- Email: [email protected]
Individual monitoring for external radiation
18
REPORT OF COMMITTEE 4
be instituted and thus on the sensitivity of this method. The upper end of the range is more difficult to define because of the uncertainty in the frequency at which events are likely to occur. If it can be assumed that an individual will be exposed to such events only a few times a year (say four to six times) the level selected should ensure that the intake is below one twentieth of the ALI, corresponding to a time integral of the air concentration of 100 DAC-hours for a working year of 2 000 h. It may also be useful to set investigation levels for other purposes, e.g. to draw attention to long-term trends. The relevant quantity may then be the estimated annual intake by an individual obtained from his personal air sampler results or the average of the individual results over a period of time for a group of workers. Particle Size Measurements (77) The AL1 and DAC are based on a standard aerosol with an activity median aerodynamic diameter (AMAD) of 1 pm and are expressed as the total activity or concentration over all particle sizes. Air samplers that are not intended to be size-selective give results that adequately reflect these total aerosol values, although their sampling efficiency falls significantly for particles of aerodynamic diameter above about 10 pm. If the air samplers in use are designed to be selective and to sample “respirable” dust, correction factors should be applied to the DAC and other DLs and should also be used in establishing any ILs and in setting authorized limits. If the AMAD of the aerosol is known to be markedly different from 1 pm, the retained fraction will differ from that of the standard aerosol and the need for correction factors should be considered.‘3’
G. INDIVIDUAL
MONITORING
FOR EXTERNAL
RADIATION
(78) The primary objectives of individual monitoring have already been mentioned in para. 35. Supplementary objectives in the case of individual monitoring for external radiation are to provide information about the trends of the doses received by workers, about conditions in workplaces and to furnish information in the event of accidental exposures. The results may also be useful in the event of epidemiological investigations.
The Design of a Monitoring Program The Scale of Service
(79) The first need in designing a program of individual monitoring is to identify the individuals for whom it must be provided. A major factor to be considered is the condition under which the workers are exposed to radiation. The Commission has specified two classes of working conditions which are described in para. 10. The Commission has recommended that workers in Working Condition A should be subject to individual monitoring. For other workers, individual monitoring is not required, the assessment of conditions in the workplace, e.g. by way of monitoring of the workplace, usually being sufficient. However, for external exposure to most types of radiation, individual monitoring is simple and it may be easier to use some additional individual monitoring rather than to adopt a comprehensive program of monitoring of the workplace to establish that the working conditions are satisfactory. (80) In reaching a decision about the appropriate choice of category for conditions of work, it is convenient to consider a range of examples.
GENERAL PRfNCIPLES OF MONITORING FOR RADIATION
PROTECTION OF WORKERS
I9
(a) Work with substantial radiation sources not enclosed in fully interlocked shields. This work clearly belongs to the category specified by Working Condition A. (b) Work similar to example (a), but where experience has shown that individual doses are almost always low but may sometimes rise to three-tenths of the annual dose-equivalent limits. This work includes operations just outside the range of conditions described in examples (c) and (d) and is discussed further in para. 81. (cl Work with adequately shielded sources with means to prevent any exposure other than through the shield. Provided that there are adequate safeguards to ensure satisfactory operation of interlocks, this work will fall into Working Condition B (but see also paras. 82 and 94). Transport workers employed by common carriers will be covered by this example because of the limited dose equivalent rates from packages and the operating procedures laid down for such work. (d) Work with small radioactive sources. It is possible to define limits of activity such that, with elementary precautions, work with such sources falls clearly into Working Condition B. These limits are suggested in Appendix A. (e) Work not involving radiation sources and taking place outside controlled areas. This work falls clearly into Working Condition B even though it may involve occasional access to controlled and supervised areas and some radiation exposure arising from nearby controlled and supervised areas. (81) Many workers covered by example (b) have to be monitored in order to assess their radiation doses, should these become significant, and to identify variations in the conditions in the workplace. Where practicable, the source of such variation should be brought under better control so that the work can be described by example (c). Widespread experience shows that certain classes of work can be controlled by establishing standards of good practice and that, thereafter, individual doses are extremely unlikely to exceed three-tenths of the appropriate annual dose-equivalent limits. Workers in these occupations, which are listed in Appendix A. do not then need individual monitoring. Workers covered by example (a) always need individual monitoring. not only for record purposes, but as part of the Iimitation of individual exposure. The information obtained will also be important in taking steps to improve operating procedures and working conditions, aimed at optimizing the level of protection of the workers. (82) The greatest difficulty in applying these examples will be in drawing the line between examples (b J and (cl. This will often depend on the assessment of the likelihood of accidental exposures and of their probable magnitude.
183 J The basic requirement of a dosemeter system for individual monitoring is that it should permit the estimation of the dose equivalent received with reasonable accuracy over the whole range of radiations and energies and of dose equivalents and dose-equivalent rates likely to be encountered in normal and abnormal operating conditions. Special dosemeters, or special components in general purpose dosemeters, may be necessary to meet this requirement in some kinds of exposure (see paras. 90-97). The necessary sensitivity and accuracy for routine measurements is discussed in the section on interpretation (para. 109).
184) An integral
part of the design of a monitoring
program
should be the specification
of
20
REPORT
OF COMMITTEE
4
how and where the dosemeters are to be worn. In general, a sufficient number of dosemeters should be used to facilitate the estimation of dose-equivalent index, both shallow and deep. However, if only one dosemeter is used, the objective is to place the dosemeter in a position where it will be representative of the most highly exposed part of the surface of the trunk. Doses to the extremities, especially to the hands, may well be somewhat higher, but unless it is likely that these doses will approach three-tenths of the appropriate dose-equivalent limit, additional dosemeters will not be needed. In special situations where installed shielding or protective clothing such as lead aprons provide significant attenuation of the incident radiation on some parts of the body, more than one dosemeter may be required. In particular, the following advice applies in medical radiology, where the use of lead aprons is common. If a single dosemeter is used it should be worn outside the apron, usually high on the trunk. The recorded result will provide information on the dose equivalent to the skin, eye, and unshielded parts of the body (though not necessarily to the hands) but will overestimate the effective dose equivalent. When the recorded values indicate annual totals approaching dose limits for effective dose equivalent or when realistic estimates of effective dose equivalent are needed as in the optimization of protection, this over-estimation may be unacceptable. Two dosemeters should then be used, one over and one under the protective apron. The interpretation of the combined results will have to depend on the local irradiation conditions and any regulatory requirements. The Choice and Type of Dosemererfor
Beta, Gamma and X-Radiation
(85) The choice of a dosemeter will depend not only on the objectives of the monitoring program but also on the method of interpretation to be used. In practice, the basic choice for beta, gamma and x radiation is between a dosemeter giving information on the dose equivalent both at the surface and at a fixed depth, usually of about of 10 mm, and a discriminating device giving some indication of the types of radiation and their effective energies. For a wide range of energies a simple two-element thermoluminescent dosemeter which exhibits small energy dependence, e.g. LiF, with one element covered by a tissue-equivalent filter provides a suitable example of the former type of dosemeter, while a multi-element dosemeter using either photographic film or thermoluminescent elements with filters of materials of different atomic numbers and thicknesses is an example of the second type. (86) As will be shown later in the section on interpretation, measurement at the surface and at a depth of 10 mm will be sufficient in almost all practical cases. Without correction it will give an over-estimate of the dose equivalent to most organs and tissues but the consequences of this over-estimate are unlikely to be serious except for radiation of low penetrating power. The dose to the skin is then likely to be limiting. It is important to note that a simple film badge or a thermoluminescent dosemeter containing elements of higher atomic number, with one filtered and one unfiltered area, does not provide an effective two-element dosemeter, because of the energy-dependent response to electromagnetic radiation of the unfiltered section. Monitoring for Neutrons (87) The principles of design of a program of individual monitoring for neutrons are the same as those for other radiations, but in practice the design may be modified by other factors. In spite of significant advances personal dosemeters are not yet satisfactory for neutrons in some energy regions and, except for thermal neutrons, the current methods of assessment are timeconsuming and expensive. However, neutron dose equivalents are often small compared with the dose-equivalent limits and contribute only a fraction of the total dose equivalent because of the presence of accompanying gamma radiation. Nevertheless, if these operations involve
GENERAL
PRINCIPLES
OF MONITORING
FOR RADIATION
PROTECTION
OF WORKERS
‘1
unshielded or only moderately shielded neutron sources, exposure to neutrons should be monitored with a simple type of dosemeter capable of providing information on the dose equivalent from neutrons of intermediate and thermal energies. (88) In some situations where the neutron spectrum in the workplace does not vary greatly, relatively simple systems such as the albedo dosemeter can be used to advantage. Albedo dosemeters measure the easily detected low energy neutrons scattered back to the dosemeter from within the body. They have the advantages of simplicity and high sensitivity to a wide range of incident neutron energies but their sensitivity varies widely with energy. They must therefore be calibrated at an energy representative of that of the neutrons expected in the workplace and, if the neutron energy spectrum is variable, as is often the case, significanl inaccuracies can occur. (89) Monitoring for incident thermal neutrons is easy to carry out, for example. by the measurement of capture gamma rays in cadmium by a photographic film. However, thermal neutrons deliver much smaller dose equivalents to tissue than does the same fluence of high energy neutrons. Monitoring for thermal neutrons will therefore be useful only in exceptional situations where the neutron spectrum is largely limited to the thermal region. Furthermore, as a result of interaction of neutrons with materials encountered in the working environment and the tissues that are irradiated a neutron dosemeter should always be accompanied by an appropriate gamma dosemeter, even where the relative contribution of neutrons to the total dose equivalent is substantial, such as with neutron generators, particle accelerators, 2s2Cf and other neutron sources. In some specific situations the estimate of neutron dose equivalent may be derived from the ratio of the dose equivalents from gamma radiation and neutron radiation. This procedure is, however, only valid if the ratio of these dose equivalents has been shown to be sufficiently constant. High neutron doses due to accidents can be crudely assessed without the aid of personal dosemeters by measuring neutron-induced activities in the body and using information on the energy spectrum of the neutrons. (201(In this context see also paras. 95-96). Operariotlal
Monitoring
with Personal
Dosemeters
(90) Operations in regions of high dose-equivalent index rate often require the use of a supplementary dosemeter to give early information about the dose equivalent incurred. Ease of reading and the provision of audible or visual warnings may be more important functions than precise dosimetry. It is important to remember that many instruments of this type are sensitive onl! to x and gamma radiation and may be seriously misleading in circumstances where substantial beta or neutron dose equivalents may be incurred. 19 I I In some operations, short-period exposures in high radiation fields may be necessary. Such operations must be very closely controlled and it will usually be appropriate to design momtormg programs involving several personal dosemeters on each worker. Direct reading and warning dosemeters may have a special part to play. These monitoring programs must be deGgned around the individual operation and therefore no generalized guidance is possible. .Spcciol .Wonitorinq
of Accidental
(91) The assessment
Esposurrs
of dose equivalent in minor accidents is adequately covered by the issue of dosemeters to workers covered by examples (a) and (b) of pal-a. 80. Dose equivalents likely to be received in these events are not such as to require special attention in the design of the dosemeter5. (931 In \ome circumstances, severe exposures are possible. Five typical situations cover
22
REPORT OF COMMITTEE 4
(a) operational errors or equipment failures may occur when large amounts of radioactive material are being transferred within or between shields. (b) failure of interlocks may occur with equipment capable of delivering large dose-equivalent rates, e.g. x-ray sets, accelerators, source activities of the order of some tens of tera bequerel or greater, or with hot cells; (c) radioactive sources used for radiographic purposes may inadvertently be left unshielded; (d) the handling of large amounts of fissile material may give rise to criticality accidents; and (e) failure of equipment or operational errors may occur in nuclear reactors or reactor fuel reprocessing plants. (94) The wearing of warning dosemeters (or dose-rate meters) will usually prevent serious exposures in cases (a) to (c) above and may aid in reducing considerably the dose equivalent incurred in cases (d) or (e). This preventive function is much more important than the mere assessment of dose equivalent provided by most personal monitors. Warning dosemeters need not be very accurate, but should be very reliable. Other personal dosemeters will be needed only if they are required by the policy indicated in paras. 80-82. (95) In the case of a criticality accident, a more reliable and forceful warning can be provided by installed equipment. The scale of the accident is difficult to forecast and it may give rise to significant dose equivalents outside the operating area in which the accident occurs. However, in the majority of those accidents that have occurred, (17)the magnitude has been in the range of 1016 to 10” fissions. Unless otherwise dictated a magnitude of 5 x 1018 fissions may be used for planning purposes. It is convenient to define three groups of workers in order to develop a monitoring policy for criticality accidents. The first group comprises those working in the operating areas where a criticality accident is possible. Doses absorbed by some of these workers may well be high in the event of an accident and it will usually be justifiable to issue them with a special dosemeter providing information on the absorbed dose due to gamma rays up to at least 10 Gy and information on the orientation of the individual in relation to the source of radiation. Furthermore, it will also be justifiable to issue special dosemeters for neutrons which make use of activation or fissile foils from which the incident neutron spectrum can be derived. From this information and the orientation of the exposed individual the absorbed dose to neutrons can be estimated. Such dosemeters would be read only in the event of an accident. The second group of workers comprises those in nearby areas. Dose equivalents may well exceed three-tenths of the annual dose-equivalent limits but will not be expected to have clinical significance. This group may contain individuals who are not normally present in the area. It is desirable to identify exposed people in this group for further investigation and to reassure others that their exposure has been low. (96) A simple indicator of neutron exposure, such as indium foil, is particularly useful for screening both these groups of individuals; it can be supplemented or substituted by simple measurement of the radiation emitted by the 24Na and 38CI in the body, although this is slower and slightly less convenient. Further indicators of severe neutron exposure are activated metal objects carried on the body. The determination of the 32P content of hair, or clothing containing wool, after exposure can provide useful information on the hardness of the neutron spectrum and on the orientation of the individual during the exposure. On some sites, the group of exposed individuals may include the majority of employees, since the possibility cannot be excluded that they may happen to be in an area, such as a roadway, adjacent to the operating area where a criticality accident occurs. On other sites, however, there may be a third group who have no access to areas adjacent to the accident area and for whom no criticality monitoring is
GENERAL
PRINCIPLES
OF MONITORING
FOR RADIATION
PROTECTION
OF WORKERS
23
necessary. Much of this and the preceding paragraph applies equally well to reactor accidents, although these involve less likelihood of severe neutron exposures. (97) A most important technique that can be used for assessing accidental exposure of workers is that of chromosome aberration analysis. (18s19)It is particularly useful for assessing exposures to x and gamma radiation for which activation measurements are not applicable. The technique can be used for absorbed doses from x or gamma radiation in excess of 0.1 Gy. It can provide the whole-body dose when exposure is essentially uniform and a rough estimate of average whole-body dose for non-uniform exposure, where an assessment by means of personal dosemeters is usually difficult and often uncertain. A further advantage is that the technique can unequivocally establish whether a substantial exposure has occurred. Furthermore, the technique has proved to be very valuable in circumstances where persons without personal dosemeters were accidentally exposed.
The Interpretation of Results (98) When a worker is exposed to an external radiation field, there is a highly complex relationship between the sources of radiation and the dose equivalent to the organs and tissues of the worker’s body. Within the workplace, the dose-equivalent index rate varies as a function of position and time whilst, within the worker’s body, the dose equivalent in an organ or tissue is related to the dose equivalent at the surface by factors such as the type and quality of the radiation, the orientation of the worker relative to the radiation field and the position and composition of the organs and tissues within the body. Several of these factors will be functions of both time and position within the workplace. (99) The concept of dose-equivalent index, as first defined by the ICRU and adopted by the (i.*) is difficult to apply in practice to the individual Commission in its Recommendations, monitoring of workers, but these difficulties are not insurmountable. (100) A dosemeter worn on the surface of the worker’s body is best regarded as a sampling device. It provides a measure of the dose equivalent to the skin in the immediate vicinity of the dosemeter and to immediately underlying tissue in this region. It does not, in general, provide an estimate of the dose equivalent to other organs and tissues. Neither does it necessarily provide an adequate estimate of the situation in the workplace, because it provides a measure of the dose equivalent only at one point on the body of each worker. (101) However, a personal dosemeter on a phantom that adequately represents the backscatter characteristics of a 30 cm diameter sphere can be calibrated in terms of the measured or calculated values of the deep and shallow dose-equivalent indices. When worn on the body of a person facing a unidirectional field of radiation, it will then, with sufficient accuracy, indicate the deep and shallow dose-equivalent indices. In the practical case, where the worker moves about the workplace, resulting effectively in a multidirectional field of incident radiation, a personal dosemeter, even a simple two element device, thus calibrated, will still, for most practical situations, provide an adequate measure of the deep and shallow dose-equivalent indices within the accuracy required as indicated in para. 109. Furthermore it should be recognized that the deep dose-equivalent index will for all likely combinations of exposure overestimate the effective dose equivalent and the overestimation may, in some cases, be substantial. (102) The use of a personal dosemeter interpreted to estimate dose equivalent close to the body surface and at a depth of about 10 mm will provide the information needed to apply para. 110 of ICRP Publication 26 insofar as it applies to external radiation and will provide for a
24
REPORT OF COMMITTEE 4
standard of protection at least as good as that recommended in para. 104 of ICRP Publication 26. (103) An alternative practical approach that may be appropriate in certain workplaces is to calibrate the personal dosemeter against a fully instrumented phantom exposed in a manner that adequately represents the movements ofworkers in the workplaces in relation to the source of radiation. (104) It is only in exceptional cases, usually associated with substantial accidental exposures, that attempts need to be made to estimate the actual organ and tissue dose equivalent. The greatest uncertainties in the assessment of organ and tissue doses from routine monitoring are caused by the fact that a small number of dosemeters, often only one, has to be taken as representative of the exposure of the whole surface of the body. Unless the dosemeter results are representative, there is little point in conducting detailed calculations of the depth dose in the human body. In these cases it may be useful to reconstruct the accident in order to assess the absorbed doses and dose equivalents in various parts of the body. Routine
Monitoring
(105) The need to use many dosemeters and the complexity of assessing the actual organ and tissue dose equivalents are prohibitive in routine individual monitoring and a simplified system based on the deep and shallow dose-equivalent indices has to be adopted. (106) The limits most commonly needed in the control of external radiation are those for effective dose equivalent, dose equivalent to skin and dose equivalent to the lens of the eye. In the majority of all practical situations a dosemeter indicating the dose equivalent at the surface and the dose equivalent at a depth of 10 mm will provide adequate control if the measurement of dose equivalent at the surface is related to the dose-equivalent limit for skin, and the measurement at depth is related to the dose-equivalent limit for the effective dose equivalent. The lens of the eye, which is intermediate in depth between the depths of the two other measurements, will then in most cases be adequately protected. It is possible to postulate situations in which the exposure is predominantly due to high energy beta radiation in which it would be theoretically possible to expose the lens of the eye above the dose-equivalent limit while exposure of the skin and the effective dose equivalent remained below the relevant limits. However, the requirement to keep all exposures as low as reasonably achievable, such as requiring workers to wear protective glasses, will prevent such situations from being of any practical significance. The position of the dosemeter is taken to be representative of the whole area of the surface of the body, or, if the radiation fields are markedly inhomogeneous, and several dosemeters are worn, each is regarded as representative of a substantial area of the surface of the body. (107) In most situations, this procedure for interpretation adequately meets the objectives of routine individual monitoring, without being unreasonably restrictive, except in instances of extremely non-uniform radiation fields. ( 108) In cases where the radiation field giving rise to the exposure is, for example, composed largely of narrow beams of radiation no practicable combination of dosemeters worn on the surface of the body can provide a representative estimate of the dose equivalents to organs and tissues of the body. Satisfactory personal monitoring is not practicable in this situation, and control must be achieved by monitoring of the workplace, and limiting access to the beams. The Accuracy
Required
in Routine
Monitoring
C109 J The uncertainties acceptable in routine individual monitoring for external radiation should be somewhat less than the investigation level and can best be expressed in relation to the
GENERAL PRINCIPLES OF MONITORING
FOR RADIATION
PROTECTION OF WORKERS
25
estimates of the annual deep and shallow dose-equivalent indices that are measured. The uncertainty in the measurement of the annual value of these quantities (or of the upper limits if a cautious interpretation is being conducted) should be reduced as far as reasonably achievable. If these quantities are of the order of the relevant annual limits, the uncertainties should not exceed a factor of 1.5 at the 95% confidence level. Where they amount to less than 10 mSv an uncertainty of a factor of 2 at the 95% confidence level is acceptable. This uncertainty includes errors due to variations in the dosemeter sensitivity with incident energy and direction of incidence, as well as intrinsic errors in the dosemeter and its calibration. It does not include uncertainties in deriving tissue or organ dose equivalents from the dosemeter results. Monitoring
of Accidental
Exposures
(110) In minor accidents, when the deep dose-equivalent index is only slightly above the limit, the organ and tissue dose equivalents themselves may still comply with the annual limit for effective dose equivalent. Information on the energy spectrum and orientation of the incident radiation may then allow more realistic estimates of these dose equivalents to be made. This information will usually be available from a knowledge of the radiation source and the methods of work, but if this is not so, and if such exposures are common, it may be worthwhile to use dosemeters which provide the necessary spectral information. Data on the orientation can be obtained by using a suitable dosemeter such as a film dosemeter, by using several dosemeters, or by interrogation of the workers. (111) In major accidents, when the absorbed doses may be high enough to require attention from a medical standpoint, an early indication of these doses will be required. In practice, uncertainties will often arise about the magnitude and distribution of the absorbed doses within the body. Resolution of such uncertainties will require a more exact determination of absorbed doses than is possible with the personal dosemeters in use and, in extreme cases, it may be necessary to reconstruct the radiation fields causing the exposures. The personal dosemeter should provide important points of reference and additional information will be available from knowledge about the accident. In the case of criticality accidents, the interpretation of absorbed doses from neutrons will be greatly aided by a knowledge of the energy spectrum and spatial distribution of the neutrons. For this reason, special energy-dependent components are usually incorporated in accident dosemeters, of which several may be worn or located at strategic positions in the workplace to give information on the orientation and extent of such exposure. When a comprehensive reconstruction of the radiation exposure is required, it will usually be appropriate to make measurements of the fundamental characteristics of the radiation fields in and around a lifelike phantom as well as to attempt the direct measurement ofthe absorbed dose at various points. For example, a knowledge of the fluence and energy spectrum at a number of points will facilitate calculations of the different components of the absorbed dose and will supplement the direct measurements of absorbed dose. Measurements of this complexity are beyond the scope of a routine personal dosemeter, which should therefore be regarded primarily as a device for assessing the scaling factor needed to adjust the reconstruction to correspond with the original accident (see also paras. 95-96). The principles and general procedures for handling accidental exposures are described in ICRP Publication 28.” 2’
H. MONITORING
FOR SKIN CONTAMINATION
( 112) One contribution to external irradiation of the body is that from skin contamination. This is never uniform and occurs preferentially on certain parts of the body. notably the hands.
REPORT OF COMMITTEE 4
be instituted and thus on the sensitivity of this method. The upper end of the range is more difficult to define because of the uncertainty in the frequency at which events are likely to occur. If it can be assumed that an individual will be exposed to such events only a few times a year (say four to six times) the level selected should ensure that the intake is below one twentieth of the ALI, corresponding to a time integral of the air concentration of 100 DAC-hours for a working year of 2 000 h. It may also be useful to set investigation levels for other purposes, e.g. to draw attention to long-term trends. The relevant quantity may then be the estimated annual intake by an individual obtained from his personal air sampler results or the average of the individual results over a period of time for a group of workers. Particle Size Measurements (77) The AL1 and DAC are based on a standard aerosol with an activity median aerodynamic diameter (AMAD) of 1 pm and are expressed as the total activity or concentration over all particle sizes. Air samplers that are not intended to be size-selective give results that adequately reflect these total aerosol values, although their sampling efficiency falls significantly for particles of aerodynamic diameter above about 10 pm. If the air samplers in use are designed to be selective and to sample “respirable” dust, correction factors should be applied to the DAC and other DLs and should also be used in establishing any ILs and in setting authorized limits. If the AMAD of the aerosol is known to be markedly different from 1 pm, the retained fraction will differ from that of the standard aerosol and the need for correction factors should be considered.‘3’
G. INDIVIDUAL
MONITORING
FOR EXTERNAL
RADIATION
(78) The primary objectives of individual monitoring have already been mentioned in para. 35. Supplementary objectives in the case of individual monitoring for external radiation are to provide information about the trends of the doses received by workers, about conditions in workplaces and to furnish information in the event of accidental exposures. The results may also be useful in the event of epidemiological investigations.
The Design of a Monitoring Program The Scale of Service
(79) The first need in designing a program of individual monitoring is to identify the individuals for whom it must be provided. A major factor to be considered is the condition under which the workers are exposed to radiation. The Commission has specified two classes of working conditions which are described in para. 10. The Commission has recommended that workers in Working Condition A should be subject to individual monitoring. For other workers, individual monitoring is not required, the assessment of conditions in the workplace, e.g. by way of monitoring of the workplace, usually being sufficient. However, for external exposure to most types of radiation, individual monitoring is simple and it may be easier to use some additional individual monitoring rather than to adopt a comprehensive program of monitoring of the workplace to establish that the working conditions are satisfactory. (80) In reaching a decision about the appropriate choice of category for conditions of work, it is convenient to consider a range of examples.
GENERAL PRfNCIPLES OF MONITORING FOR RADIATION
PROTECTION OF WORKERS
I9
(a) Work with substantial radiation sources not enclosed in fully interlocked shields. This work clearly belongs to the category specified by Working Condition A. (b) Work similar to example (a), but where experience has shown that individual doses are almost always low but may sometimes rise to three-tenths of the annual dose-equivalent limits. This work includes operations just outside the range of conditions described in examples (c) and (d) and is discussed further in para. 81. (cl Work with adequately shielded sources with means to prevent any exposure other than through the shield. Provided that there are adequate safeguards to ensure satisfactory operation of interlocks, this work will fall into Working Condition B (but see also paras. 82 and 94). Transport workers employed by common carriers will be covered by this example because of the limited dose equivalent rates from packages and the operating procedures laid down for such work. (d) Work with small radioactive sources. It is possible to define limits of activity such that, with elementary precautions, work with such sources falls clearly into Working Condition B. These limits are suggested in Appendix A. (e) Work not involving radiation sources and taking place outside controlled areas. This work falls clearly into Working Condition B even though it may involve occasional access to controlled and supervised areas and some radiation exposure arising from nearby controlled and supervised areas. (81) Many workers covered by example (b) have to be monitored in order to assess their radiation doses, should these become significant, and to identify variations in the conditions in the workplace. Where practicable, the source of such variation should be brought under better control so that the work can be described by example (c). Widespread experience shows that certain classes of work can be controlled by establishing standards of good practice and that, thereafter, individual doses are extremely unlikely to exceed three-tenths of the appropriate annual dose-equivalent limits. Workers in these occupations, which are listed in Appendix A. do not then need individual monitoring. Workers covered by example (a) always need individual monitoring. not only for record purposes, but as part of the Iimitation of individual exposure. The information obtained will also be important in taking steps to improve operating procedures and working conditions, aimed at optimizing the level of protection of the workers. (82) The greatest difficulty in applying these examples will be in drawing the line between examples (b J and (cl. This will often depend on the assessment of the likelihood of accidental exposures and of their probable magnitude.
183 J The basic requirement of a dosemeter system for individual monitoring is that it should permit the estimation of the dose equivalent received with reasonable accuracy over the whole range of radiations and energies and of dose equivalents and dose-equivalent rates likely to be encountered in normal and abnormal operating conditions. Special dosemeters, or special components in general purpose dosemeters, may be necessary to meet this requirement in some kinds of exposure (see paras. 90-97). The necessary sensitivity and accuracy for routine measurements is discussed in the section on interpretation (para. 109).
184) An integral
part of the design of a monitoring
program
should be the specification
of
20
REPORT
OF COMMITTEE
4
how and where the dosemeters are to be worn. In general, a sufficient number of dosemeters should be used to facilitate the estimation of dose-equivalent index, both shallow and deep. However, if only one dosemeter is used, the objective is to place the dosemeter in a position where it will be representative of the most highly exposed part of the surface of the trunk. Doses to the extremities, especially to the hands, may well be somewhat higher, but unless it is likely that these doses will approach three-tenths of the appropriate dose-equivalent limit, additional dosemeters will not be needed. In special situations where installed shielding or protective clothing such as lead aprons provide significant attenuation of the incident radiation on some parts of the body, more than one dosemeter may be required. In particular, the following advice applies in medical radiology, where the use of lead aprons is common. If a single dosemeter is used it should be worn outside the apron, usually high on the trunk. The recorded result will provide information on the dose equivalent to the skin, eye, and unshielded parts of the body (though not necessarily to the hands) but will overestimate the effective dose equivalent. When the recorded values indicate annual totals approaching dose limits for effective dose equivalent or when realistic estimates of effective dose equivalent are needed as in the optimization of protection, this over-estimation may be unacceptable. Two dosemeters should then be used, one over and one under the protective apron. The interpretation of the combined results will have to depend on the local irradiation conditions and any regulatory requirements. The Choice and Type of Dosemererfor
Beta, Gamma and X-Radiation
(85) The choice of a dosemeter will depend not only on the objectives of the monitoring program but also on the method of interpretation to be used. In practice, the basic choice for beta, gamma and x radiation is between a dosemeter giving information on the dose equivalent both at the surface and at a fixed depth, usually of about of 10 mm, and a discriminating device giving some indication of the types of radiation and their effective energies. For a wide range of energies a simple two-element thermoluminescent dosemeter which exhibits small energy dependence, e.g. LiF, with one element covered by a tissue-equivalent filter provides a suitable example of the former type of dosemeter, while a multi-element dosemeter using either photographic film or thermoluminescent elements with filters of materials of different atomic numbers and thicknesses is an example of the second type. (86) As will be shown later in the section on interpretation, measurement at the surface and at a depth of 10 mm will be sufficient in almost all practical cases. Without correction it will give an over-estimate of the dose equivalent to most organs and tissues but the consequences of this over-estimate are unlikely to be serious except for radiation of low penetrating power. The dose to the skin is then likely to be limiting. It is important to note that a simple film badge or a thermoluminescent dosemeter containing elements of higher atomic number, with one filtered and one unfiltered area, does not provide an effective two-element dosemeter, because of the energy-dependent response to electromagnetic radiation of the unfiltered section. Monitoring for Neutrons (87) The principles of design of a program of individual monitoring for neutrons are the same as those for other radiations, but in practice the design may be modified by other factors. In spite of significant advances personal dosemeters are not yet satisfactory for neutrons in some energy regions and, except for thermal neutrons, the current methods of assessment are timeconsuming and expensive. However, neutron dose equivalents are often small compared with the dose-equivalent limits and contribute only a fraction of the total dose equivalent because of the presence of accompanying gamma radiation. Nevertheless, if these operations involve
GENERAL
PRINCIPLES
OF MONITORING
FOR RADIATION
PROTECTION
OF WORKERS
‘1
unshielded or only moderately shielded neutron sources, exposure to neutrons should be monitored with a simple type of dosemeter capable of providing information on the dose equivalent from neutrons of intermediate and thermal energies. (88) In some situations where the neutron spectrum in the workplace does not vary greatly, relatively simple systems such as the albedo dosemeter can be used to advantage. Albedo dosemeters measure the easily detected low energy neutrons scattered back to the dosemeter from within the body. They have the advantages of simplicity and high sensitivity to a wide range of incident neutron energies but their sensitivity varies widely with energy. They must therefore be calibrated at an energy representative of that of the neutrons expected in the workplace and, if the neutron energy spectrum is variable, as is often the case, significanl inaccuracies can occur. (89) Monitoring for incident thermal neutrons is easy to carry out, for example. by the measurement of capture gamma rays in cadmium by a photographic film. However, thermal neutrons deliver much smaller dose equivalents to tissue than does the same fluence of high energy neutrons. Monitoring for thermal neutrons will therefore be useful only in exceptional situations where the neutron spectrum is largely limited to the thermal region. Furthermore, as a result of interaction of neutrons with materials encountered in the working environment and the tissues that are irradiated a neutron dosemeter should always be accompanied by an appropriate gamma dosemeter, even where the relative contribution of neutrons to the total dose equivalent is substantial, such as with neutron generators, particle accelerators, 2s2Cf and other neutron sources. In some specific situations the estimate of neutron dose equivalent may be derived from the ratio of the dose equivalents from gamma radiation and neutron radiation. This procedure is, however, only valid if the ratio of these dose equivalents has been shown to be sufficiently constant. High neutron doses due to accidents can be crudely assessed without the aid of personal dosemeters by measuring neutron-induced activities in the body and using information on the energy spectrum of the neutrons. (201(In this context see also paras. 95-96). Operariotlal
Monitoring
with Personal
Dosemeters
(90) Operations in regions of high dose-equivalent index rate often require the use of a supplementary dosemeter to give early information about the dose equivalent incurred. Ease of reading and the provision of audible or visual warnings may be more important functions than precise dosimetry. It is important to remember that many instruments of this type are sensitive onl! to x and gamma radiation and may be seriously misleading in circumstances where substantial beta or neutron dose equivalents may be incurred. 19 I I In some operations, short-period exposures in high radiation fields may be necessary. Such operations must be very closely controlled and it will usually be appropriate to design momtormg programs involving several personal dosemeters on each worker. Direct reading and warning dosemeters may have a special part to play. These monitoring programs must be deGgned around the individual operation and therefore no generalized guidance is possible. .Spcciol .Wonitorinq
of Accidental
(91) The assessment
Esposurrs
of dose equivalent in minor accidents is adequately covered by the issue of dosemeters to workers covered by examples (a) and (b) of pal-a. 80. Dose equivalents likely to be received in these events are not such as to require special attention in the design of the dosemeter5. (931 In \ome circumstances, severe exposures are possible. Five typical situations cover
22
REPORT OF COMMITTEE 4
(a) operational errors or equipment failures may occur when large amounts of radioactive material are being transferred within or between shields. (b) failure of interlocks may occur with equipment capable of delivering large dose-equivalent rates, e.g. x-ray sets, accelerators, source activities of the order of some tens of tera bequerel or greater, or with hot cells; (c) radioactive sources used for radiographic purposes may inadvertently be left unshielded; (d) the handling of large amounts of fissile material may give rise to criticality accidents; and (e) failure of equipment or operational errors may occur in nuclear reactors or reactor fuel reprocessing plants. (94) The wearing of warning dosemeters (or dose-rate meters) will usually prevent serious exposures in cases (a) to (c) above and may aid in reducing considerably the dose equivalent incurred in cases (d) or (e). This preventive function is much more important than the mere assessment of dose equivalent provided by most personal monitors. Warning dosemeters need not be very accurate, but should be very reliable. Other personal dosemeters will be needed only if they are required by the policy indicated in paras. 80-82. (95) In the case of a criticality accident, a more reliable and forceful warning can be provided by installed equipment. The scale of the accident is difficult to forecast and it may give rise to significant dose equivalents outside the operating area in which the accident occurs. However, in the majority of those accidents that have occurred, (17)the magnitude has been in the range of 1016 to 10” fissions. Unless otherwise dictated a magnitude of 5 x 1018 fissions may be used for planning purposes. It is convenient to define three groups of workers in order to develop a monitoring policy for criticality accidents. The first group comprises those working in the operating areas where a criticality accident is possible. Doses absorbed by some of these workers may well be high in the event of an accident and it will usually be justifiable to issue them with a special dosemeter providing information on the absorbed dose due to gamma rays up to at least 10 Gy and information on the orientation of the individual in relation to the source of radiation. Furthermore, it will also be justifiable to issue special dosemeters for neutrons which make use of activation or fissile foils from which the incident neutron spectrum can be derived. From this information and the orientation of the exposed individual the absorbed dose to neutrons can be estimated. Such dosemeters would be read only in the event of an accident. The second group of workers comprises those in nearby areas. Dose equivalents may well exceed three-tenths of the annual dose-equivalent limits but will not be expected to have clinical significance. This group may contain individuals who are not normally present in the area. It is desirable to identify exposed people in this group for further investigation and to reassure others that their exposure has been low. (96) A simple indicator of neutron exposure, such as indium foil, is particularly useful for screening both these groups of individuals; it can be supplemented or substituted by simple measurement of the radiation emitted by the 24Na and 38CI in the body, although this is slower and slightly less convenient. Further indicators of severe neutron exposure are activated metal objects carried on the body. The determination of the 32P content of hair, or clothing containing wool, after exposure can provide useful information on the hardness of the neutron spectrum and on the orientation of the individual during the exposure. On some sites, the group of exposed individuals may include the majority of employees, since the possibility cannot be excluded that they may happen to be in an area, such as a roadway, adjacent to the operating area where a criticality accident occurs. On other sites, however, there may be a third group who have no access to areas adjacent to the accident area and for whom no criticality monitoring is
GENERAL
PRINCIPLES
OF MONITORING
FOR RADIATION
PROTECTION
OF WORKERS
23
necessary. Much of this and the preceding paragraph applies equally well to reactor accidents, although these involve less likelihood of severe neutron exposures. (97) A most important technique that can be used for assessing accidental exposure of workers is that of chromosome aberration analysis. (18s19)It is particularly useful for assessing exposures to x and gamma radiation for which activation measurements are not applicable. The technique can be used for absorbed doses from x or gamma radiation in excess of 0.1 Gy. It can provide the whole-body dose when exposure is essentially uniform and a rough estimate of average whole-body dose for non-uniform exposure, where an assessment by means of personal dosemeters is usually difficult and often uncertain. A further advantage is that the technique can unequivocally establish whether a substantial exposure has occurred. Furthermore, the technique has proved to be very valuable in circumstances where persons without personal dosemeters were accidentally exposed.
The Interpretation of Results (98) When a worker is exposed to an external radiation field, there is a highly complex relationship between the sources of radiation and the dose equivalent to the organs and tissues of the worker’s body. Within the workplace, the dose-equivalent index rate varies as a function of position and time whilst, within the worker’s body, the dose equivalent in an organ or tissue is related to the dose equivalent at the surface by factors such as the type and quality of the radiation, the orientation of the worker relative to the radiation field and the position and composition of the organs and tissues within the body. Several of these factors will be functions of both time and position within the workplace. (99) The concept of dose-equivalent index, as first defined by the ICRU and adopted by the (i.*) is difficult to apply in practice to the individual Commission in its Recommendations, monitoring of workers, but these difficulties are not insurmountable. (100) A dosemeter worn on the surface of the worker’s body is best regarded as a sampling device. It provides a measure of the dose equivalent to the skin in the immediate vicinity of the dosemeter and to immediately underlying tissue in this region. It does not, in general, provide an estimate of the dose equivalent to other organs and tissues. Neither does it necessarily provide an adequate estimate of the situation in the workplace, because it provides a measure of the dose equivalent only at one point on the body of each worker. (101) However, a personal dosemeter on a phantom that adequately represents the backscatter characteristics of a 30 cm diameter sphere can be calibrated in terms of the measured or calculated values of the deep and shallow dose-equivalent indices. When worn on the body of a person facing a unidirectional field of radiation, it will then, with sufficient accuracy, indicate the deep and shallow dose-equivalent indices. In the practical case, where the worker moves about the workplace, resulting effectively in a multidirectional field of incident radiation, a personal dosemeter, even a simple two element device, thus calibrated, will still, for most practical situations, provide an adequate measure of the deep and shallow dose-equivalent indices within the accuracy required as indicated in para. 109. Furthermore it should be recognized that the deep dose-equivalent index will for all likely combinations of exposure overestimate the effective dose equivalent and the overestimation may, in some cases, be substantial. (102) The use of a personal dosemeter interpreted to estimate dose equivalent close to the body surface and at a depth of about 10 mm will provide the information needed to apply para. 110 of ICRP Publication 26 insofar as it applies to external radiation and will provide for a
24
REPORT OF COMMITTEE 4
standard of protection at least as good as that recommended in para. 104 of ICRP Publication 26. (103) An alternative practical approach that may be appropriate in certain workplaces is to calibrate the personal dosemeter against a fully instrumented phantom exposed in a manner that adequately represents the movements ofworkers in the workplaces in relation to the source of radiation. (104) It is only in exceptional cases, usually associated with substantial accidental exposures, that attempts need to be made to estimate the actual organ and tissue dose equivalent. The greatest uncertainties in the assessment of organ and tissue doses from routine monitoring are caused by the fact that a small number of dosemeters, often only one, has to be taken as representative of the exposure of the whole surface of the body. Unless the dosemeter results are representative, there is little point in conducting detailed calculations of the depth dose in the human body. In these cases it may be useful to reconstruct the accident in order to assess the absorbed doses and dose equivalents in various parts of the body. Routine
Monitoring
(105) The need to use many dosemeters and the complexity of assessing the actual organ and tissue dose equivalents are prohibitive in routine individual monitoring and a simplified system based on the deep and shallow dose-equivalent indices has to be adopted. (106) The limits most commonly needed in the control of external radiation are those for effective dose equivalent, dose equivalent to skin and dose equivalent to the lens of the eye. In the majority of all practical situations a dosemeter indicating the dose equivalent at the surface and the dose equivalent at a depth of 10 mm will provide adequate control if the measurement of dose equivalent at the surface is related to the dose-equivalent limit for skin, and the measurement at depth is related to the dose-equivalent limit for the effective dose equivalent. The lens of the eye, which is intermediate in depth between the depths of the two other measurements, will then in most cases be adequately protected. It is possible to postulate situations in which the exposure is predominantly due to high energy beta radiation in which it would be theoretically possible to expose the lens of the eye above the dose-equivalent limit while exposure of the skin and the effective dose equivalent remained below the relevant limits. However, the requirement to keep all exposures as low as reasonably achievable, such as requiring workers to wear protective glasses, will prevent such situations from being of any practical significance. The position of the dosemeter is taken to be representative of the whole area of the surface of the body, or, if the radiation fields are markedly inhomogeneous, and several dosemeters are worn, each is regarded as representative of a substantial area of the surface of the body. (107) In most situations, this procedure for interpretation adequately meets the objectives of routine individual monitoring, without being unreasonably restrictive, except in instances of extremely non-uniform radiation fields. ( 108) In cases where the radiation field giving rise to the exposure is, for example, composed largely of narrow beams of radiation no practicable combination of dosemeters worn on the surface of the body can provide a representative estimate of the dose equivalents to organs and tissues of the body. Satisfactory personal monitoring is not practicable in this situation, and control must be achieved by monitoring of the workplace, and limiting access to the beams. The Accuracy
Required
in Routine
Monitoring
C109 J The uncertainties acceptable in routine individual monitoring for external radiation should be somewhat less than the investigation level and can best be expressed in relation to the
GENERAL PRINCIPLES OF MONITORING
FOR RADIATION
PROTECTION OF WORKERS
25
estimates of the annual deep and shallow dose-equivalent indices that are measured. The uncertainty in the measurement of the annual value of these quantities (or of the upper limits if a cautious interpretation is being conducted) should be reduced as far as reasonably achievable. If these quantities are of the order of the relevant annual limits, the uncertainties should not exceed a factor of 1.5 at the 95% confidence level. Where they amount to less than 10 mSv an uncertainty of a factor of 2 at the 95% confidence level is acceptable. This uncertainty includes errors due to variations in the dosemeter sensitivity with incident energy and direction of incidence, as well as intrinsic errors in the dosemeter and its calibration. It does not include uncertainties in deriving tissue or organ dose equivalents from the dosemeter results. Monitoring
of Accidental
Exposures
(110) In minor accidents, when the deep dose-equivalent index is only slightly above the limit, the organ and tissue dose equivalents themselves may still comply with the annual limit for effective dose equivalent. Information on the energy spectrum and orientation of the incident radiation may then allow more realistic estimates of these dose equivalents to be made. This information will usually be available from a knowledge of the radiation source and the methods of work, but if this is not so, and if such exposures are common, it may be worthwhile to use dosemeters which provide the necessary spectral information. Data on the orientation can be obtained by using a suitable dosemeter such as a film dosemeter, by using several dosemeters, or by interrogation of the workers. (111) In major accidents, when the absorbed doses may be high enough to require attention from a medical standpoint, an early indication of these doses will be required. In practice, uncertainties will often arise about the magnitude and distribution of the absorbed doses within the body. Resolution of such uncertainties will require a more exact determination of absorbed doses than is possible with the personal dosemeters in use and, in extreme cases, it may be necessary to reconstruct the radiation fields causing the exposures. The personal dosemeter should provide important points of reference and additional information will be available from knowledge about the accident. In the case of criticality accidents, the interpretation of absorbed doses from neutrons will be greatly aided by a knowledge of the energy spectrum and spatial distribution of the neutrons. For this reason, special energy-dependent components are usually incorporated in accident dosemeters, of which several may be worn or located at strategic positions in the workplace to give information on the orientation and extent of such exposure. When a comprehensive reconstruction of the radiation exposure is required, it will usually be appropriate to make measurements of the fundamental characteristics of the radiation fields in and around a lifelike phantom as well as to attempt the direct measurement ofthe absorbed dose at various points. For example, a knowledge of the fluence and energy spectrum at a number of points will facilitate calculations of the different components of the absorbed dose and will supplement the direct measurements of absorbed dose. Measurements of this complexity are beyond the scope of a routine personal dosemeter, which should therefore be regarded primarily as a device for assessing the scaling factor needed to adjust the reconstruction to correspond with the original accident (see also paras. 95-96). The principles and general procedures for handling accidental exposures are described in ICRP Publication 28.” 2’
H. MONITORING
FOR SKIN CONTAMINATION
( 112) One contribution to external irradiation of the body is that from skin contamination. This is never uniform and occurs preferentially on certain parts of the body. notably the hands.