Radiation Hazard evaluation and Controls

8/6/2019 Radiation Hazard evaluation and Controls

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Radiation Hazard : Biological Effects and

Dose LimitsEvaluation : Assessment of hazard

Control: Methods of reducinghazard below thedose limits


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Nature of effect :

Deterministic effect

Probabilistic effect

Who is affected :Exposed individual (somatic)

Progeny of exposed person

(hereditary)When effect appears:Immediate (hours days)

Delayed (months years)


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BIOLOGICAL EFFECTS OFRADIATION

Deterministic effects:

Temporary Sterility

NVD

Epilation

Skin burn

Cataract

Lethality


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BIOLOGICAL EFFECTS OFRADIATION (CONTD.)

Probabilistic effects: Cancer

Leukaemia Hereditary effect

No threshold dose

is defined !!


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Determination of the radiation hazard

that may result from exposure toradiation.

Assessment of the dose received by the

exposed persons


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Safety and security of sources

Protection of personnel and public Safe work practices

.


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Internal Exposure

-Resulting from intake of radioactivematerial

External Exposure-Resulting from source outsidethe human body


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Entry of radioactive material intothe body

Inhalation Ingestion InjectionSource of radiation inside the

exposed person. Source hasto be flushed out of the body.

Working with sealed sourcesunlikely to cause internalexposure.


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Source outside the exposed person Exposure ceases as the source is

removed Shielding interposed between source and

exposed person can reduce the dose

Working with sealed sources likely tocause external exposure.


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Alpha has a range of a few cm in air.

Beta has a range of a few cm in Al /

perspex Gamma can penetrate great distances

Hazard Alpha Beta Gamma

External Nil Not high Very high

Internal Very high Very high High


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Personnel monitoring TLD, Pocket dosimeter

Record total dose (mSv)

Area monitoringSurvey meter (e.g. GM survey meter)

Display dose-rate (mSv/h or mR/h)

Contamination monitorDisplay reading in Bq/cm2


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INTERNAL EXPOSURE &

SEALED SOURCES

Working with sealed sources unlikely

to cause internal exposure.


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External hazard from alpha radiation is nil.

External hazard from beta not severe

External hazard from gamma can besevere

Working with sealed sources likely to

cause external exposure.


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Source mGyh-1 GBq-1 Rh-1 Ci-1

60 Co 0.31 1.32137Cs 0.07 0.3131I 0.05 0.22203Hg 0.03 0.1324Na 0.42 1.84


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Time Distance

Shielding Activity limitNucleonic gauges are designed to meet stringent

safety standards. Dose-rate on the exterior is

low.Individual monitoring not recommended.

Area monitoring necessary.


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Reduce period of exposure to radiation toreduce the dose received from source.


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Increase distance from source to decreaseexposure-rate.

I1 d12

= I2 d22

Double the distance from the source;dose-rate falls to the original value.

Halve the distance from the source; dose-rate increase to 4 times the original value.


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HVT reduces dose-rate to half the original value.

TVT reduces dose-rate to a tenth of the original value.

.Source 16 mSv/h8 mSv/h

0.4 mSv/h

1 HVL

4 mSv/h2 HVL

1 TVL+2 HVLHVL = HVTTVL = TVT


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Exposure-rate at 1 m from a gammasource = Activity x gamma ray constant

X = q x GRC Sv/h


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For beta and gamma = 4 Bq per sq. cm

For alpha = 0.4 Bq per sq. cm


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High Z shielding of beta radiation givesrise to bremsstrahlung radiation

Ideal beta shields are perspex and Al


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High electron density materials are suitable formost of the gamma sources

Lead, concrete, depleted uranium-------------------------------------------------------HVL TVL

Source cm Pb cm Pb

-------------------------------------------------------60Co 1.2 4

137 Cs 0.7 2.2


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Neutron sources commonly used in industrialprocess control (e.g. moisture content in coke infurnaces) and well logging are 252Cf and 241Am-

Be. Fast neutron flux of 3.7 n cm-2 s-1 corresponds toa dose-rate of 10 Sv /h.

Hydrogenous materials like paraffin are used for

shielding neutrons. Such materials slow downfast neutrons which are then absorbed by boronreleasing particles detected by conventionalradiation detectors.


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Procure only gauges approved by AERB

Ensure safety and security of sourceduring use and storage Maintenance by authorized service

engineer

Shutter OFF when not in use Minimize occupancy near installation


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Procure and use area monitors

Generally individual monitoring recommendedonly for well-logging personnel

Maintain a logbook of operations Handle emergency carefully (time, distance,

shielding) and inform AERB immediately.

After useful life, source should be sent fordisposal with permission from AERB.

Never keep a source that you do not need.


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To reduce the dose to the individual- Reduce the time of exposure

Increase the distance from the source Interpose a shielding material between

source and expose person

Limit the activity handled


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Exposure-rate at 1 m from a gammasource = Activity x gamma ray constant

X = q x GRC Sv/h


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Example set 11. What is the exposure-rate at 1 m from a cobalt 60 source of activity 220

GBq?

q = 220 GBq

GRC for cobalt 60 = 0.31 mGy h-1GBq-1 at 1 m

Dose rate at 1 m = q x GRC mGy / h

= 220 x 0.31 mGy / h= 68.2 mGy / h

2. What will be dose received by a person who spends five hours at 1 mfrom the source?

Dose received = dose-rate x period of exposure

Dose received = 68.2 mGy/h x 5 hours

= 341 mGy


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Example set 2

3. At 1 m from a 60Co source the dose-rate is 62 mGy/h. What is the

activity of the source?

Let the activity be q GBq.

Dose rate at 1 m from the source = q x 0.31 mGy/h

i.e. q x 0.31 = 62 mGy/hq = 200 GBq

4. A person spent 7 hours at a place. His TLD recorded a total dose of 840 mGy.

What is the dose-rate at the place?

5. The dose-rate at1

m from a source is 40 mGy/h. What is the dose-rate at 2m?6. The dose-rate at 2 m from a source is 36 mGy/h. What is the dose-rate at 3 m?

7. The dose-rate at 5 m from a source is 225 mGy/h. What is the dose-rate at 3 m?


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Example set 38. What is the distance required to reduce the dose rate from a

137

Cssource of activity 50 GBq to 0.1 mGy/h?

I1 = q x GRC mGy/h d1 = 1 m

I2 = 0.1 mGy/h d2 = ?

The GRC of137Cs is 0.072 mGy/h per GBq at 1 m.

9. What is the thickness of lead required for a storage container housing

a 60Co source of activity 6.5 GBq so that at 1 m from the source position, the

dose rate does not exceed 0.01 mSv/h?

Dose-rate at 1 m from the source without shielding = 6.5 x 0.31 mSv/h = 2 mSv/h

Shielding required to reduce this dose-rate to 0.01 mSv/h is 2 TVL + 1 HVL

Lead shielding thickness = 2 x 4 + 1.2 = 9.2 cm

10. Can a 137Cs of activity 13, 500 GBq be stored in the above container without

exceeding the dose limit of 0.01 mSv/h at 1 m from the source position ?

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Radiation Hazard evaluation and Controls