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Categorisation of radiological emergencies for planning intervention
PRINCIPLES
FOR WERVENTION
FOR PROTECTION
OF THE PUBLIC
7
include multi-attribute utility or multi-criteria outranking analysis, cost-benefit and extended cost-benefit analysis. The principal merit of these techniques is to help to rational& the decision process by making explicit all the factors involved and allowing an appreciation of their relative importance. (32) One approach to deriving intervention levels in quantitative terms by the processes of justification and optimisation is to place a monetary value on averted dose. Several methods have been developed for assessing the cost for society of loss of life expectancy; these are reviewed in ICRPPublications37 and 55 (1983,1989). One method commonly used in the past, the “human capital” approach, has been criticised as being irrelevant because most people value safety by their aversion to the prospect of their own death rather than because of concern for their future contribution to the national economy. Another method which is becoming more widely preferred because of its greater relevance and validity is the “willingness to pay” approach, based on direct interviews with a representative sample of the general public. The International Atomic Energy Agency has published a recommendation for assessing such a value for transboundary radiation exposure (IAEA 1985b). (33) It is of great importance that decision makers inform the public of all aspects of their decisions, especially when the interventions are chosen mainly for political, social and/or economic reasons rather than health protection grounds. Otherwise the public may be misled and the radiological protection efforts will be mistrusted.
4. CATEGORISATION OF RADIOLOGICAL EMERGENCIES FOR PLANNING INTERVENTION (34) The basic principles apply to all types of radiological emergencies. However, there will be differences in their time of application and choice of interventions depending upon the type of accident. For example radiological emergencies involving lost or damaged sources may occur at sites that cannot be identified in advance and it may be difficult to identify the possibly affected area. (35) Different protective actions will be applicable depending on the nature of the accident, its duration in time and the area affected. In planning and implementing intervention, each protective action needs to be considered in turn; the advantages and disadvantages of each are analysed. The results from such analysis should then be used to develop a strategy for intervention which may involve more than one protective action. 4.1. Types of Radiological Emergencies (36) There is a variety of conceivable accidents to be considered in the preparation of emergency plans: those occurring at nuclear facilities, for example power reactors and other fuel cycle facilities involving criticality or chemical reactions and release of radioactive materials; and those occurring at radiological facilities or involving other radiation sources, such as from the medical, industrial and commercial use of radionuclides. Accidents in the transportation of radioactive materials may also lead to the release of radionuclides to the environment. (37) Severe accidents at nuclear facilities can result in abnormal on-site exposures, containment failure and release of radioactive materials to the atmosphere or directly to surface or subsurface waters. The risks of exposure of the general public and workers from releases to the atmosphere are generally considered to be greater than those from releases to the hydrosphere because of the higher doses resulting and shorter times needed to transfer radionuclides to man and because of the relatively greater difficulty in implementing
8
REPORT OF A TASK GROUP OF COMMITTEE 4
appropriate protective measures. Nevertheless, the exposure pathways resulting from accidental discharges to the hydrosphere need to be considered in the preparation of emergency response plans, although at a lesser degree of detail than for atmospheric pathways. (38) Other radiological emergencies can result if radioisotope sources are mishandled, inappropriately released to public access, lost or have their containment, seal or shielding integrity damaged during their production, transport or use. Of the serious radiation accidents reported over the past 45 years, about two-thirds of the events involved non-nuclear facilities (industry, research, medical) (IAEA, 1988). The spatial and temporal extent of such accidents generally will be limited relative to a major accident occurring at nuclear facilities; however, experience has indicated that such accidents may result in significant radiation doses to members of the public, occasionally leading to early deaths. (39) No single accident type or sequence of events can be used as a basis for development of emergency response plans. In the case of nuclear facilities, the type of plant and its potential for release of different radionuclides will influence the emergency response plan developed specificallyfor that plant and its site. The off-siteconsequences of the range of predicted accident sequences can form a basis on which detailed emergency plans are prepared. Accidents at certain nuclear fuel cycle facilities may have as their main consequence the release of materials which are chemically toxic and for which the radiological contribution to the hazard is minimal. Response plans for radiological emergencies involving lost sources, or involving transport of radioactive materials, will need to have broad applicability as the site at which the accident may occur will not be known in advance.
4.2. Exposure Pathways (40) The consequences of a radiological emergency can vary considerably in magnitude, being dependent upon such factors as the particular type and nature of the accident, the total amount of radioactive material and the different radionuclides involved, the energy with which they are dispersed into the environment, the nature of the surrounding environment and the mechanisms of radionuclide dispersion and transfer (Alpert et al., 1986;USNRC, 1975).In most cases, there will be a limited set of exposure routes requiring consideration. The relative importance of each potential route of exposure to the total accidental exposure can vary, being affected in particular by the type of accident and by its temporal and spatial characteristics. These aspects need to be considered in the development of emergency response plans and the implementation of protective actions. (41) A severe accident at a nuclear facility, for example, can lead to the atmospheric release of noble gases, radioiodines and/or particulate fission and activation products. Gamma radiation from radioactive material in the plume can result in whole-body exposures; inhalation of radionuclides in the plume will result in internal irradiation of organs and tissues. The change in dose rates with time after the release will depend on the particular composition of radionuclides in the release and the meteorological conditions at the site. Other radiological emergencies typically involve a single radionuclide, such as cobalt-60, caesium-137 or iridium-192. The physical and chemical properties of the particular radioactive source is an important factor in the dose received and its distribution in the body. (42) When planning for emergency response, it is important to identify possible exposure pathways and to evaluate their relative importance. Different protective actions may need to be implemented to avoid or reduce the radiation dose, depending particularly upon the pathway by which exposure is liable to occur, as well as upon the body organs or tissues which are liable
PRINCIPLES FOR INTERVENTION FOR PROTECTION OF THE PUBLIC
9
to be irradiated and the dose that is projected to be received. These protective actions are discussed in Section 5. (43) Exposure of the public and workers to radiation following an accident may be either external or internal, or both and may be incurred by various pathways (see Table 1). External exposures can result directly from the source or facility and from radionuclides contained in releases from the source (for example, airborne plumes), deposited onto surfaces, or transferred from one surface to another through contamination of skin and clothing. Internal exposures can result from inhalation of radionuclides in the plume or of resuspended radionuclides and from ingestion of activity through the consumption of contaminated food and water or by contact with contaminated materials. Internal exposures may also result from absorption of radionuclides through skin or wounds. The overall risk to the individual represents the summation of effects of external and internal exposures.
Table 1. Principal exposure pathways of relevance to radiological emergencies External exposure from:
source or facility Plume Radionuclide contamination on surfaces Radionuclide contamination of skin and clothing
Internal exposure from:
Inhalation of radionuclides in plume Inhalation of resuspended radionuclides Ingestion of contaminated food and water Ingestion of radionuclides from wntaminated materials Absorption through skin and wounds
(44) Some members of the public who are not involved in an accident or recovery operations, but whose normal employment is undertaken in an area contaminated by the accident require special consideration. They may be exposed via pathways which are additional to those normally considered for the public. General classes of such individuals include farmers on contaminated land, fishermen in contaminated waters and workers in vital industries in contaminated areas, but other groups must be considered such as those involved in waste treatment or in handling contaminated items such as air filters. (45) The consequences of an accidental release of radionuclides will also be influenced by the particular characteristics of the affected environment. For example, the external radiation dose can vary considerably between urban and rural environments, depending on such factors as the type and amount of deposition, the surface characteristics and the land use and living habits of the exposed population. Seasonal variations in environmental conditions (for example, crop growth stage, presence of snow cover, availability of pasture) can also influence the levels of contamination in agricultural produce in areas where deposition of radionuclides has occurred, thus strongly influencing internal exposures resulting from ingestion of contaminated food (NEA, 1989a, 1991; WHO 1987).
43. Temporal and Spatial Aspects of Radiological Emergencies (46) Categorisation of radiological emergencies in terms of their temporal and spatial aspects can be useful in the development of appropriate emergency response plans. Routes of exposure
10
REPORT OF A TASK GROUP OF COMMITTEE 4
may differ at various times in an accident sequence, such differences most likely requiring the
implementation of different protective actions. For example, external exposure to and inhalation from, a radioactive plume can be of more immediate significance than would be external exposure to deposited radionuclides and ingestion of contaminated food. This is due principally to the shorter times involved in the possibility of exposure from the plume and thus the need for making urgent decisions on implementing protective actions. The spatial extent of contamination will be determined principally by the natural processes such as wind and rain which control the transfer of radionuclides through the environment. Variability inherent in these processes can, therefore, result in heterogeneous spatial patterns of deposition and radionuclide concentrations, which must be considered for emergency response plating purposes. While an understanding of the temporal and spatial aspects of radiological emergencies is useful for planning purposes, these aspects are only one input (NEA 1989b, 1990). (47) The following time stages can be considered for the purpose of planning intervention for a radiological emergency: a pre-release stage, a release stage and a post-release stage. The prerelease stage is that period from the time when potential or actual accidental exposure is recognised, to the time when significant amounts of radioactive material are released or the source is brought under control. During this stage and in the tist few hours of the release stage, urgent decisions on measures to avert doses to the public and workers may be necessary. These decisions can be based on scenarios which have been identified in advance concerning the occurrence of abnormal events and source conditions. Initial results of environmental monitoring are unlikely to be available to aid decisions and the prediction of future developments may be subject to substantial uncertainties. For this reason, emergency response plans need to include procedures for implementing protective measures which are based on information about the condition of the facility or source, any measurements of released material and meteorological conditions and the possible pathways for exposure. If the pre-release stage is very short, only limited off-site actions may be feasible before the commencement of the release. (48) For extended releases, an immediate concern is exposures arising from passage of an airborne plume; subsequent concerns will include exposures arising from deposited radionuclides. The results of environmental monitoring should become available during or shortly after the release stage to assist in making appropriate decisions on the implementation of protective actions. (49) In the post-release stage there will be both decisions made concerning the implementation of further protective actions and about the return of normal living conditions. This stage may extend over a prolonged period of months or years. (50) The major routes of exposures which exist in the post-release stage will need to be reassessed at regular intervals to determine if continuation of protective actions previously introduced is still justified. (51) In the post-release stage, there are a number of social, economic and technical factors which are more important in making decisions than in the earlier stages. These factors depend upon the particular spatial and temporal aspects of the accident. The nature of the land use and
living habits in the area affected, the size of the population evacuated or relocated, the time of year, the ease of decontamination and the perceptions and attitudes of the population in returning to their homes are some of the factors that must be considered and balanced in the decision-making process. A process of optimisation should be used for the periodic review for the continuation of protective actions when specific information on the nature and consequences of the accident is available. Further discussion of these aspects is provided in Section 5.
FOR WERVENTION
FOR PROTECTION
OF THE PUBLIC
7
include multi-attribute utility or multi-criteria outranking analysis, cost-benefit and extended cost-benefit analysis. The principal merit of these techniques is to help to rational& the decision process by making explicit all the factors involved and allowing an appreciation of their relative importance. (32) One approach to deriving intervention levels in quantitative terms by the processes of justification and optimisation is to place a monetary value on averted dose. Several methods have been developed for assessing the cost for society of loss of life expectancy; these are reviewed in ICRPPublications37 and 55 (1983,1989). One method commonly used in the past, the “human capital” approach, has been criticised as being irrelevant because most people value safety by their aversion to the prospect of their own death rather than because of concern for their future contribution to the national economy. Another method which is becoming more widely preferred because of its greater relevance and validity is the “willingness to pay” approach, based on direct interviews with a representative sample of the general public. The International Atomic Energy Agency has published a recommendation for assessing such a value for transboundary radiation exposure (IAEA 1985b). (33) It is of great importance that decision makers inform the public of all aspects of their decisions, especially when the interventions are chosen mainly for political, social and/or economic reasons rather than health protection grounds. Otherwise the public may be misled and the radiological protection efforts will be mistrusted.
4. CATEGORISATION OF RADIOLOGICAL EMERGENCIES FOR PLANNING INTERVENTION (34) The basic principles apply to all types of radiological emergencies. However, there will be differences in their time of application and choice of interventions depending upon the type of accident. For example radiological emergencies involving lost or damaged sources may occur at sites that cannot be identified in advance and it may be difficult to identify the possibly affected area. (35) Different protective actions will be applicable depending on the nature of the accident, its duration in time and the area affected. In planning and implementing intervention, each protective action needs to be considered in turn; the advantages and disadvantages of each are analysed. The results from such analysis should then be used to develop a strategy for intervention which may involve more than one protective action. 4.1. Types of Radiological Emergencies (36) There is a variety of conceivable accidents to be considered in the preparation of emergency plans: those occurring at nuclear facilities, for example power reactors and other fuel cycle facilities involving criticality or chemical reactions and release of radioactive materials; and those occurring at radiological facilities or involving other radiation sources, such as from the medical, industrial and commercial use of radionuclides. Accidents in the transportation of radioactive materials may also lead to the release of radionuclides to the environment. (37) Severe accidents at nuclear facilities can result in abnormal on-site exposures, containment failure and release of radioactive materials to the atmosphere or directly to surface or subsurface waters. The risks of exposure of the general public and workers from releases to the atmosphere are generally considered to be greater than those from releases to the hydrosphere because of the higher doses resulting and shorter times needed to transfer radionuclides to man and because of the relatively greater difficulty in implementing
8
REPORT OF A TASK GROUP OF COMMITTEE 4
appropriate protective measures. Nevertheless, the exposure pathways resulting from accidental discharges to the hydrosphere need to be considered in the preparation of emergency response plans, although at a lesser degree of detail than for atmospheric pathways. (38) Other radiological emergencies can result if radioisotope sources are mishandled, inappropriately released to public access, lost or have their containment, seal or shielding integrity damaged during their production, transport or use. Of the serious radiation accidents reported over the past 45 years, about two-thirds of the events involved non-nuclear facilities (industry, research, medical) (IAEA, 1988). The spatial and temporal extent of such accidents generally will be limited relative to a major accident occurring at nuclear facilities; however, experience has indicated that such accidents may result in significant radiation doses to members of the public, occasionally leading to early deaths. (39) No single accident type or sequence of events can be used as a basis for development of emergency response plans. In the case of nuclear facilities, the type of plant and its potential for release of different radionuclides will influence the emergency response plan developed specificallyfor that plant and its site. The off-siteconsequences of the range of predicted accident sequences can form a basis on which detailed emergency plans are prepared. Accidents at certain nuclear fuel cycle facilities may have as their main consequence the release of materials which are chemically toxic and for which the radiological contribution to the hazard is minimal. Response plans for radiological emergencies involving lost sources, or involving transport of radioactive materials, will need to have broad applicability as the site at which the accident may occur will not be known in advance.
4.2. Exposure Pathways (40) The consequences of a radiological emergency can vary considerably in magnitude, being dependent upon such factors as the particular type and nature of the accident, the total amount of radioactive material and the different radionuclides involved, the energy with which they are dispersed into the environment, the nature of the surrounding environment and the mechanisms of radionuclide dispersion and transfer (Alpert et al., 1986;USNRC, 1975).In most cases, there will be a limited set of exposure routes requiring consideration. The relative importance of each potential route of exposure to the total accidental exposure can vary, being affected in particular by the type of accident and by its temporal and spatial characteristics. These aspects need to be considered in the development of emergency response plans and the implementation of protective actions. (41) A severe accident at a nuclear facility, for example, can lead to the atmospheric release of noble gases, radioiodines and/or particulate fission and activation products. Gamma radiation from radioactive material in the plume can result in whole-body exposures; inhalation of radionuclides in the plume will result in internal irradiation of organs and tissues. The change in dose rates with time after the release will depend on the particular composition of radionuclides in the release and the meteorological conditions at the site. Other radiological emergencies typically involve a single radionuclide, such as cobalt-60, caesium-137 or iridium-192. The physical and chemical properties of the particular radioactive source is an important factor in the dose received and its distribution in the body. (42) When planning for emergency response, it is important to identify possible exposure pathways and to evaluate their relative importance. Different protective actions may need to be implemented to avoid or reduce the radiation dose, depending particularly upon the pathway by which exposure is liable to occur, as well as upon the body organs or tissues which are liable
PRINCIPLES FOR INTERVENTION FOR PROTECTION OF THE PUBLIC
9
to be irradiated and the dose that is projected to be received. These protective actions are discussed in Section 5. (43) Exposure of the public and workers to radiation following an accident may be either external or internal, or both and may be incurred by various pathways (see Table 1). External exposures can result directly from the source or facility and from radionuclides contained in releases from the source (for example, airborne plumes), deposited onto surfaces, or transferred from one surface to another through contamination of skin and clothing. Internal exposures can result from inhalation of radionuclides in the plume or of resuspended radionuclides and from ingestion of activity through the consumption of contaminated food and water or by contact with contaminated materials. Internal exposures may also result from absorption of radionuclides through skin or wounds. The overall risk to the individual represents the summation of effects of external and internal exposures.
Table 1. Principal exposure pathways of relevance to radiological emergencies External exposure from:
source or facility Plume Radionuclide contamination on surfaces Radionuclide contamination of skin and clothing
Internal exposure from:
Inhalation of radionuclides in plume Inhalation of resuspended radionuclides Ingestion of contaminated food and water Ingestion of radionuclides from wntaminated materials Absorption through skin and wounds
(44) Some members of the public who are not involved in an accident or recovery operations, but whose normal employment is undertaken in an area contaminated by the accident require special consideration. They may be exposed via pathways which are additional to those normally considered for the public. General classes of such individuals include farmers on contaminated land, fishermen in contaminated waters and workers in vital industries in contaminated areas, but other groups must be considered such as those involved in waste treatment or in handling contaminated items such as air filters. (45) The consequences of an accidental release of radionuclides will also be influenced by the particular characteristics of the affected environment. For example, the external radiation dose can vary considerably between urban and rural environments, depending on such factors as the type and amount of deposition, the surface characteristics and the land use and living habits of the exposed population. Seasonal variations in environmental conditions (for example, crop growth stage, presence of snow cover, availability of pasture) can also influence the levels of contamination in agricultural produce in areas where deposition of radionuclides has occurred, thus strongly influencing internal exposures resulting from ingestion of contaminated food (NEA, 1989a, 1991; WHO 1987).
43. Temporal and Spatial Aspects of Radiological Emergencies (46) Categorisation of radiological emergencies in terms of their temporal and spatial aspects can be useful in the development of appropriate emergency response plans. Routes of exposure
10
REPORT OF A TASK GROUP OF COMMITTEE 4
may differ at various times in an accident sequence, such differences most likely requiring the
implementation of different protective actions. For example, external exposure to and inhalation from, a radioactive plume can be of more immediate significance than would be external exposure to deposited radionuclides and ingestion of contaminated food. This is due principally to the shorter times involved in the possibility of exposure from the plume and thus the need for making urgent decisions on implementing protective actions. The spatial extent of contamination will be determined principally by the natural processes such as wind and rain which control the transfer of radionuclides through the environment. Variability inherent in these processes can, therefore, result in heterogeneous spatial patterns of deposition and radionuclide concentrations, which must be considered for emergency response plating purposes. While an understanding of the temporal and spatial aspects of radiological emergencies is useful for planning purposes, these aspects are only one input (NEA 1989b, 1990). (47) The following time stages can be considered for the purpose of planning intervention for a radiological emergency: a pre-release stage, a release stage and a post-release stage. The prerelease stage is that period from the time when potential or actual accidental exposure is recognised, to the time when significant amounts of radioactive material are released or the source is brought under control. During this stage and in the tist few hours of the release stage, urgent decisions on measures to avert doses to the public and workers may be necessary. These decisions can be based on scenarios which have been identified in advance concerning the occurrence of abnormal events and source conditions. Initial results of environmental monitoring are unlikely to be available to aid decisions and the prediction of future developments may be subject to substantial uncertainties. For this reason, emergency response plans need to include procedures for implementing protective measures which are based on information about the condition of the facility or source, any measurements of released material and meteorological conditions and the possible pathways for exposure. If the pre-release stage is very short, only limited off-site actions may be feasible before the commencement of the release. (48) For extended releases, an immediate concern is exposures arising from passage of an airborne plume; subsequent concerns will include exposures arising from deposited radionuclides. The results of environmental monitoring should become available during or shortly after the release stage to assist in making appropriate decisions on the implementation of protective actions. (49) In the post-release stage there will be both decisions made concerning the implementation of further protective actions and about the return of normal living conditions. This stage may extend over a prolonged period of months or years. (50) The major routes of exposures which exist in the post-release stage will need to be reassessed at regular intervals to determine if continuation of protective actions previously introduced is still justified. (51) In the post-release stage, there are a number of social, economic and technical factors which are more important in making decisions than in the earlier stages. These factors depend upon the particular spatial and temporal aspects of the accident. The nature of the land use and
living habits in the area affected, the size of the population evacuated or relocated, the time of year, the ease of decontamination and the perceptions and attitudes of the population in returning to their homes are some of the factors that must be considered and balanced in the decision-making process. A process of optimisation should be used for the periodic review for the continuation of protective actions when specific information on the nature and consequences of the accident is available. Further discussion of these aspects is provided in Section 5.