international atomic energy agency assessment of occupational exposure due to intakes of...
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International Atomic Energy Agency
ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF
RADIONUCLIDES
Interpretation of Measurement Results
International Atomic Energy Agency
Introduction
International Atomic Energy Agency
Measurements for internal dose assessment
Direct measurement - the use of detectors placed external to the body to detect ionizing radiation emitted by radioactive material contained in the body.
Indirect measurement - the analysis of excreta, or other biological materials, or physical samples to estimate the body content of radioactive material.
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Measurements for internal dose assessment
Direct or indirect measurements provide information about the radionuclides present in: The body, Parts of the body, e.g specific organs or
tissues, A biological sample or A sample from the working environment.
These data are likely to be used first for an estimation of the intake of the radionuclide
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Measurements for internal dose assessment
Biokinetic models are used for this purpose. Measurements of body activity can also be
used to estimate dose rates directly Calculation of committed doses from direct
measurements still involves the assumption of a biokinetic model,
If sufficient measurements are available to determine retention functions, biokinetic models may not be needed
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Estimatedintake
DirectMeasurements
(In vivo)
Body/organcontent, M
IndirectMeasurements
Excretion rate, M
Air concentration
Doserate
Committedeffective dose
m(t)
DAC-hr
e(g)j
m(t)
Interpretation of monitoring measurements
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Estimate of intake
Where M is the measured body content or excretion rate, m(t) is the fraction of the intake retained in the whole body (direct measurement) or having been excreted from the body in a single day (indirect measurement) – retention or excretion fraction - at time t (usually in days) after intake.
)t(mM
Intake
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Estimate of intake
The ICRP has published default values of m(t) in Publication 78
When significant intakes may have occurred, more refined calculations based on individual specific parameters (special dosimetry) should be made
If multiple measurements are available, a single best estimate of intake is obtained by the method of least squares.
When more than 10% of the measurements could be attributed to previous evaluated intakes a correction should be performed.
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• The ICRP Publication 78 “Individual Monitoring for internal exposure of workers - replacement of ICRP Publication 54” provides a general guidance on the design of individual monitoring programmes and the interpretation of results of estimates of intakes of radionuclides by workers.
• A reference worker is assumed in relation to the biokinetic models and the parameter values describing the scenario of contamination. Radionuclides are selected for their potential importance in occupational exposure.
• This publication replaces the previous one ICRP Publication 54 “Individual Monitoring for intakes of radionuclides by workers: design and interpretation” taking into account:
- new protection quantities and new set of exposure conditions (ICRP 60)
- new general principles for radiation protection of workers (ICRP 75)
- respiratory tract model of ICRP 66
- revised biokinetic models when available for selected radionuclides
Implementing biokinetic modelsImplementing biokinetic models
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• Basic assumption for a reference worker in ICRP 78:
– Adult male
– Normal nose breathing at light work
– Breathing rate 1.2 m3/h
– Inhaled aerosol with Activity Median Aerodynamic Diameter (AMAD) 5 µm
– Regional Deposition [%]
ET1 34
ET2 40
BB 1.8
BB 1.1
AI 5.3
total 82
Implementing biokinetic modelsImplementing biokinetic models
ICRP 78 CURVES AND DATAICRP 78 CURVES AND DATA
The data and curves available in ICRP 78 refers to these specific conditions of exposure!
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Implementing biokinetic modelsImplementing biokinetic models
ICRP 78 CURVES AND DATAICRP 78 CURVES AND DATA
– Description of the model
– Standard assumption for transfer into systemic phase
– Dose coefficients
– Other informations
In relation to the radionuclide other significant information are available: monitoring techniques (as for Pu), etc.
• General information :
e(50)
ALI=0.02/e(50)ALI=0.02/e(50)
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CaesiumCaesium
• Model : Non-recycling model
Transfer compartment
Compartment A
Compartment B
Urinary bladder
ULI
Urine
Faeces
LLI
T1/2b= ln(2)/12 [d]
10 %
90 %
80 %
80 % 20 %
20 %
T1/2b = 2 d
T1/2b = 110 d
H, P, Cr, Mn, Co, Zn, Rb, Zr, Ru, Ag, Sb, Ce, Hg, Cf are as well
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• Retention : (Bq per Bq intake)
• Excretion : (Bq/d per Bq intake)
Implementing biokinetic modelsImplementing biokinetic models
ICRP 78 CURVES AND DATAICRP 78 CURVES AND DATA
Special monitoring(inhalation)
Routine monitoring(inhalation)
Special monitoring(ingestion and injection)
• Data :
m(t)
m(T/2)
t
T
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Retention or excretion fraction – m(t)
Depends on:
Route of intake
Absorption type, i.e. chemical form; Type F (fast), Type M (moderate), or Type S (slow)
Measurement and sample type
Direct Whole body Lungs Thyroid
Indirect Urine Faeces
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Retention fraction example – 60Co
Intake may be through inhalation, ingestion or injection (wounds)
Assigned two absorption types – M and S
Assigned two f1 values for ingestion – 0.01 and 0.05
ICRP 78 considers 4 possibilities for measurement Direct
Whole body Lungs
Indirect Urine Faeces
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60Co Routine Monitoring Retention FractionsInhalation
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60Co Retention Fractions - Inhalation
Type M
Type S
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60Co Routine Special Retention FractionsInhalation
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60Co Retention Fractions - Ingestion
f1 = 0.1 f1 = 0.05
Special Monitoring
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60Co Retention Fractions - InjectionSpecial Monitoring
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Intake Estimates - An Example
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Estimate of intake - an example
Occupational exposure to radioiodine occurs in various situations
I-131 is a common short lived iodine isotope: Half-life = 8 d particles - average energy 0.19 MeV - main emission 0.364 MeV Rapidly absorbed in blood following intake Concentrates in the thyroid Excreted predominantly in urine
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Estimate of intake - an example
After intake, I-131 may be detected directly in the thyroid, or indirectly in urine samples
If occupational exposure to I-131 can occur, a routine monitoring programme is needed
Based on direct thyroid measurement or
Indirect monitoring of urine or workplace samples
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Estimate of intake - an example
Choice of monitoring method depends on various factors: Availability of instrumentation Relative costs of the analyses Sensitivity that is needed
Direct measurement of activity in the thyroid offers the most accurate dose assessment
Other methods may be adequate and may be better suited to the circumstances
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Estimate of intake - an example
Chemical form of the radionuclide is a key parameter in establishing biokinetics
All common forms of iodine are readily taken up by the body
For inhalation of particulate iodine, lung absorption type F is assumed
Elemental iodine vapour is assigned to class SR-1 with absorption type F
Absorption of iodine from the gastrointestinal tract is assumed to be complete, i.e. f1 = 1.
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Dose coefficients
Inhalation Ingestion e(g)inh
(Sv/Bq) Radionuclide
Type /
form (a) AMAD
1 m AMAD 5 m
f1
e(g)ing (Sv/Bq)
I-125 F 5.3 E-09 7.3 E-09 1.0 1.5 E-08 V 1.4 E-08(b)
I-131 F 7.6 E-09 1.1 E-08 1.0 2.2 E-08 V 2.0 E-08(b)
(a) For lung absorption types see para. 6.16 of RS-G-1.2(b) For inhalation of gases and vapours, the AMAD does not apply for this form.
2.0 E-08
1.4 E-08
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Biokinetic model for systemic iodine
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Radioiodine biokinetics
30% of iodine reaching the blood is assumed transported to the thyroid
The other 70% is excreted directly in urine
Biological half-time in blood is taken to be 6 h
Iodine incorporated into thyroid hormones leaves the gland with a biological half-life of 80 d and enters other tissues
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Radioiodine biokinetics
Iodine is retained in these tissues with a biological half-life of 12 d.
Most iodine (80%) is subsequently released and available in the circulation for uptake by the thyroid or direct urinary excretion
Remainder is excreted via the large intestine in the faeces
The physical half-life of I-131 is short, so this recycling is not important for committed effective dose.
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131I intake - Thyroid monitoring
A routine monitoring programme
14 day monitoring period
Thyroid content of 3000 Bq 131I is detected in a male worker
Based on workplace situation, exposures are assumed due to inhalation of particulates
Intakes by ingestion would lead to the same pattern of retention and excretion
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131I intake - Thyroid monitoring
Intake pattern is not known
Assume an acute intake occurred in the middle ofthe monitoring period
From the biokinetic model, 7.4% of the radioactivity inhaled in a particulate (type F) form with a default AMAD of 5 is retained in the thyroid after 7 d
from table A.6.17 (Thyroid) in ICRP 78
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131I intake - Thyroid monitoring
Time after intake, d
Ret
enti
on
, B
qVapor particle
0.074
7
or table A.6.17 in ICRP 78
Special monitoringSpecial monitoring
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131I intake - Thyroid monitoring
Thus, m(7) = 0.074, and
Application of the dose coefficients given in the BSS and in the previous table gives,
A committed effective dose of 0.45 mSv
(4.1•104 Bq 1.1•10-8 Sv/Bq 103
mSv/Sv)
This dose may require follow-up investigation
kBqm
MIntake 41
074.0
000,3
7
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131I intake - Urine measurement
One day after the direct thyroid measurement, the worker has a 24-h urine sample
Sample assay shows 30 Bq of 131I
From the biokinetic model for a type F particulate, m(8) for daily urinary excretion is 1.1 E-04
from table A.6.17 (dairy urinary excretion) in ICRP 78
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131I intake - Urine measurement
A committed effective dose of 3 mSv
(2.7•105 Bq 1.1•10-8 Sv/Bq 103
mSv/Sv)
For this example no account is taken of any previous intakes
kBq270101.1
30)8(m
MIntake
4
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131I intake - Workplace air measurements
Workplace air measurements showed 131I concentrations that were low but variable
Maximum concentrations between 10 and 20 kBq/m3 (12 to 25 times the DAC) for short periods several times in several locations
At the default breathing rate of 1.2 m3/h, worker could receive an intake of 24 kBq in one hour without respiratory protection
DAC; Derived Air Concentration
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Derived air concentrations
DAC (Bq/m3) Radionuclide Type/form AMAD
1 m AMAD 5 m
Gas/vapour
Sb-125 F 6 E+03 5 E+03 M 2 E+03 3 E+03 I-125 F 2 E+03 1 E+03 V 6 E+02 I-131 F 1 E+03 8 E+02 V 4 E+02 Cs-134 F 1 E+03 9 E+02
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131I intake - Workplace air measurements
If worker had worked for one hour without respiratory protection, or
Somewhat longer with limited respiratory protection
The intake estimated from air monitoring would be consistent with that determined by bioassay (direct and indirect) measurements
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131I intake - Dose assessment
Intake discrepancy suggests at least one of the default assumptions is not correct
Significant individual differences in uptake and metabolism cannot generally account for discrepancies of nearly a factor of 10
The rate of 131I excretion in urine decreases markedly with time after intake - a factor of more than 1000 over the monitoring period
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131I - Daily urinary excretion after inhalation
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131I intake - Dose assessment
Assumption of the time of intake is a probable source of error
If the intake occurred 3 days before the urine sample was submitted
Intake estimated from the urine measurement would be 21 kBq
Intake from the thyroid measurement would be 25 kBq
The agreement would be satisfactory
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131I intake - Dose assessment
From the biokinetic model, the fraction of inhaled 131I retained in the thyroid only changes by about a factor of 3 over the monitoring period
Without more information, the new assumption is more reliable for dose assessment
The committed effective dose for this example would then be 0.27 mSv
A 2nd urine sample obtained after a few more days should be used to verify this conclusion.
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131I intake - Dose assessment
Committed effective dose from thyroid monitoring is relatively insensitive to assumptions about the time of intake
However, there is rapid change in urinary excretion with time after exposure
Result - direct measurement provides a more reliable basis for interpreting routine radioiodine monitoring measurements
Urine screening may still be adequate to detect significant intakes
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131I intake - Dose assessment
Air concentrations that substantially exceed a DAC should trigger individual monitoring
However, because of direct dependence on: Period of exposure Breathing rates Levels of protection and Other factors known only approximately
Intake based on air monitoring for 131I are less reliable than from individual measurements