international atomic energy agency assessment of occupational exposure due to intakes of...

International Atomic Energy Agency

ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF

RADIONUCLIDES

Interpretation of Measurement Results


Introduction


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.



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



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


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


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


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.


• 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


• 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


ICRP 78 CURVES AND DATAICRP 78 CURVES AND DATA

The data and curves available in ICRP 78 refers to these specific conditions of exposure!




– 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)


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


• Retention : (Bq per Bq intake)

• Excretion : (Bq/d per Bq intake)



Special monitoring(inhalation)

Routine monitoring(inhalation)

Special monitoring(ingestion and injection)

• Data :

m(t)

m(T/2)

t

T


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


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


60Co Routine Monitoring Retention FractionsInhalation


60Co Retention Fractions - Inhalation

Type M

Type S


60Co Routine Special Retention FractionsInhalation


60Co Retention Fractions - Ingestion

f1 = 0.1 f1 = 0.05

Special Monitoring


60Co Retention Fractions - InjectionSpecial Monitoring


Intake Estimates - An Example


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



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



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



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.


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


Biokinetic model for systemic iodine


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


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.


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



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



Time after intake, d

Ret

enti

on

, B

qVapor particle

0.074

7

or table A.6.17 in ICRP 78

Special monitoringSpecial 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


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


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


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


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


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


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


131I - Daily urinary excretion after inhalation



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



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.



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



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

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