophylaxis 50*-500** Relocation 10 msv per month for prolonged exposure * Level of dose below which the introduction of the countermeasure is not considered justified. ** Level of dose at which the implementation of countermeasure will almost always be justified. 5

6 6. Emergency planning: some considerations Due to the widespread use of radioactive sources, it is quite impossible to preclude their employment in a radiological dispersion device (RDD). The increase of the security level for the higher-risk sources should be considered and specific emergency preparedness in response to such events be implemented [5, 6]. The characteristics of a terrorist attack and the evolution of a dirty bomb event appear to be quite different compared to nuclear accidents, on which current emergency plans are generally based, and a suitable response would be warranted by developing specific emergency arrangements. An urban densely populated area could represent the scene of the event, and no prerelease phase (time between the beginning of an accident and the start of the release) can be planned to starting up in advance protective measures averting doses to the involved population. Finally, the presence of radioactive material may be unknown and the failure of the early recognition of a radiological risk could preclude the timely implementation of the countermeasures. Following an event of explosion of unknown origin, the intervention of well trained first responding team would be of fundamental importance to detect radioactive contamination and delimit the affected area. In fact, for emergency response purposes the identification of the extension of the impact area represents a critical task. The distances within which the effective doses exceed the intervention levels (table II) have been evaluated for the maximum activities of the applications of concern (R = 1). In figure 3 the maximum distances which the different countermeasures could be extended to, are shown > 5mSv > 50mSv > 500mSv Distance (m) Industrial Gauges Industrial Radiography HDR/MDR Brachytherapy Sr-90 Industrial Gauges Well Logging Industrial Gauges LDR Brachytherapy HDR/MDR Brachytherapy Ir-192 Industrial Radiography Ir-192 HDR/MDR Brachytherapy Po-210 Static Electricity eliminator Ra-226 LDR Brachytherapy Am-241/Be Industrial Detectors Am-241/Be Well Logging Am-241 Bone Densitometry Am-241 Industrial Application Cf-252 LDR Brachytherapy Cf-252 Well Logging FIG. 3. Emergency protective action distances 6

7 Finally in figure 4 the maximum distances for relocation intervention, according to ICRP criteria (table II), are reported for the most significant radionuclides. The calculations have been carried out taking into account the exposure both from ground irradiation and from inhalation of resuspended radionuclides Distance (m) Industrial Gauges Industrial Radiography HDR/MDR Brachytherapy Well Logging Industrial Gauges LDR Brachytherapy HDR/MDR Brachytherapy Am-241/Be Industrial Detectors Am-241/Be Well Logging Am-241 Bone Densitometry Am-241 Industrial Application FIG. 4. Distances of relocation implementation 7. Concluding remarks The radiological impact of an explosive RDD comes essentially from inhalation both of material dispersed in the environment and of resuspended alpha emitting radionuclides. The evaluation of inhalation doses for the most exposed age class has allowed obtaining a numerical ranking of source applications related to dose levels for emergency interventions to reduce the stochastic effects. The most disruptive actions (evacuation or relocation) should be adopted only for a limited number of sources. In particular, excluding only a few applications, the evacuation should be limited within some hundreds of meters, whereas the relocation should be confined within 1 km. For a better estimation a specific dispersion model should be developed describing the radioactivity diffusion and ground deposition in a complex environment like an urban area. Besides the possible presence of high activity debris due to an incomplete aerosol production of the radioactive material during the blast should be considered and related to the potential deterministic effects both for the population and for the first responder teams. The evaluations are referred to the worst case. Indeed an airborne release fraction (ARF) and a respirable fraction (RF) of 100% have been assumed. Moreover the F atmospheric diffusion category is assumed as precautionary measure. As an example, if the most probable neutral atmospheric condition and a faster wind speed are assumed, the protective actions should be extended to distances about 4 times lower. Then an accurate knowledge of weather conditions play an important role in the emergency response. 7

8 Reference 1. International Commission on Radiological Protection, Principles for Intervention for Protection of the Public in a Radiological Emergency, Publication 63, Annals of ICRP 22, No. 4, Pergamon Press, Oxford and New York (1993). 2. International Atomic Energy Agency, Categorization of radioactive sources, IAEA-TECDOC- 1344, Vienna (2003). 3. International Atomic Energy Agency, Preparedness and Response for a Nuclear or Radiological Emergency, Safety Standards Series No. GS-R-2, IAEA, Vienna (2002). 4. International Atomic Energy Agency, Generic procedures for assessment and response during a radiological emergency, IAEA-TECDOC-1162, Vienna (2000). 5. International Atomic Energy Agency, Security of radioactive sources, IAEA-TECDOC-1355, Vienna (2003). 6. International Atomic Energy Agency, Method for Developing Arrangements for Response to Nuclear or Radiological Emergency Updating IAEA-TECDOC-953, IAEA-EPR-METHOD 2003, Vienna (2003). 8