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Health Consequences of Releases
CHAPTER
HEALTH CONSEQUENCES OF RELEASES
7
7.1 THE PRINCIPLES OF HEALTH PROTECTION AND SAFETY The principles of radiation protection and safety as summarized by the IAEA (INSAG 12) and based on ICRP (2007) are •
• •
• • • • •
A practice which entails or that could entail exposure to radiation should only be adopted if it yields sufficient benefit to the exposed individuals or to society to outweigh the radiation detriment it causes or could cause (justification principle). Individual doses due to the combination of exposures from all relevant practices should not exceed specified dose limits (limitation principle). Radiation sources and installations should be provided with the best available protection and safety measures under the prevailing circumstances, so that the magnitudes and likelihood of exposures and the number of individuals exposed be as low as reasonably achievable, economic and social factors being taken into account, and the doses they deliver and the risks they entail be constrained [optimization principle or as low as reasonably achievable (ALARA)]. Radiation exposures that are not part of a practice should be reduced by intervention when this is justified, and the intervention measures should be optimized. The legal person authorized to engage in a practice involving a source of radiation should bear the primary responsibility for protection and safety. A safety culture should be inculcated that governs the attitudes and behavior in relation to protection and safety of all the individuals and organizations dealing with sources of radiation. In depth defensive measures should be incorporated into the design and operating procedures for radiation sources to compensate for potential failures in protection and safety measures. Protection and safety should be ensured by sound management and good engineering, quality assurance, training and qualification of personnel, comprehensive safety assessments, and attention to lessons learned from experience and research.
7.2 SOME QUANTITIES, TERMS, AND UNITS OF MEASURE OF HEALTH PHYSICS Absorbed dose: the average energy imparted by an ionizing radiation to the mass unity of a matter, unit of measure: gray (Gy) 5 1 J/kg. Nuclear Safety. DOI: https://doi.org/10.1016/B978-0-12-818326-7.00007-X © 2020 Elsevier Ltd. All rights reserved.
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Dose: This term has two meanings: • •
a measure of the quantity of radiation present in a radiation field or given by this field: notion expressed by the word “exposure”; a measure of the radiation received or absorbed by a target.
Effective dose: The summation of the tissue equivalent doses, each multiplied by the appropriate tissue weighting factor. Equivalent dose: The dose absorbed by a tissue or organ, multiplied for the pertinent radiation type weighting factor. Unit of measure: sievert (Sv) 5 1 J/kg (sometimes, the previous unit, the rem 5 1/100 Sv, is still used). Genetic effects: The effects on genetic material of somatic or germ cells, used in an imprecise way as a synonym of “hereditary effects.” Hereditary effects: The effects which manifest themselves in descendants of the exposed individual. Nonstocastic (deterministic) effects: The effects for which generally a threshold level of dose exists above which the severity of the effect is greater for a higher dose. Radiation weighting factor: A multiplication factor for the absorbed dose which accounts for the relative effectiveness of the various types of radiation in inducing health effects (see Table 7.1). Radioactivity: The radioactivity of a sample is the number of disintegrations per second. Unit of measure: becquerel (Bq) 5 disintegrations second21 [sometimes, the previous unit, the curie (Ci) 5 37 GBq is still used (1 TBq, a frequently used unit, is thus equal to about 27 Ci)]. Somatic effects: The effects that manifest themselves in the exposed individual. Stochastic effects: The radiation effects, generally occurring without a threshold level of dose, whose probability is proportional to the dose and whose severity is independent of the dose. Tissue weighting factors: To account for the different sensitivity of organs and tissues to the induction of stochastic effects of radiation (see Table 7.2).
Table 7.1 Radiation Weighting Factors. Type and Energy Range of Radiation
Radiation Weighting Factor
Photons Electrons and muons Neutrons ,10 keV Neutrons 10 100 keV Neutrons 100 keV 2 MeV Neutrons 2 20 MeV Neutrons .20 MeV Protons (except recoil protons) .20 MeV α-Particles, fissile fragments, heavy nuclei
1 1 5 10 20 10 5 5 20
7.3 TYPES OF EFFECTS OF RADIATION DOSES AND LIMITS
105
Table 7.2 Tissue Weighting Factors. Tissue or Organ
Weighting Factor
Gonads Bone marrow (red) Colon Lung Stomach Bladder Breast Liver Oesophagus Thyroid Skin Bone surface Remainder
0.2 0.12 0.12 0.12 0.12 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.05
7.3 TYPES OF EFFECTS OF RADIATION DOSES AND LIMITS So far as the deterministic effects are concerned, the following brief and imprecise facts should be remembered: • • •
The lethal dose at 50% probability (LD50) is equal to about 3 5 Gy, in the absence of a good medical assistance. Impairment of vision may happen between 1 and 10 Gy, according to the type of radiation (high or low linear energy transfer). Permanent sterility may occur between 2.5 and 6 Gy. For the stochastic effects in the population, the following, again brief, reference data should be noted:
• • • • •
Death risk for low doses 5 5 3 1022/Sv. Risk of serious effects in descendants 5 0.5 1.3 3 1022/Sv. As far as the limits adopted in many countries by law are concerned, we have: for workers: 20 mSv for solar year (effective dose); for the population: 10 μSv/year for each practice.
These limits hold for normal operation of the plants and not for accidents. Indeed, other limits for accidents do not exist except those fixed by the local regulatory body, case by case, or for classes of plants and of sources, for example, for a nuclear power station, the most recent trend in Italy was to prevent the overcoming of the reference values for short-term evacuation of the population (taken as equal to 1 rem, which is the lowest value named in foreign and international guidelines) in case of a severe accident. Moreover, it is usual to define a design limit for the collective dose of workers: the present value in many countries is of the order of 1 Sv person/year.
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CHAPTER 7 HEALTH CONSEQUENCES OF RELEASES
7.4 EVALUATION OF THE HEALTH CONSEQUENCES OF RELEASES As elsewhere in this book, here only simple methods and orders of magnitude are listed which can be useful for quick dose evaluations for preliminary decisions: more precise methods are described in the references and in the abundant literature in the field.
7.4.1 EVALUATION OF INHALATION DOSES FROM RADIOACTIVE IODINE The following is a simple formula that can be used for a quick evaluation. It is most easily remembered if the old units of measurement are used (curie, rem, etc.).1 D 5 10 3 χ 3 R
where D is the effective dose for adults (rem) (for children a multiplication factor ranging from 5 to 10, according to age, has to be used), χ is the cloud concentration (s/m3) (see also Chapter 6: The Dispersion of Radioactivity Releases), and R are the curies of iodine-131 released.2 The dose calculated for all the iodine isotopes (not just for iodine-131) could result in a dose of the order of double that calculated for iodine-131 only. The dose to the thyroid is equal to about 20 times the one here calculated.
7.4.2 EVALUATION OF DOSES DUE TO SUBMERSION IN A RADIOACTIVE CLOUD In some cases the term “submersion doses” may not be appropriate because what is generally meant with this expression are the doses of direct radiation from a cloud of radioactive substances traveling in the vicinity. Here xenon-133 (important for accidents to reactors or to gaseous waste decay tanks) and tritium (e.g., 3H, important for fusion machines) are considered. The doses are roughly: For xenon-133 D5
χ3R 300
which can give lower dose values than other models [this has been taken from Commission of European Communities (CEC) documents]. In order to take into account the finite dimensions of the cloud, the calculated doses should be multiplied by a factor (,1) which, for ground release and for F category ranges from 0.1 at 1 km to 0.7 at 100 km. For tritium (skin irradiation, inhalation): D 5 0:03 3 χ 3 R
1 Relaxation moment! Concerning the use of obsolete units of measure, the subtle truth contained in a popular joke comes to mind. It concerns a professor, very popular with his students, who, answering a question about the reason why he taught so many incorrect notions in his lessons, replied, “This way they understand better!.” 2 It is worth recalling from Chapter 6, The Dispersion of Radioactivity Releases, that χ can conservatively be assumed to equal 1024 1023 s/m3 (Pasquill category F with a wind speed of 2 m/s) at 1 km and variable for other distances as the inverse of the ratio of the distances raised to the power of 1.5 2.
7.4 EVALUATION OF THE HEALTH CONSEQUENCES OF RELEASES
107
Table 7.3 Ground-shine Dose (Caesium-137). First Year
Second Year
0 50 Years
1.2 mSv 120 mrem
800 μSv 80 mrem
16 mSv 1600 mrem
7.4.3 EVALUATION OF THE DOSES OF RADIATION FROM CAESIUM-137 DEPOSITED ON THE GROUND (“GROUND-SHINE” DOSE) The figures of interest for any practical case can be extrapolated from the data shown in Table 7.3, which gives the dose at various times after the deposition of 1 kBq/m2. For contamination deriving from an accident to a reactor, the radiation doses from the ground due to caesium-137 are generally more important than the contribution of other isotopes.
7.4.4 EVALUATION OF THE DOSE DUE TO DEPOSITION OF PLUTONIUM ON THE GROUND A deposition of plutonium might happen as the result of an accident to a space vehicle (238Pu) (see Chapter 26, Nuclear Facilities on Satellites) or because of a very violent accident to a nuclear reactor (239Pu and 240Pu). Plutonium isotopes are highly radiotoxic but plutonium is highly insoluble and in general the highest risk originates from the inhalation of very fine dusts (B5 μm). The conversion factor for the inhalation dose to adult is, for plutonium-238, in average conditions, 4.6 3 1025 Sv/Bq.AR29 Similar figures apply to plutonium-239 and plutonium-240. The mechanisms by which the plutonium might be inhaled are to be evaluated case by case. The specific activity of plutonium-238 is 6.44 3 105 Bq/μg and 2300 Bq/μg for plutonium-239.
7.4.5 INDICATIVE EVALUATION OF LONG DISTANCE DOSES FOR VERY SERIOUS ACCIDENTS TO NUCLEAR REACTORS Fig. 7.1 gives a first impression of possible effective committed doses. It shows data from the Chernobyl, Windscale, and Three Mile Island accidents (collected by G. Santarossa), together with a subjective evaluation of the effects of a maximum severe accident “reasonably” conceivable for a present and future reactor.
7.4.6 DIRECT RADIATION DOSES It is often useful to have an idea of the possible radiation fields caused by a point source. The approximate formula to remember is the following (see Note 1): Rhm 5 0:6 3 C 3 E
where Rhm is rems per hour at 1 m distance in air, C is the source curies, and E is the energy of the emitted radiation (MeV). Fig. 7.2 can also be of help.3 3 As an example, one curie of cobalt-60, which emits γ-radiation at a total of 2.5 MeV, delivers about 1.5 rem/h at the distance of 1 m.
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CHAPTER 7 HEALTH CONSEQUENCES OF RELEASES
Dose (mSv)
100
10
1
0.1
1
10
100
1000
FIGURE 7.1 Long range doses from accidents.
1 Ci
1 Mev
1m
FIGURE 7.2 Activity dose relationship.
0.6 rem h
Range (km)
7.4 EVALUATION OF THE HEALTH CONSEQUENCES OF RELEASES
Air
1000 m
Water
34 cm
Concrete
16.5 cm
Glass (Ce or Pb)
5–15 cm
Steel
5 cm
Lead
3 cm
FIGURE 7.3 Thicknesses of materials for reduction of 10 in γ-ray intensity.
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CHAPTER 7 HEALTH CONSEQUENCES OF RELEASES
Remember that α-rays are stopped by the thickness of a simple sheet of paper, while β-rays can penetrate several centimeters into human body tissue. γ-rays or neutrons can penetrate much deeper into matter. Fig. 7.3 shows the thickness of various materials able to reduce the intensity of γ-rays by a factor of 10. As can be seen, there is a certain inverse proportionality to the material density.
REFERENCES ICRP, 2007. Recommendations of the International Commission on Radiological Protection. ICRP Publication 103, Pergamon Press. INSAG-12, 1999. Basic Safety Principles for Nuclear Power Plants.
HEALTH CONSEQUENCES OF RELEASES
7
7.1 THE PRINCIPLES OF HEALTH PROTECTION AND SAFETY The principles of radiation protection and safety as summarized by the IAEA (INSAG 12) and based on ICRP (2007) are •
• •
• • • • •
A practice which entails or that could entail exposure to radiation should only be adopted if it yields sufficient benefit to the exposed individuals or to society to outweigh the radiation detriment it causes or could cause (justification principle). Individual doses due to the combination of exposures from all relevant practices should not exceed specified dose limits (limitation principle). Radiation sources and installations should be provided with the best available protection and safety measures under the prevailing circumstances, so that the magnitudes and likelihood of exposures and the number of individuals exposed be as low as reasonably achievable, economic and social factors being taken into account, and the doses they deliver and the risks they entail be constrained [optimization principle or as low as reasonably achievable (ALARA)]. Radiation exposures that are not part of a practice should be reduced by intervention when this is justified, and the intervention measures should be optimized. The legal person authorized to engage in a practice involving a source of radiation should bear the primary responsibility for protection and safety. A safety culture should be inculcated that governs the attitudes and behavior in relation to protection and safety of all the individuals and organizations dealing with sources of radiation. In depth defensive measures should be incorporated into the design and operating procedures for radiation sources to compensate for potential failures in protection and safety measures. Protection and safety should be ensured by sound management and good engineering, quality assurance, training and qualification of personnel, comprehensive safety assessments, and attention to lessons learned from experience and research.
7.2 SOME QUANTITIES, TERMS, AND UNITS OF MEASURE OF HEALTH PHYSICS Absorbed dose: the average energy imparted by an ionizing radiation to the mass unity of a matter, unit of measure: gray (Gy) 5 1 J/kg. Nuclear Safety. DOI: https://doi.org/10.1016/B978-0-12-818326-7.00007-X © 2020 Elsevier Ltd. All rights reserved.
103
104
CHAPTER 7 HEALTH CONSEQUENCES OF RELEASES
Dose: This term has two meanings: • •
a measure of the quantity of radiation present in a radiation field or given by this field: notion expressed by the word “exposure”; a measure of the radiation received or absorbed by a target.
Effective dose: The summation of the tissue equivalent doses, each multiplied by the appropriate tissue weighting factor. Equivalent dose: The dose absorbed by a tissue or organ, multiplied for the pertinent radiation type weighting factor. Unit of measure: sievert (Sv) 5 1 J/kg (sometimes, the previous unit, the rem 5 1/100 Sv, is still used). Genetic effects: The effects on genetic material of somatic or germ cells, used in an imprecise way as a synonym of “hereditary effects.” Hereditary effects: The effects which manifest themselves in descendants of the exposed individual. Nonstocastic (deterministic) effects: The effects for which generally a threshold level of dose exists above which the severity of the effect is greater for a higher dose. Radiation weighting factor: A multiplication factor for the absorbed dose which accounts for the relative effectiveness of the various types of radiation in inducing health effects (see Table 7.1). Radioactivity: The radioactivity of a sample is the number of disintegrations per second. Unit of measure: becquerel (Bq) 5 disintegrations second21 [sometimes, the previous unit, the curie (Ci) 5 37 GBq is still used (1 TBq, a frequently used unit, is thus equal to about 27 Ci)]. Somatic effects: The effects that manifest themselves in the exposed individual. Stochastic effects: The radiation effects, generally occurring without a threshold level of dose, whose probability is proportional to the dose and whose severity is independent of the dose. Tissue weighting factors: To account for the different sensitivity of organs and tissues to the induction of stochastic effects of radiation (see Table 7.2).
Table 7.1 Radiation Weighting Factors. Type and Energy Range of Radiation
Radiation Weighting Factor
Photons Electrons and muons Neutrons ,10 keV Neutrons 10 100 keV Neutrons 100 keV 2 MeV Neutrons 2 20 MeV Neutrons .20 MeV Protons (except recoil protons) .20 MeV α-Particles, fissile fragments, heavy nuclei
1 1 5 10 20 10 5 5 20
7.3 TYPES OF EFFECTS OF RADIATION DOSES AND LIMITS
105
Table 7.2 Tissue Weighting Factors. Tissue or Organ
Weighting Factor
Gonads Bone marrow (red) Colon Lung Stomach Bladder Breast Liver Oesophagus Thyroid Skin Bone surface Remainder
0.2 0.12 0.12 0.12 0.12 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.05
7.3 TYPES OF EFFECTS OF RADIATION DOSES AND LIMITS So far as the deterministic effects are concerned, the following brief and imprecise facts should be remembered: • • •
The lethal dose at 50% probability (LD50) is equal to about 3 5 Gy, in the absence of a good medical assistance. Impairment of vision may happen between 1 and 10 Gy, according to the type of radiation (high or low linear energy transfer). Permanent sterility may occur between 2.5 and 6 Gy. For the stochastic effects in the population, the following, again brief, reference data should be noted:
• • • • •
Death risk for low doses 5 5 3 1022/Sv. Risk of serious effects in descendants 5 0.5 1.3 3 1022/Sv. As far as the limits adopted in many countries by law are concerned, we have: for workers: 20 mSv for solar year (effective dose); for the population: 10 μSv/year for each practice.
These limits hold for normal operation of the plants and not for accidents. Indeed, other limits for accidents do not exist except those fixed by the local regulatory body, case by case, or for classes of plants and of sources, for example, for a nuclear power station, the most recent trend in Italy was to prevent the overcoming of the reference values for short-term evacuation of the population (taken as equal to 1 rem, which is the lowest value named in foreign and international guidelines) in case of a severe accident. Moreover, it is usual to define a design limit for the collective dose of workers: the present value in many countries is of the order of 1 Sv person/year.
106
CHAPTER 7 HEALTH CONSEQUENCES OF RELEASES
7.4 EVALUATION OF THE HEALTH CONSEQUENCES OF RELEASES As elsewhere in this book, here only simple methods and orders of magnitude are listed which can be useful for quick dose evaluations for preliminary decisions: more precise methods are described in the references and in the abundant literature in the field.
7.4.1 EVALUATION OF INHALATION DOSES FROM RADIOACTIVE IODINE The following is a simple formula that can be used for a quick evaluation. It is most easily remembered if the old units of measurement are used (curie, rem, etc.).1 D 5 10 3 χ 3 R
where D is the effective dose for adults (rem) (for children a multiplication factor ranging from 5 to 10, according to age, has to be used), χ is the cloud concentration (s/m3) (see also Chapter 6: The Dispersion of Radioactivity Releases), and R are the curies of iodine-131 released.2 The dose calculated for all the iodine isotopes (not just for iodine-131) could result in a dose of the order of double that calculated for iodine-131 only. The dose to the thyroid is equal to about 20 times the one here calculated.
7.4.2 EVALUATION OF DOSES DUE TO SUBMERSION IN A RADIOACTIVE CLOUD In some cases the term “submersion doses” may not be appropriate because what is generally meant with this expression are the doses of direct radiation from a cloud of radioactive substances traveling in the vicinity. Here xenon-133 (important for accidents to reactors or to gaseous waste decay tanks) and tritium (e.g., 3H, important for fusion machines) are considered. The doses are roughly: For xenon-133 D5
χ3R 300
which can give lower dose values than other models [this has been taken from Commission of European Communities (CEC) documents]. In order to take into account the finite dimensions of the cloud, the calculated doses should be multiplied by a factor (,1) which, for ground release and for F category ranges from 0.1 at 1 km to 0.7 at 100 km. For tritium (skin irradiation, inhalation): D 5 0:03 3 χ 3 R
1 Relaxation moment! Concerning the use of obsolete units of measure, the subtle truth contained in a popular joke comes to mind. It concerns a professor, very popular with his students, who, answering a question about the reason why he taught so many incorrect notions in his lessons, replied, “This way they understand better!.” 2 It is worth recalling from Chapter 6, The Dispersion of Radioactivity Releases, that χ can conservatively be assumed to equal 1024 1023 s/m3 (Pasquill category F with a wind speed of 2 m/s) at 1 km and variable for other distances as the inverse of the ratio of the distances raised to the power of 1.5 2.
7.4 EVALUATION OF THE HEALTH CONSEQUENCES OF RELEASES
107
Table 7.3 Ground-shine Dose (Caesium-137). First Year
Second Year
0 50 Years
1.2 mSv 120 mrem
800 μSv 80 mrem
16 mSv 1600 mrem
7.4.3 EVALUATION OF THE DOSES OF RADIATION FROM CAESIUM-137 DEPOSITED ON THE GROUND (“GROUND-SHINE” DOSE) The figures of interest for any practical case can be extrapolated from the data shown in Table 7.3, which gives the dose at various times after the deposition of 1 kBq/m2. For contamination deriving from an accident to a reactor, the radiation doses from the ground due to caesium-137 are generally more important than the contribution of other isotopes.
7.4.4 EVALUATION OF THE DOSE DUE TO DEPOSITION OF PLUTONIUM ON THE GROUND A deposition of plutonium might happen as the result of an accident to a space vehicle (238Pu) (see Chapter 26, Nuclear Facilities on Satellites) or because of a very violent accident to a nuclear reactor (239Pu and 240Pu). Plutonium isotopes are highly radiotoxic but plutonium is highly insoluble and in general the highest risk originates from the inhalation of very fine dusts (B5 μm). The conversion factor for the inhalation dose to adult is, for plutonium-238, in average conditions, 4.6 3 1025 Sv/Bq.AR29 Similar figures apply to plutonium-239 and plutonium-240. The mechanisms by which the plutonium might be inhaled are to be evaluated case by case. The specific activity of plutonium-238 is 6.44 3 105 Bq/μg and 2300 Bq/μg for plutonium-239.
7.4.5 INDICATIVE EVALUATION OF LONG DISTANCE DOSES FOR VERY SERIOUS ACCIDENTS TO NUCLEAR REACTORS Fig. 7.1 gives a first impression of possible effective committed doses. It shows data from the Chernobyl, Windscale, and Three Mile Island accidents (collected by G. Santarossa), together with a subjective evaluation of the effects of a maximum severe accident “reasonably” conceivable for a present and future reactor.
7.4.6 DIRECT RADIATION DOSES It is often useful to have an idea of the possible radiation fields caused by a point source. The approximate formula to remember is the following (see Note 1): Rhm 5 0:6 3 C 3 E
where Rhm is rems per hour at 1 m distance in air, C is the source curies, and E is the energy of the emitted radiation (MeV). Fig. 7.2 can also be of help.3 3 As an example, one curie of cobalt-60, which emits γ-radiation at a total of 2.5 MeV, delivers about 1.5 rem/h at the distance of 1 m.
108
CHAPTER 7 HEALTH CONSEQUENCES OF RELEASES
Dose (mSv)
100
10
1
0.1
1
10
100
1000
FIGURE 7.1 Long range doses from accidents.
1 Ci
1 Mev
1m
FIGURE 7.2 Activity dose relationship.
0.6 rem h
Range (km)
7.4 EVALUATION OF THE HEALTH CONSEQUENCES OF RELEASES
Air
1000 m
Water
34 cm
Concrete
16.5 cm
Glass (Ce or Pb)
5–15 cm
Steel
5 cm
Lead
3 cm
FIGURE 7.3 Thicknesses of materials for reduction of 10 in γ-ray intensity.
109
110
CHAPTER 7 HEALTH CONSEQUENCES OF RELEASES
Remember that α-rays are stopped by the thickness of a simple sheet of paper, while β-rays can penetrate several centimeters into human body tissue. γ-rays or neutrons can penetrate much deeper into matter. Fig. 7.3 shows the thickness of various materials able to reduce the intensity of γ-rays by a factor of 10. As can be seen, there is a certain inverse proportionality to the material density.
REFERENCES ICRP, 2007. Recommendations of the International Commission on Radiological Protection. ICRP Publication 103, Pergamon Press. INSAG-12, 1999. Basic Safety Principles for Nuclear Power Plants.