OCCUPATIONAL EXPOSURE AND TYPES OF EXPOSURE SITUATION 2.1. Occupational exposure is the exposure of workers incurred during the course of their work, regardless of the exposure situation. For the purpose of establishing practical requirements for protection and safety, GSR Part 3 [2]
distinguishes between three different types of exposure situation: planned exposure situations, emergency exposure situations and existing exposure situations. As stated in para. 1.20 of GSR Part 3 [2]:
“(a) A planned exposure situation is a situation of exposure that arises from the planned operation of a source or from a planned activity that results in an exposure due to a source. Since provision for protection and safety can be made before embarking on the activity concerned, the associated exposures and their likelihood of occurrence can be
restricted from the outset. The primary means of controlling exposure in planned exposure situations is by good design of facilities, equipment and operating procedures, and by training….
(b) An emergency exposure situation is a situation of exposure that arises as a result of an accident, a malicious act or any other unexpected event, and requires prompt action in order to avoid or to reduce adverse consequences. Preventive measures and mitigatory actions have to be considered before an emergency exposure situation arises.
However, once an emergency exposure situation actually arises, exposures can be reduced only by implementing protective actions.
(c) An existing exposure situation is a situation of exposure that already exists when a decision on the need for control needs to be taken.
Existing exposure situations include situations of exposure to natural background radiation. They also include situations of exposure due to residual radioactive material that derives from past practices that were not subject to regulatory control or that remains after an emergency exposure situation.”
This Safety Guide gives guidance on the protection of workers in each of these three types of exposure situation.
2.2. As stated in para. 1.21 of GSR Part 3 [2]:
“The descriptions that are given in para. 1.20 of the three types of exposure situation are not always sufficient to determine unequivocally which type of exposure situation applies for particular circumstances. For instance, the transitions from an emergency exposure situation to an existing exposure situation may occur progressively over time; and some exposures due to natural sources may have some characteristics of both planned exposure situations and existing exposure situations. In these Standards, the most appropriate type of exposure situation for particular circumstances has been determined by taking practical considerations into account.”
2.3. Reference is made to potential exposure in para. 1.20(a) of GSR Part 3 [2]
as follows:
“In planned exposure situations, exposure at some level can be expected to occur. If exposure is not expected to occur with certainty, but could result from an accident or from an event or a sequence of events that may occur but is not certain to occur, this is referred to as ‘potential exposure’.
…….
“If an event or a sequence of events that has been considered in the assessment of potential exposure does actually occur, it may be treated either as a planned exposure situation or, if an emergency has been declared, as an emergency exposure situation.”
2.4. Some exposures are excluded from the scope of GSR Part 3 [2].
Paragraph 1.42 of GSR Part 3 [2] states that:
“These Standards apply to all situations involving radiation exposure that is amenable to control. Exposures deemed to be not amenable to control are excluded from the scope of these Standards.8
“8 It is generally accepted, for example, that it is not feasible to control 40K in the body or cosmic radiation at the surface of the Earth.”
Guidance is given in Section 3 on the components of exposure due to natural sources of radiation that may be required to be subject to control as occupational exposure.
PRINCIPLES OF RADIATION PROTECTION
2.5. The three general principles of radiation protection, which concern justification, optimization of protection and application of dose limits (limitation of doses), are expressed in Safety Principles 4–6 and 10 of the Fundamental Safety Principles [1]. In terms of Requirement 1 of GSR Part 3 [2], those “with responsibilities for protection and safety shall ensure that the principles of radiation protection are applied for all exposure situations.”
Justification
2.6. Paragraphs 2.8 and 2.9 of GSR Part 3 [2] state that:
“2.8. For planned exposure situations, each party with responsibilities for protection and safety shall ensure, when relevant requirements apply to that party, that no practice is undertaken unless it is justified.
“2.9. For emergency exposure situations and existing exposure situations, each party with responsibilities for protection and safety shall ensure, when
relevant requirements apply to that party, that protective actions or remedial actions are justified and are undertaken in such a way as to achieve the objectives set out in a protection strategy.”
2.7. In planned exposure situations, this means that no practice or source within a practice should be authorized unless the practice produces sufficient benefit to the exposed individuals or to society to offset the harm (including radiation detriment) that it might cause; that is, unless the practice is justified, with societal, economic and other relevant factors having been taken into account.
2.8. The process of determining whether a practice is justified involves consideration of all radiation doses received by workers and members of the public. In general, the assumption made in this Safety Guide is that the process of justification has already taken place and that the contribution of occupational exposure to the total radiation detriment has been taken into account. The subject of justification for planned exposure situations is therefore not considered in detail in this Safety Guide. Guidance on justification is given in IAEA Safety Standards Series No. GSG-5, Justification of Practices, Including Non-medical Human Imaging [4].
Optimization
2.9. Paragraph 2.10 of GSR Part 3 [2] states that:
“For all exposure situations, each party with responsibilities for protection and safety shall ensure, when relevant requirements apply to that party, that protection and safety is optimized9.
“9 ‘Protection and safety is optimized’ means that optimization of protection and safety has been applied and the result of that process has been implemented.”
2.10. In planned exposure situations, in relation to exposures due to any particular source within a practice, protection and safety is required to be optimized in order that the magnitude of individual doses, the number of people exposed and the likelihood of incurring exposures all be kept as low as reasonably achievable, economic and societal factors being taken into account, with the restriction that the doses to individuals delivered by the source be subject to dose constraints.
This principle is of particular importance for the implementation of radiation protection measures in the workplace and therefore underlies much of the more detailed guidance given in Section 3.
Dose limitation
2.11. Paragraph 2.11 of GSR Part 3 [2] states that:
“For planned exposure situations other than for medical exposure, each party with responsibilities for protection and safety shall ensure that, when relevant requirements apply to that party, specified dose limits are not exceeded.”
2.12. Dose limits apply only in planned exposure situations. In such situations, the exposure of individuals should be restricted so that neither the total effective dose nor the total equivalent dose to relevant tissues or organs, caused by possible combinations of exposures due to authorized practices, exceeds any relevant dose limit.
2.13. The limit on effective dose represents the level above which the risk of stochastic effects due to radiation exposure is considered to be unacceptable. For localized exposure of the lens of the eye, the extremities and the skin, this limit on effective dose is not sufficient to ensure the avoidance of deterministic effects.
Limits on equivalent dose to these tissues and organs are, therefore, specified for such situations.
2.14. Guidance on the application of the dose limits for occupational exposure is given in Section 3.
RESPONSIBILITIES The government
2.15. The responsibilities of the government6 with regard to protection and safety are set out in paras 2.13–2.28 of GSR Part 3 [2]. These include:
(a) Establishing an effective legal and regulatory framework for protection and safety in all exposure situations;
(b) Establishing legislation that meets specified requirements;
(c) Establishing an independent regulatory body with the necessary legal authority, competence and resources;
6 Since States have different legal structures, the term ‘government’ here is to be understood in a broad sense and, accordingly, is interchangeable with the term ‘State’.
(d) Establishing requirements for education and training in protection and safety;
(e) Ensuring that arrangements are in place for the provision of technical services and education and training services.
The regulatory body
2.16. The responsibilities of the regulatory body with regard to protection and safety are set out in paras 2.29–2.38 of GSR Part 3 [2]. These include:
(a) Establishing requirements for applying the principles of radiation protection;
(b) Establishing a regulatory system that meets specified requirements;
(c) Ensuring the application of the requirements for education and training in protection and safety;
(d) Ensuring that mechanisms are in place for the dissemination of lessons learned from incidents and accidents;
(e) Setting acceptance and performance criteria for sources and equipment with implications for protection and safety;
(f) Making provision for the establishment and maintenance of records.
2.17. The responsibilities of the regulatory body specific to occupational exposure in planned exposure situations are set out in paras 3.69–3.73 of GSR Part 3 [2].
The regulatory body is responsible for establishing and enforcing requirements for ensuring that protection and safety is optimized, ensuring that applicable dose limits are complied with, and monitoring and recording occupational exposures.
Employers, registrants and licensees
2.18. Requirement 4 of GSR Part 3 [2] states that “The person or organization responsible for facilities and activities that give rise to radiation risks shall have the prime responsibility for protection and safety....” In planned exposure situations, employers, registrants and licensees (hereinafter referred to simply as the ‘management’) are responsible for ensuring that protection and safety is optimized, that applicable dose limits are complied with, and that appropriate radiation protection programmes are established and implemented. Guidance on the content of the radiation protection programme is given in Section 3.
Compliance by workers
2.19. Requirement 22 of GSR Part 3 [2] states that “Workers shall fulfil their obligations and carry out their duties for protection and safety.” This
requirement reflects the fact that workers can, by their own actions, contribute to protection and safety for themselves and for others at work. The requirements on workers in this regard are listed in para. 3.83 of GSR Part 3 [2] and relate to: following rules and procedures; using monitoring equipment and personal protective equipment; cooperating in programmes for workers’ health surveillance and programmes for dose assessment; and accepting instruction and training.
Workers are also required to provide relevant information to the management and to act in a responsible manner with regard to protection and safety.
GRADED APPROACH
2.20. Paragraph 2.31 of GSR Part 3 [2] provides the basis for the graded approach to the control of exposure:
“The regulatory body shall adopt a graded approach to the implementation of the system of protection and safety, such that the application of regulatory requirements is commensurate with the radiation risks associated with the exposure situation.”
It is required as a general responsibility of the government to ensure that the overall application of the principles of radiation protection is in line with the graded approach (see para. 2.18 of GSR Part 3 [2]).
2.21. Requirement 6 of GSR Part 3 [2] refers to the graded approach in the more specific context of planned exposure situations:
“The application of the requirements of these Standards in planned exposure situations shall be commensurate with the characteristics of the practice or the source within a practice, and with the likelihood and magnitude of exposures.”
2.22. An important feature of the graded approach in planned exposure situations is the provision for exemption and clearance. Requirement 8 of GSR Part 3 [2]
states that:
“The government or the regulatory body shall determine which practices or sources within practices are to be exempted from some or all of the requirements of these Standards. The regulatory body shall
approve which sources, including materials and objects, within notified practices or authorized practices may be cleared from regulatory control.”
MANAGEMENT SYSTEM
2.23. Requirement 5 of GSR Part 3 [2] states that “The principal parties shall ensure that protection and safety are effectively integrated into the overall management system of the organizations for which they are responsible.”
For occupational exposure in planned exposure situations, the principal party is the employer. For emergency exposure situations or existing exposure situations, the principal parties are those persons or organizations designated to deal with the situation.
2.24. In terms of paras 2.47–2.52 of GSR Part 3 [2], the “principal parties shall demonstrate commitment to protection and safety at the highest levels within the organizations” and “shall ensure that the management system…is designed and applied to enhance protection and safety” while maintaining coherence between measures for protection and safety and other measures, such as those addressing operational performance and security.
2.25. Specific actions should be taken to provide the necessary degree of confidence in the measures taken for achieving protection and safety and to ensure their regular assessment and review. A safety culture should be promoted and maintained at all levels within the organization. The management system should also address human factors by supporting good performance and good practices to prevent human and organizational failures, with attention being given to the design of equipment, the development of operating procedures, limits and conditions, as appropriate, training and the use of safety systems to mitigate consequences of human error.
2.26. More detailed requirements and guidance on the management system for facilities and activities are given IAEA Safety Standards Series Nos GSR Part 2, Leadership and Management for Safety [5], and GS-G-3.1, Application of the Management System for Facilities and Activities [6], and also in International Labour Organization guidelines [7]. Recommendations and guidance on the management system for providers of technical services in relation to protection and safety is given in Section 8.
DOSIMETRIC QUANTITIES
2.27. The dosimetric quantities recommended for radiation protection purposes (the protection quantities), and in which the dose limits are expressed in GSR Part 3 [2], are the equivalent dose HT in tissue or organ T and the effective dose E.
2.28. The basic physical quantities include the particle fluence Φ, the kerma K and the absorbed dose D.
2.29. The determination of equivalent dose HT in tissue or organ T involves the use of a radiation weighting factor wR as a multiplier of absorbed dose for radiation type R to reflect the relative biological effectiveness (RBE) of the radiation in inducing stochastic effects at low doses:
T R T, R
R
H =
å
w D× (1)where DT, R is the average absorbed dose in tissue or organ T for radiation type R.
2.30. The determination of effective dose E involves the use of a tissue weighting factor wT as a multiplier of the equivalent dose for tissue T to account for the different sensitivities of different tissues and organs to the induction of stochastic effects:
T T
T
E=
å
w H× (2)which, on substituting for HT from Eq. (1), gives:
T R T, R
T R
E=
å å
w × w D× (3)2.31. The recommended values of wR and wT are based on a review of published biological and epidemiological studies, and are given in the definitions of terms in GSR Part 3 [2].
2.32. The protection quantities E and HT relate to the sum of the effective doses or equivalent doses, respectively, received from exposure due to external sources within a given time period, and the committed effective doses or committed equivalent doses, respectively, from exposure due to intakes of radionuclides
occurring within the same time period. The total effective dose E received or committed during a given time period can be estimated from the operational quantities by using the following equation:
( ) ( ) ( )
p j,ing j,ing j,inh j,inh
j j
E H d@ +
å
e g ×I +å
e g ×I (4)where
Hp(d) is the personal dose equivalent in soft tissue at an appropriate depth d below a specified point on the body during a given time period;
e(g)j,ing is the committed effective dose per unit intake by ingestion for radionuclide j by the group of age g during the same time period;
e(g)j,inh is the committed effective dose per unit intake by inhalation for radionuclide j by the group of age g during the same time period;
Ij,ing is the intake via ingestion of radionuclide j during the same time period;
and Ij,inh is the intake via inhalation of radionuclide j during the same time period.
For occupational exposure, the appropriate values of e(g)j,ing and e(g)j,inh are those for workers.
2.33. The dose limits are such that deterministic effects will not occur. For situations that could lead to severe deterministic effects (e.g. emergency exposure situations), the RBE of different types of radiation in causing severe deterministic effects should be considered. The recommended dosimetric quantity is the RBE weighted absorbed dose DT in tissue or organ T. The determination of RBE weighted absorbed dose involves the use of tissue specific and radiation specific factors RBET, R as multipliers of absorbed dose in a tissue or organ to take account of the RBE in causing the development of severe deterministic health effects from a given absorbed dose that is delivered in a tissue or organ by a given type of radiation. Recommended values of RBET, R for the development of selected severe deterministic effects are based on a review of published biological studies and are given in the definitions of terms in GSR Part 3 [2]. The use of effective dose is inappropriate for the assessment of tissue reactions. In such situations, it is necessary to estimate absorbed dose and to take into account the appropriate RBE as the basis for any assessment of radiation effects.
Operational quantities for individual monitoring in external dosimetry 2.34. Since radiation protection quantities cannot be measured directly, the ICRU introduced operational quantities for practical use in radiation protection where
exposure due to external sources is concerned. Definitions of these quantities can be found in GSR Part 3 [2] and Ref. [8]. The operational quantities provide an estimate of effective or equivalent dose in a way that avoids underestimation and overestimation in most radiation fields encountered in practice. Radiation quality factors Q(L) are used in calculating the operational dose equivalent quantities used in monitoring [3]. The quality factor characterizes the biological effectiveness of the radiation type based on the ionization density along the tracks of charged particles in tissue. Q is defined as a function of the unrestricted linear energy transfer L∞ (often denoted as L or linear energy transfer, LET) of charged particles in water. A detailed evaluation of the numerical relationship between the physical quantities, radiation protection quantities and operational quantities was conducted by a joint task group of the ICRP and ICRU [9]. The conceptual relationship between those quantities is illustrated in Fig. 1 [9].
2.35. Strongly penetrating radiation and weakly penetrating radiation are differentiated by the ICRU [10]. If, for a given orientation of the body in a uniform and unidirectional radiation field, the equivalent dose received by any small area of the sensitive layer of the skin is less than ten times larger than the effective dose, the radiation is said to be strongly penetrating. If the equivalent dose is more than ten times larger than the effective dose, the radiation is said to be weakly penetrating.
Compared by measurement and calculations (using and anthropomorphic phantoms)wR, wT
Operational quantities
• Ambient dose equivalent, H*(10)
• Directional dose equivalent, H'(0.07, Ω)
• Personal dose equivalent, HP (d )
FIG. 1. Conceptual relationship between physical quantities, radiation protection quantities and operational quantities used for radiation protection purposes.