- Email: [email protected]
The basis of radiation safety
6
REPORT OF A TASK GROUP OF COMMITTEE
4
always introduce an uncertainty which is difficult to assess quantitatively. Nevertheless, the result of the safety analysis, sometimes expressed by the inverse value of the estimated event frequency, may be seen as a useful figure of merit in comparisons of safety results. 2.3. Dose Response Relationship (28) For potential exposure situations the consideration of the basic dose response relationship used for stochastic effects must be extended into the range of high doses where deterministic effects also occur. It must also be recognised that the radiation and tissue weighting factors used to derive effective dose are not applicable to deterministic effects and that these doses should be absorbed doses rather than equivalent doses and be expressed in grays (Gy) rather than in sieverts (Sv). (29) At levels of effective dose below about 0.1 Sv, only stochastic effects are expected to occur and the probability of their occurrence is assumed to be directly proportional to the effective dose. The relationship of probability of harm to dose is therefore linear without threshold in this range. A nominal proportionality coefficient of 5 x 10m2 Sv-’ for the probability of fatal cancer in the general population, given a dose and dose rate effectiveness factor (DDREF) of two for low dose or dose rate, is used by the Commission in ZCZW Publication 60. (30) For absorbed doses higher than approximately
0.5 Gy, delivered over a short period, some deterministic effects begin to occur in addition to stochastic effects. The dose response relationship for attributable death approximates a sigmoid curve, although the exact shape depends on a number of factors, such as the dose rate and the distribution of the exposure in time. For a dose to the whole body of approximately 3 Gy, the probability of death is about 0.5 in the absence of medical attention. For acute doses higher than about 6 Gy, delivered over a short period, practically all irradiated individuals will suffer an acute radiation syndrome and are likely to eventually die as a consequence of the irradiation.
3. THE BASIS OF RADIATION
SAFETY
3.1. Aims (31) Safety is a complex concept that has been used with different connotations. Typically, it has been linked to the concepts of protection and security and used to denote reliability, prudent caution and freedom from danger. Early toxicologists used the concept of a safe dose to indicate an amount of harmful substance that was below the level (threshold) where toxicity could be manifested; but with the knowledge of genotoxics, which are currently assumed to have no threshold for effect, toxicologists modified the concept to reflect an amount for which the likelihood of toxic effects is kept below a prescribed level. Technologists and engineers, on the other hand, generally use the term safety to denote accident prevention. Accident preventive measures are typically intended to reduce the probability of an accident rather than eliminate the potential for an accident. (32) Historically, the discipline of radiation protection has dealt mainly with the unconditional limitation of radiation doses from anticipatable, “normal” exposures to manmade radiation sources while the discipline of nuclear safety has dealt mainly with the prevention of nuclear accidents and, should they occur, with the mitigation of their consequences by technological or other means. Not surprisingly, these differences in approach have created problems of interpretation on the ultimate safety objectives. Although they are often perceived to be separate and entirely different, the two disciplines are, in fact,
PROTECTION
FROM POTENTIAL
EXPOSURE
7
complementary. They form the two parts of a continuous control regime which should cover all exposure situations, i.e., both normal and potential exposures in all types of practices. For the purposes of this report, radiation protection and the safety of radiation sources, e.g., nuclear safety, are therefore considered in a unified framework titled “radiation safety”. The principles for assessment of potential exposure are equally applicable to complex installations and to simpler installations and practices, although complexity in practical application may vary widely. (33) The system of radiological protection recommended by the Commission for proposed and continuing practices is based on the following general principles. (a) No practice involving exposures to radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation detriment it causes. (The justification of a practice.) (b) In relation to any particular source within a practice, the magnitude of individual doses, the number of people exposed and the likelihood of incurring exposures where these are not certain to be received should all be kept as low as reasonably achievable, economic and social factors being taken into account. This procedure should be constrained by restrictions on the doses to individuals (dose constraints), or the risks to individuals in the case of potential exposures (risk constraints), so as to limit the inequity likely to result from the inherent economic and social judgments. (The optimisation of protection.) (c) The exposure of individuals resulting from the combination of all the relevant practices should be subject to dose limits, or to some control of risk in the case of potential exposures. These are aimed at ensuring that no individual is exposed to radiation risks that are judged to be unacceptable from these practices in any normal circumstances. Not all sources are susceptible to control by action at the source and it is necessary to specify the sources to be included as relevant before selecting a dose limit. (Individual dose and risk limits.) (ICRP, 1991).
3.2. Source- and Individual-Related Assessments (34) It is convenient to think of the processes that may cause potential exposures as a network of potential events and situations; the situations defining the context in which the events may occur. Each part of the network starts from a source. There is a potential for events or processes occurring which may cause the source to deliver radiation or to release radioactive materials. Radiation or radioactive material then passes along environmental pathways, involving sequences and processes which may be very complex in the natural environment, with some pathways being common to many sources. Occurrence probabilities and transport process characteristics can usually be assigned to each environmental transport pathway. Combinations of these events, processes and pathways are usually called “event sequences” or “scenarios” in risk assessments. In many cases, the term “event sequence” is used as being equivalent to “scenario”. However, “scenario” is a wider concept as it implies a more explicit specification of the context in which the event sequence occurs, i.e., a scenario is defined as a specific combination of events, features and processes starting from an initiating event and leading to radiological consequences. (35) Individuals, possibly many individuals, may be at risk as a result of potential exposures from several scenarios associated with a single original source. Also, since there can be many sources, some individuals may be exposed to radiation risks from more than one source. The issue of whether individuals will actually be exposed and whether, given an exposure,
8
REPORT
OF A TASK GROUP
OF COMMITTEE
4
individuals will actually incur harm, is not known with certainty and thus must be represented by a probability or combination of probabilities. (36) Assessments of the effectiveness of the safety measures can be made from two different viewpoints. If the assessment is made from the standpoint of exposures from a single source, then it is termed a source-related assessment. However, the assessment could also be made from the standpoint of an individual who is exposed to one or many sources. This second case, which includes exposures from all relevant sources exposing a given individual, is termed an indiuidualrelated assessment. (37) A source-related
assessment may include a number of scenarios leading to exposure of individuals via various pathways. For example, there may be scenarios leading to direct exposure to radiation from the source and scenarios leading to inhalation or ingestion exposures through releases of radioactive material to air or water. Thus, the scenarios of exposure include both the initiating events and processes and the subsequent environmental and other pathways to exposure. (38) A particular case of source-related assessment is the estimation of the risk associated assessments are essential when potential with particular scenarios. Such scenario-related exposures are involved. They allow a judgment to be made of whether all reasonable steps have been taken to reduce risks from radiation attributable to those scenarios. (39) The source-related assessment is a combination of assessments of all plausible scenarios and takes account of both the magnitude of individual risks attributable to that source and of the number of individuals exposed, but will not consider the contributions from other sources. Individual-related assessments involve the determination of the total risk to individuals from all the relevant sources in order to determine whether the risk to any individual is unacceptably high. In most potential exposure situations, it will not be feasible to perform individual-related assessments. Moreover, for many sources, the number and complexity of scenarios will be such that source-related assessments are also difficult. For that reason, requirements for situations involving potential exposures may need to be based on scenario-related assessments of a particular source. (40) Ideally, the assessment of risk should include all scenarios with a potential of causing exposure to radiation; the result of this assessment being some measure of total risk to be compared with a risk judged tolerable by society. For technically complex sources, it is impossible to include all scenarios. The practical approach is to base a risk assessment on the sequences that cover the major contributors to risk as far as reasonably achievable. 3.3. Basic Principles (41) The relevant parts of the fundamental principles in the system of radiological protection specifically applicable to potential exposure situations are as follows: (42) Justification of Practice: No practice involving exposures to radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation detriment it may cause. (43) The justification requirement simply specifies that, in order to permit the introduction or accept the continuation of a practice, more benefit than detriment should be expected. When moving from normal exposures to the wider scope of potential exposure, the practical application of the principle of justification becomes more complicated. For some scenarios, the probability of occurrence can be very low but, if an event anticipated by the scenario occurs, the consequences could be judged as being unacceptably high. Such cases should be included in an assessment of justification and the tolerability of the event considered in terms of both the
PROTECTION
FROM POTENTIAL
EXPOSURE
9
probabilities and the consequences involved. Typically, the final decision on the tolerability of such low probability-high consequence scenarios as a part of a justification of a practice is taken at the political level or under explicit political guidance or authorisation. Justification should not necessarily be applied in a prescriptive manner. Rather, it could form part of a general declaration of principles and authorities should retain the ability to exclude the introduction of a practice or to discontinue a practice on the basis of justification arguments. (44) Optimisation of Protection: In relation to any particular source within a justified practice, the likelihood of incurring exposures, the magnitude of individual doses and the number of people exposed should all be kept as low as reasonably achievable, economic and social factors being taken into account. (45) Once a practice has been justified and adopted, the next step is to determine how best to use resources for safety measures to keep as low as reasonable, under the prevailing circumstances, the potential radiological consequences as well as their likelihood of occurrence. The ultimate level of safety applied to a radiation source results from a choice among feasible alternative safety options. Those individuals or groups making decisions should satisfy themselves that the most appropriate safety option under the prevailing circumstances has been selected. As safety measures are increased, the occurrence probability or the potential radiological consequences themselves will logically be decreased. However, if the next increment of safety requires a deployment of resources or causes an increase in the social cost that is disproportionate to the resultant reduction in the probability or the magnitude of the radiological consequence, it is not in society’s interest for that step to be taken. The safety measures can then be said to be optimised and the remaining risks to be as low as reasonably achievable, economic and social factors having been taken into account. (46) Individual Risk Limitation: The optimisation procedure should be constrained by restrictions on the risks to individuals so as to limit the inequity which may result from the inherent economic and social judgments. Thus, some control of individual risk should be established, aimed at ensuring that no individual is exposed to radiation risks that are judged to be unacceptable from a justified practice. (47) An optimum safety option arrived at on the basis of an unconstrained optimisation process may not be acceptable unless it meets the requirement that no individual shall be expected to be subjected to a probability of radiation harm greater than a pre-established level. These individual levels should be established before optimising. Because risks above the individual risk limit are considered to be unacceptable, a risk constraint, lower than the limit, may be established as part of the analysis. The risk constraint is particularly important when assessing the acceptability of an event sequence or scenario, since the risk associated with a particular scenario is only part of the overall risk to an individual from all scenarios and sources. The establishment of a constraint can also serve the purpose of a priori apportionment of an overall risk limit to any particular source, scenario, or event sequence since an individual could be potentially exposed from a number of sources and practices. Furthermore, a constraint can provide an additional measure of caution when applying probability values with large distributions or uncertainties (see paragraph 30). (48) Technical and Managerial Principles: Implementation of a radiation safety programme should be based upon a number of practical technical and managerial principles in addition to the fundamental principles discussed above. These principles are generally applicable to the safe siting, design, construction, operation, decommissioning and final disposal of radiation sources, commensurate with the risk, and have been discussed and applied in a number of contexts, notably in the area of nuclear safety. (49) The first and most important technical principle is that there should be layers (i.e.,
10
REPORT OF A TASK GROUP OF COMMITTEE 4
structures, components, systems, procedures, or combinations thereof) of overlapping safety provisions (“defence-in-depth”). A given level of protection is best provided by a combination of layers rather than trying to achieve a high level of protection with only one layer. One reason for this is that low overall probabilities of failure are most easily achieved by a combination of independent protective layers such that the probabilities of failure are multiplicative. Another is that unforeseen failure modes have less effect on the overall protection when there are many independent protective layers. (50) This principle of “defence-in-depth” must be applied to any significant radiation source as a means to compensate for potential human and mechanical failures. The degree of defencein-depth is normally graded according to the complexity and size of the radiation source as well as the potential consequences in the event of an accident. Corollaries to this “defence-in-depth” principle are the concepts of accident prevention and accident mitigation. Thus, the first emphasis should be placed on the measures that serve to prevent accidents and then further measures should be available to reduce substantially the effects of the accident, should it occur. (51) In addition to the fundamental principle of “defence-in-depth,” there are other technical principles which are essential to achieving radiation safety. These include the following: -The design, construction and operation of radiation sources should be based on sound engineering principles and practices which are proven by testing and experience, subject to the need for research and innovation to improve safety and which are reflected in approved codes and standards and other appropriately documented instruments. -A comprehensive system of quality assurance should ensure, with high confidence, that the design, construction and operation of sources meet specified requirements. -All personnel who can affect radiation safety should be trained and qualified to perform their duties. The possibility of human errors should be taken into account as one of the primary contributors to many events and steps taken, i.e., by facilitating correct decisions and inhibiting wrong decisions, for reducing this contribution and providing means for detecting and correcting or compensating for such errors. -Safety assessments should be conducted as an integral part of design, construction and operation of a radiation source. These assessments should be well documented and independently reviewed and systematically updated in the light of any significant new safety information. A feedback mechanism should be established so that weaknesses in safety based on operating experience can be taken into account in subsequent design and construction. Close co-operation among designers, manufacturers and operators is essential for safety improvements. (52) In addition to the technical principles, an essential managerial principle for all individuals and organisations is to establish a consistent and pervading approach to safety which governs all of the actions associated with construction, operation and the ultimate disposal of radiation sources. This principle has been referred to as a “safety culture” and is defined in the context of nuclear safety (IAEA, 1988b) as “that assembly of characteristics and attitudes in organisations and individuals which establish that, as an overriding priority, . . . safety issues receive the attention warranted by their significance . . . [it] refers to the personal dedication and accountability of all individuals engaged in any activity which has a bearing on the safety of nuclear power plants.” However, the principle is generally applicable to all sources and practices and serves to emphasise both personal attitudes and habits of thought and organisational approaches and priorities. Thus, the application of this principle requires that all duties important to safety be carried out correctly, with alertness, due thought and full knowledge, sound judgment and a proper sense of accountability. Furthermore, it implies a
PROTECTION
FROM POTENTIAL
EXPOSURE
11
learning attitude in all organisations concerned, taking into account all relevant operating experience as well as new research results as a basis for safety improvements and reassessments (IAEA, 1991). (53) The primary responsibility for achieving and maintaining a satisfactory control of radiation exposures rests squarely on the management bodies of the institutions conducting the operations giving rise to the exposures. When equipment or plants are designed and supplied by other institutions, they, in turn, have a responsibility to see that the items supplied will be satisfactory, if used as intended. Governments have the responsibility to set up regulatory agencies, which then have the responsibility for providing a regulatory, and often also an advisory, framework to emphasise the responsibilities of the management bodies while, at the same time, setting and enforcing overall standards of protection. They may also have to take direct responsibility when, as with exposures to many natural sources, there is no relevant management body (ICRP, 1991).
4. PROTECTION FROM POTENTIAL EXPOSURE: PRACTICAL APPLICATIONS 4.1. Justification of Practice (54) As has been indicated in Section 3, justification of the practice should not necessarily be applied in a prescriptive manner. In the discussions of the methods of assessment and applications of concepts which follow, it is assumed that the practice has already been justified.
4.2. Optimisation of Radiation Safety (55) The safety level associated with sources of potential exposure is usually established by the designers or operators of the installation and judged by competent governmental organisations. Far from being a straightforward process related to objective radiation safety aspects alone, the decision and judgmental processes in reality reflect cultural perspectives, national traditions, social values and professional attitudes. It is also recognised that safety levels should be adjusted to maximise the net benefit to the individual or to society. This is not a simple process because the objectives of the individual or groups of individuals and those of society may not coincide. (56) The judgments required in optimising the radiation safety measures are not purely quantitative: they reflect preferences between detriments of different kinds and their probabilities of occurrence and between the deployment of resources and the radiation detriment. The process of optimising the safety measures should therefore be carefully structured. It should be applied to the design and implementation of the safety measures following the justification of the practice. It is here that reductions in potential doses and their probability of occurrence are most likely to be achievable in cost-effective ways. (57) The first step towards safety optimisation is the identification of the relevant factors to be taken into account in the process. The optimisation of potential exposure situations will necessarily involve consideration of factors such as the probability of the exposure, the distribution of risks, the total consequences should the exposure occur and the safety efforts that may be applied (sometimes expressed in terms of the costs of safety). (58) Additionally, it should be recognised when assessing the total, i.e., collective detriment: (1) that potential exposure situations can result in doses producing deterministic effects and
REPORT OF A TASK GROUP OF COMMITTEE
4
always introduce an uncertainty which is difficult to assess quantitatively. Nevertheless, the result of the safety analysis, sometimes expressed by the inverse value of the estimated event frequency, may be seen as a useful figure of merit in comparisons of safety results. 2.3. Dose Response Relationship (28) For potential exposure situations the consideration of the basic dose response relationship used for stochastic effects must be extended into the range of high doses where deterministic effects also occur. It must also be recognised that the radiation and tissue weighting factors used to derive effective dose are not applicable to deterministic effects and that these doses should be absorbed doses rather than equivalent doses and be expressed in grays (Gy) rather than in sieverts (Sv). (29) At levels of effective dose below about 0.1 Sv, only stochastic effects are expected to occur and the probability of their occurrence is assumed to be directly proportional to the effective dose. The relationship of probability of harm to dose is therefore linear without threshold in this range. A nominal proportionality coefficient of 5 x 10m2 Sv-’ for the probability of fatal cancer in the general population, given a dose and dose rate effectiveness factor (DDREF) of two for low dose or dose rate, is used by the Commission in ZCZW Publication 60. (30) For absorbed doses higher than approximately
0.5 Gy, delivered over a short period, some deterministic effects begin to occur in addition to stochastic effects. The dose response relationship for attributable death approximates a sigmoid curve, although the exact shape depends on a number of factors, such as the dose rate and the distribution of the exposure in time. For a dose to the whole body of approximately 3 Gy, the probability of death is about 0.5 in the absence of medical attention. For acute doses higher than about 6 Gy, delivered over a short period, practically all irradiated individuals will suffer an acute radiation syndrome and are likely to eventually die as a consequence of the irradiation.
3. THE BASIS OF RADIATION
SAFETY
3.1. Aims (31) Safety is a complex concept that has been used with different connotations. Typically, it has been linked to the concepts of protection and security and used to denote reliability, prudent caution and freedom from danger. Early toxicologists used the concept of a safe dose to indicate an amount of harmful substance that was below the level (threshold) where toxicity could be manifested; but with the knowledge of genotoxics, which are currently assumed to have no threshold for effect, toxicologists modified the concept to reflect an amount for which the likelihood of toxic effects is kept below a prescribed level. Technologists and engineers, on the other hand, generally use the term safety to denote accident prevention. Accident preventive measures are typically intended to reduce the probability of an accident rather than eliminate the potential for an accident. (32) Historically, the discipline of radiation protection has dealt mainly with the unconditional limitation of radiation doses from anticipatable, “normal” exposures to manmade radiation sources while the discipline of nuclear safety has dealt mainly with the prevention of nuclear accidents and, should they occur, with the mitigation of their consequences by technological or other means. Not surprisingly, these differences in approach have created problems of interpretation on the ultimate safety objectives. Although they are often perceived to be separate and entirely different, the two disciplines are, in fact,
PROTECTION
FROM POTENTIAL
EXPOSURE
7
complementary. They form the two parts of a continuous control regime which should cover all exposure situations, i.e., both normal and potential exposures in all types of practices. For the purposes of this report, radiation protection and the safety of radiation sources, e.g., nuclear safety, are therefore considered in a unified framework titled “radiation safety”. The principles for assessment of potential exposure are equally applicable to complex installations and to simpler installations and practices, although complexity in practical application may vary widely. (33) The system of radiological protection recommended by the Commission for proposed and continuing practices is based on the following general principles. (a) No practice involving exposures to radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation detriment it causes. (The justification of a practice.) (b) In relation to any particular source within a practice, the magnitude of individual doses, the number of people exposed and the likelihood of incurring exposures where these are not certain to be received should all be kept as low as reasonably achievable, economic and social factors being taken into account. This procedure should be constrained by restrictions on the doses to individuals (dose constraints), or the risks to individuals in the case of potential exposures (risk constraints), so as to limit the inequity likely to result from the inherent economic and social judgments. (The optimisation of protection.) (c) The exposure of individuals resulting from the combination of all the relevant practices should be subject to dose limits, or to some control of risk in the case of potential exposures. These are aimed at ensuring that no individual is exposed to radiation risks that are judged to be unacceptable from these practices in any normal circumstances. Not all sources are susceptible to control by action at the source and it is necessary to specify the sources to be included as relevant before selecting a dose limit. (Individual dose and risk limits.) (ICRP, 1991).
3.2. Source- and Individual-Related Assessments (34) It is convenient to think of the processes that may cause potential exposures as a network of potential events and situations; the situations defining the context in which the events may occur. Each part of the network starts from a source. There is a potential for events or processes occurring which may cause the source to deliver radiation or to release radioactive materials. Radiation or radioactive material then passes along environmental pathways, involving sequences and processes which may be very complex in the natural environment, with some pathways being common to many sources. Occurrence probabilities and transport process characteristics can usually be assigned to each environmental transport pathway. Combinations of these events, processes and pathways are usually called “event sequences” or “scenarios” in risk assessments. In many cases, the term “event sequence” is used as being equivalent to “scenario”. However, “scenario” is a wider concept as it implies a more explicit specification of the context in which the event sequence occurs, i.e., a scenario is defined as a specific combination of events, features and processes starting from an initiating event and leading to radiological consequences. (35) Individuals, possibly many individuals, may be at risk as a result of potential exposures from several scenarios associated with a single original source. Also, since there can be many sources, some individuals may be exposed to radiation risks from more than one source. The issue of whether individuals will actually be exposed and whether, given an exposure,
8
REPORT
OF A TASK GROUP
OF COMMITTEE
4
individuals will actually incur harm, is not known with certainty and thus must be represented by a probability or combination of probabilities. (36) Assessments of the effectiveness of the safety measures can be made from two different viewpoints. If the assessment is made from the standpoint of exposures from a single source, then it is termed a source-related assessment. However, the assessment could also be made from the standpoint of an individual who is exposed to one or many sources. This second case, which includes exposures from all relevant sources exposing a given individual, is termed an indiuidualrelated assessment. (37) A source-related
assessment may include a number of scenarios leading to exposure of individuals via various pathways. For example, there may be scenarios leading to direct exposure to radiation from the source and scenarios leading to inhalation or ingestion exposures through releases of radioactive material to air or water. Thus, the scenarios of exposure include both the initiating events and processes and the subsequent environmental and other pathways to exposure. (38) A particular case of source-related assessment is the estimation of the risk associated assessments are essential when potential with particular scenarios. Such scenario-related exposures are involved. They allow a judgment to be made of whether all reasonable steps have been taken to reduce risks from radiation attributable to those scenarios. (39) The source-related assessment is a combination of assessments of all plausible scenarios and takes account of both the magnitude of individual risks attributable to that source and of the number of individuals exposed, but will not consider the contributions from other sources. Individual-related assessments involve the determination of the total risk to individuals from all the relevant sources in order to determine whether the risk to any individual is unacceptably high. In most potential exposure situations, it will not be feasible to perform individual-related assessments. Moreover, for many sources, the number and complexity of scenarios will be such that source-related assessments are also difficult. For that reason, requirements for situations involving potential exposures may need to be based on scenario-related assessments of a particular source. (40) Ideally, the assessment of risk should include all scenarios with a potential of causing exposure to radiation; the result of this assessment being some measure of total risk to be compared with a risk judged tolerable by society. For technically complex sources, it is impossible to include all scenarios. The practical approach is to base a risk assessment on the sequences that cover the major contributors to risk as far as reasonably achievable. 3.3. Basic Principles (41) The relevant parts of the fundamental principles in the system of radiological protection specifically applicable to potential exposure situations are as follows: (42) Justification of Practice: No practice involving exposures to radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation detriment it may cause. (43) The justification requirement simply specifies that, in order to permit the introduction or accept the continuation of a practice, more benefit than detriment should be expected. When moving from normal exposures to the wider scope of potential exposure, the practical application of the principle of justification becomes more complicated. For some scenarios, the probability of occurrence can be very low but, if an event anticipated by the scenario occurs, the consequences could be judged as being unacceptably high. Such cases should be included in an assessment of justification and the tolerability of the event considered in terms of both the
PROTECTION
FROM POTENTIAL
EXPOSURE
9
probabilities and the consequences involved. Typically, the final decision on the tolerability of such low probability-high consequence scenarios as a part of a justification of a practice is taken at the political level or under explicit political guidance or authorisation. Justification should not necessarily be applied in a prescriptive manner. Rather, it could form part of a general declaration of principles and authorities should retain the ability to exclude the introduction of a practice or to discontinue a practice on the basis of justification arguments. (44) Optimisation of Protection: In relation to any particular source within a justified practice, the likelihood of incurring exposures, the magnitude of individual doses and the number of people exposed should all be kept as low as reasonably achievable, economic and social factors being taken into account. (45) Once a practice has been justified and adopted, the next step is to determine how best to use resources for safety measures to keep as low as reasonable, under the prevailing circumstances, the potential radiological consequences as well as their likelihood of occurrence. The ultimate level of safety applied to a radiation source results from a choice among feasible alternative safety options. Those individuals or groups making decisions should satisfy themselves that the most appropriate safety option under the prevailing circumstances has been selected. As safety measures are increased, the occurrence probability or the potential radiological consequences themselves will logically be decreased. However, if the next increment of safety requires a deployment of resources or causes an increase in the social cost that is disproportionate to the resultant reduction in the probability or the magnitude of the radiological consequence, it is not in society’s interest for that step to be taken. The safety measures can then be said to be optimised and the remaining risks to be as low as reasonably achievable, economic and social factors having been taken into account. (46) Individual Risk Limitation: The optimisation procedure should be constrained by restrictions on the risks to individuals so as to limit the inequity which may result from the inherent economic and social judgments. Thus, some control of individual risk should be established, aimed at ensuring that no individual is exposed to radiation risks that are judged to be unacceptable from a justified practice. (47) An optimum safety option arrived at on the basis of an unconstrained optimisation process may not be acceptable unless it meets the requirement that no individual shall be expected to be subjected to a probability of radiation harm greater than a pre-established level. These individual levels should be established before optimising. Because risks above the individual risk limit are considered to be unacceptable, a risk constraint, lower than the limit, may be established as part of the analysis. The risk constraint is particularly important when assessing the acceptability of an event sequence or scenario, since the risk associated with a particular scenario is only part of the overall risk to an individual from all scenarios and sources. The establishment of a constraint can also serve the purpose of a priori apportionment of an overall risk limit to any particular source, scenario, or event sequence since an individual could be potentially exposed from a number of sources and practices. Furthermore, a constraint can provide an additional measure of caution when applying probability values with large distributions or uncertainties (see paragraph 30). (48) Technical and Managerial Principles: Implementation of a radiation safety programme should be based upon a number of practical technical and managerial principles in addition to the fundamental principles discussed above. These principles are generally applicable to the safe siting, design, construction, operation, decommissioning and final disposal of radiation sources, commensurate with the risk, and have been discussed and applied in a number of contexts, notably in the area of nuclear safety. (49) The first and most important technical principle is that there should be layers (i.e.,
10
REPORT OF A TASK GROUP OF COMMITTEE 4
structures, components, systems, procedures, or combinations thereof) of overlapping safety provisions (“defence-in-depth”). A given level of protection is best provided by a combination of layers rather than trying to achieve a high level of protection with only one layer. One reason for this is that low overall probabilities of failure are most easily achieved by a combination of independent protective layers such that the probabilities of failure are multiplicative. Another is that unforeseen failure modes have less effect on the overall protection when there are many independent protective layers. (50) This principle of “defence-in-depth” must be applied to any significant radiation source as a means to compensate for potential human and mechanical failures. The degree of defencein-depth is normally graded according to the complexity and size of the radiation source as well as the potential consequences in the event of an accident. Corollaries to this “defence-in-depth” principle are the concepts of accident prevention and accident mitigation. Thus, the first emphasis should be placed on the measures that serve to prevent accidents and then further measures should be available to reduce substantially the effects of the accident, should it occur. (51) In addition to the fundamental principle of “defence-in-depth,” there are other technical principles which are essential to achieving radiation safety. These include the following: -The design, construction and operation of radiation sources should be based on sound engineering principles and practices which are proven by testing and experience, subject to the need for research and innovation to improve safety and which are reflected in approved codes and standards and other appropriately documented instruments. -A comprehensive system of quality assurance should ensure, with high confidence, that the design, construction and operation of sources meet specified requirements. -All personnel who can affect radiation safety should be trained and qualified to perform their duties. The possibility of human errors should be taken into account as one of the primary contributors to many events and steps taken, i.e., by facilitating correct decisions and inhibiting wrong decisions, for reducing this contribution and providing means for detecting and correcting or compensating for such errors. -Safety assessments should be conducted as an integral part of design, construction and operation of a radiation source. These assessments should be well documented and independently reviewed and systematically updated in the light of any significant new safety information. A feedback mechanism should be established so that weaknesses in safety based on operating experience can be taken into account in subsequent design and construction. Close co-operation among designers, manufacturers and operators is essential for safety improvements. (52) In addition to the technical principles, an essential managerial principle for all individuals and organisations is to establish a consistent and pervading approach to safety which governs all of the actions associated with construction, operation and the ultimate disposal of radiation sources. This principle has been referred to as a “safety culture” and is defined in the context of nuclear safety (IAEA, 1988b) as “that assembly of characteristics and attitudes in organisations and individuals which establish that, as an overriding priority, . . . safety issues receive the attention warranted by their significance . . . [it] refers to the personal dedication and accountability of all individuals engaged in any activity which has a bearing on the safety of nuclear power plants.” However, the principle is generally applicable to all sources and practices and serves to emphasise both personal attitudes and habits of thought and organisational approaches and priorities. Thus, the application of this principle requires that all duties important to safety be carried out correctly, with alertness, due thought and full knowledge, sound judgment and a proper sense of accountability. Furthermore, it implies a
PROTECTION
FROM POTENTIAL
EXPOSURE
11
learning attitude in all organisations concerned, taking into account all relevant operating experience as well as new research results as a basis for safety improvements and reassessments (IAEA, 1991). (53) The primary responsibility for achieving and maintaining a satisfactory control of radiation exposures rests squarely on the management bodies of the institutions conducting the operations giving rise to the exposures. When equipment or plants are designed and supplied by other institutions, they, in turn, have a responsibility to see that the items supplied will be satisfactory, if used as intended. Governments have the responsibility to set up regulatory agencies, which then have the responsibility for providing a regulatory, and often also an advisory, framework to emphasise the responsibilities of the management bodies while, at the same time, setting and enforcing overall standards of protection. They may also have to take direct responsibility when, as with exposures to many natural sources, there is no relevant management body (ICRP, 1991).
4. PROTECTION FROM POTENTIAL EXPOSURE: PRACTICAL APPLICATIONS 4.1. Justification of Practice (54) As has been indicated in Section 3, justification of the practice should not necessarily be applied in a prescriptive manner. In the discussions of the methods of assessment and applications of concepts which follow, it is assumed that the practice has already been justified.
4.2. Optimisation of Radiation Safety (55) The safety level associated with sources of potential exposure is usually established by the designers or operators of the installation and judged by competent governmental organisations. Far from being a straightforward process related to objective radiation safety aspects alone, the decision and judgmental processes in reality reflect cultural perspectives, national traditions, social values and professional attitudes. It is also recognised that safety levels should be adjusted to maximise the net benefit to the individual or to society. This is not a simple process because the objectives of the individual or groups of individuals and those of society may not coincide. (56) The judgments required in optimising the radiation safety measures are not purely quantitative: they reflect preferences between detriments of different kinds and their probabilities of occurrence and between the deployment of resources and the radiation detriment. The process of optimising the safety measures should therefore be carefully structured. It should be applied to the design and implementation of the safety measures following the justification of the practice. It is here that reductions in potential doses and their probability of occurrence are most likely to be achievable in cost-effective ways. (57) The first step towards safety optimisation is the identification of the relevant factors to be taken into account in the process. The optimisation of potential exposure situations will necessarily involve consideration of factors such as the probability of the exposure, the distribution of risks, the total consequences should the exposure occur and the safety efforts that may be applied (sometimes expressed in terms of the costs of safety). (58) Additionally, it should be recognised when assessing the total, i.e., collective detriment: (1) that potential exposure situations can result in doses producing deterministic effects and