EVALUATION OF OCCUPATIONAL DOSE IN

NIGERIA

GODWIN B. EKONG

MONITORING AND TECHNICAL SERVICES UNIT

DEPARTMENT OF RADIOLOGICAL SAFETY

Friday, June

30, 20171

Contents

Preambles

Introduction to Occupational Dose

Benefits for Dose Evaluation

Legal Basis of Dose Monitoring

Method of Dose Evaluation and Results

Major concerns & Way Forward

Conclusion

ReferencesFriday, June

30, 2017

2

PREAMBLES

Fall outs from dose presentation of year 2014 and 2015:

Evaluate backlog of occupational dose from 2012 to 2016 to

span over 5 years for good assessment

Separate occupational dose for activities from facilities inline

with current requirement of General Safety Requirements 3

How dose records from Dosimetry Service Providers can be

standardized.

Friday, June

30, 20173

Introduction to Occupational Dose

Occupational exposure monitoring is a systematic process of evaluating the Risk(Potential, Real) associated with working in any activities involving ionizingradiation.

These are in both Radiological and Nuclear activities:

• Medical - diagnostic radiology, nuclear medicine, Radiotherapy

• Industrial - industrial radiography, well logging, nuclear gauges

• Gamma Irradiation Facility,

• nuclear fission reaction – Research Reactor

• Other Activities – Transport, Freight Forwarding, RSA etc.

Cumulative dose - The total dose that an occupationally exposed worker receives as aresult of repeated exposures to ionizing radiation to the same portion of the body, orto the whole body, over time.

Collective dose is a measure of the extent to which a group of people or a populationhas been exposed. It is equal to the sum of the individual doses. Expressed in man-Sieverts.

Exposures from these activities result in both Internal and External dose

Friday, June

30, 20174

Introduction to Occupational Dose

contd.

However, Occupational dose excludes doses from:

• Members of public

• sources of natural background,

• Medically treated patient with radioactive materials,

• Comforters

• Member in medical research programs[1] Friday, Ju

ne

30, 2017

5

Introduction to Occupational Dose

contd.

Quantifying health risk, ICRU, ICRP and IAEA equates quantities & units to riskfrom radiation exposures.

Absorbed Dose (D) measurement – dE/dm (1J/Kg = Gy)

D is weighted by Radiation weighting (WR) factor for each radiation type in Equivalent Dose (HT) = ΣWR.DR,T and

HT is weighted with Radio- sensitivity of different organs and tissues (WT) being exposed in Effective Dose (HE) = ΣWT.HT. Unit is in Sieverts.

Max. Risk (1/1000) → 10-3yr-1; Risk/Sieverts → 5x10-2Sv-1

Maximum dose to the risk → 10-3yr-1/ 5x10-2Sv-1 → 0.2x10-1Sv/yr

→ 20mSv/yr

Dose Limits [2,3] Friday, June

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6

Introduction to Occupational Dose

contd.

The rationale behind the system of dose limits is to ensure that the:

Dose received by occupational worker is below the level for deterministic effect

that requires a least amount dose for appearance, and

Probability of any stochastic effect is small enough to be acceptable to the

individual [4].

To help in limiting radiological hazards, System of area classification is

always instituted.

These areas are: Uncontrolled, Supervised, Controlled, Restricted Area [6].

Constant survey is necessary once these areas are designated to confirm the

correctness of the each classified area. This forms major part of risk assessment

process.

The use of active /passive devises are used for radiation monitoring and dose

measurement in designated area.Friday, June

30, 20177

BENEFITS OF OCCUPATIONAL DOSE EVALUATION

Assessing the effectiveness of our regulatory system

Timely investigation of overexposure on radiation workers

Prepare the Regulatory Authority during matters arising

from litigation

Provides information for research and

Epidemiological studiesFriday, June

30, 2017

8

Introduction to Occupational Dose

contd.Dose measurement devises accredited so far in Nigeria are:

THERMO LUMINESCENCE DOSIMETER (TLD) is a type of personal passive radiation detection device that measures ionizingradiation exposure by measuring the intensity of visible light emitted from a crystal in the detector when the crystal isheated in a TLD reader. This phenomenon is based on existing imperfections in the doped crystal lattice structure of thedetection device, hence their ability to trap electrons released by incident ionizing radiation. The heat applied to the TLDsubsequently releases the trapped electrons and their excess energy appear as light photons. The intensity of lightemitted is dependent upon the radiation exposure [7,8,9]. The two most common types of TLDs are and lithium fluoridedoped with magnesium or titanium (LiF:Mg,Ti) and Calcium Sulfate doped with thulium (CaSO4:Tm). This device iscurrently most commonly used in Nigeria although it has its limitations.

INSTADOSE DOSIMETER is a passive radiation detection device which comprises a rugged USB dosimeter utilizing direct ion storage technology. Components in the dosimeter include ion gas chamber and a non-volatile analog memory element (MOSFET). The principle of operation is photon interaction with wall material of the chamber, thereby generating secondary electrons which in turn ionize gas in the chamber. The resulting ions are separated via potential difference between electrode and wall, and then the ions are stored into the analog memory element cell. At read out, the ion conductivity over the MOSFET which is proportional to the charge is measured non-destructively. This breakthrough technology provides radiation workers with a precise measurement of radiation dose and includes accurate long-term exposure tracking via built-in memory chip that stores each user's identity with an embedded unique serial code that is assigned to the user. The fully automated transfer of data minimizes the chance of human error and misidentification. This device has gained accreditation in Nigeria for use. [10,11]

Friday, June

30, 20179

ATOMTEX (Optically Stimulated Luminescence)

Similar to the TLD, Atomtex is also a passive radiation detection device which also utilizes theimperfections (impurities or defects) in the crystal lattice of detector material (commonlyquartz, feldspar, carbon doped Aluminium oxide - Al2O3:C) to trap electrons that have been set freeby incident ionizing radiation. The principle of operation involves production of electron-hole pairsbetween the conduction and valence bands by the incident ionizing radiation and the subsequententrapment of these electrons. However unlike the TLD which uses heat to stimulate trappedelectrons, the atomtex dosimeter uses light source at a particular frequeuncy to stimulate thetrapped electrons to go into the conduction band where recombination with holes mayoccur, giving rise to the emission of light. The intensity of light emitted is dependentupon the radiation exposure [12]. OSL is an advanced technology in radiation monitoring offeringseveral advantages such as fast dose assessment, dosimeter archiving, non-destructivereadouts, high degree of environmental stability and much more.

Application Submitted for Accreditation

DOSICARD badge

is a real-time active electronic dosimeter which comprises a silicon diode detector accompaniedwith analog and digital circuitry. This device also encompasses a microcontroller with a large non-volatile memory, LCD display and audio & visual alarms. The use of silicon semiconductor detectorsallows for charges to be released by incident ionizing radiation resulting in measurable electriccurrent. Dosicard has the capability to generate visual, audible and vibratory alarms whenpredefined dose and dose rate thresholds are reached. Itgives direct access to the user for doses received with a confidential password [13]. Friday, June

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10

NNRA’s STAND ON DOSE MONITORING

NSRP Act 1995 No.19

Section 25 – the Authority shall in the performance of its functions and

for the protection of radiation workers and the general public

ensure that …

b. – the dose equivalent to shall in no way exceed the established

limits prescribed by the Authority

NiBIRR 2003

Regulations 47. - (1) Every employer shall ensure that -

(a) in respect of each of his employees who is designated as a

classified person, an assessment is made of all doses of ionizing

radiation received by such employee which are likely to be

significant; and Friday, June

30, 2017

11

NNRA’s STAND ON DOSE MONITORING

(b) such assessments are recorded and reported to theAuthority.

(2) For the purposes of paragraph (1) of thisregulation, the employer shall make suitablearrangements with one or more authorised DosimetryService Providers for –

…….(b) the making and maintenance of dose records relatingto each classified person.

Friday, June

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METHOD OF EVALUATION & RESULTS

The occupational dose records from read worn dosimeters were submitted by authorized DosimetryService Provider to the Authority on a quarterly/yearly basis spanning from 2012 to 2016.

In the evaluation process:

Doses from individual radiation worker were collated and analyzed, and Maximum dose derived;

Radiation workers doses were summed and averaged from each facility;

Average Doses from facilities were collated and averaged for each occupational groups formCollective dose;

[Collective Dose (CD) = (NRW X AD of Occ. Grps X 10-3)manSv]

Collective doses from different occupational groups were averaged to form Annual collective doses;

Total numbers of radiation workers (NRW) for each year were derived.Friday, June

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Friday, June

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15

OCCUPATIONAL DOSE FOR YEAR 2012

Occupational

Groups

Radiation

Workers

Total Dose of

Occ. Groups

Highest

Dose

(mSv/yr)

Average Dose

(mSv/yr)

Collective

Dose

(manSv)

Ave.

Collective

Dose

(manSv)

IR 185 104.60 6.56 0.57 0.10

0.1

DR 23 25.43 5.32 1.1 0.03

WL 167 57.33 3.61 0.34 0.06

RR 32 79.38 3.62 2.48 0.08

GIF 15 14.32 1.25 0.95 0.01

RT 153 186.60 1.45 1.22 0.19

NM - - - - 0.00

Ac 191 58.68 4.06 0.31 0.06

NG 25 31.40 4.17 1.26 0.03

Friday, June

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16

OCCUPATIONAL DOSE FOR YEAR 2016

Occupational

Groups

Radiation

Workers

Total Dose of

Occ. Groups

Highest Dose

(mSv/yr)

Average Dose

(mSv/yr)

Collective Dose

(manSv)

Ave. Collective

Dose

(manSv)

IR319 248.33 4.51 0.79 0.25

0.1

DR203 118.14 2.69 0.58 0.15

WL150 127.11 2.95 0.85 0.15

RR44 81.82 2.44 1.86 0.08

GIF17 35.19 2.61 2.07 0.04

RT146 59.84 8.21 0.41 0.06

NM1 0.04 0.04 0.04 0.01

Ac16 2.44 0.40 0.15 0.01

NG36 41.23 6.71 0.87 0.04

Friday, June

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0.00

2.00

4.00

6.00

8.00

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12.00

14.00

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Highest doses received by radiation workers in 2012

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

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Collective Dose of Occ. Grps 2012 (manSv)

(1.25 –6.26)mSv/yr for 2012

Dose Limit

Friday, June

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0.00

2.00

4.00

6.00

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Highest doses received by radiation workers in 2013

Dose Limit

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

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Collective Dose of Occ. Grps 2013 (manSv)

(0.62 – 16.10)mSv/yr for 2013

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0.00

2.00

4.00

6.00

8.00

10.00

12.00

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18.00

20.00

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Highest doses received by radiation workers in 2014

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

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Collective Dose of Occ. Grps 2014 (manSv)

(0.44 – 5.49)mSv/yr for 2014

Dose Limit

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0.00

2.00

4.00

6.00

8.00

10.00

12.00

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18.00

20.00

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Highest dose received by radiation workers in 2015

Dose Limit

-0.10

0.10

0.30

0.50

0.70

0.90

1.10

1.30

1.50

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Collective Dose of Occ. Grps in 2015 (man Sv)

(0.87 – 8.62)mSv/yr for 2015

Friday, June

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0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

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Highest dose received by radiation workers in 2016Dose Limit

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

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Collective Dose of Occ. Grps in 2016 (manSv)

(0.40 – 8.21)mSv/yr for 2016

Friday, June

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Collective doses of all occupational groups from 2012 – 2016 indicates that the risk due to

radiation is very low :

0.1manSv of 766 radiation workers for 2012, 0.1manSv of 938 radiation workers for

2013, 0.1manSv of 607 radiation workers for 2014, 0.2manSv of 1332 radiation workers for 2015

and 0.1manSv of 896 radiation workers for 2016.

STANDARDIZATION OF OUR DOSIMETRY SYSTEM

Proficiency Testing Standard of Dosimetry System (Lesson from past experience)

Applies for intending DSPs applying for accreditation for the first time or renewal ofaccreditation, those introducing new models of dosimeters and others on routine or periodiccalibration testing [14].

Establishes type test conditions and performance criteria for evaluating personnel dosimeters

• Response

• Reproducibility

• Repeatability

• Linearity

Ensure that accredited Dosimetry Service Provider (DSP) meet the technical requirement and

Implement quality assurance measures, in accordance with the mandate of NNRA to protect livesfrom the harmful effects of ionizing radiation

Draft National Standard for Testing Dosimetry System (Already reviewed and submitted to theManagement)

Friday, June

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MAJOR CONCERNS & WAY FORWARD

Dose reporting by the DSPs

• Discrepancies in reporting,

• Complex medical facilities combining doses of DR, NM and RT together

• DSPs sending records from unauthorised facilities

• Some DSPs are unaccredited by the NNRA, yet practising

Dose records for Regulators (NNRA Staff) are presently not available in theregistry

Need for ICT support for NDR:

• Eliminate manual data processing,

• quick data extraction,

• hosting of NDR

Need for stakeholders meeting to discuss overarching issues on dosimetry forimprovement

Acquisition of Irradiators for proficiency testing at the NIRPR, Ibadan(Cs, Co, X-ray etc)

Staff training on internal dosimetry and auditing of proficiency testing reportfrom DSPs (Request for one staff from NIRPR)

Friday, June

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CONCLUSION

Occupational exposure monitoring in Nigeria has begun and doses

received by an individual while performing duties associated

with exposures to radiation sources has been collated from 2012

to 2016 .

Evaluation of the dose records submitted were analyzed results

shows that limits were below 20mSv per year as stipulated.

Collective doses of occupational groups shows risk due to radiation

exposures was very low.

Major concerns not limited to those highlighted above needs to be

properly addressed for effective Dose Registry System in Nigeria. Friday, June

30, 201725

APPRECIATION

Director, Radiological Safety

Dose evaluation committee for sorting out data from archives

Staff of Radiological Monitoring and Technical Services Unit

Your best wishes

Friday, June

30, 2017

26

REFERENCES1. https://www.nrc.gov/reading-rm/basic-ref/glossary/occupational-dose.html last assessed 18th May 2017

2. Nuclear Safety and Radiation Protection Act 19 of 1995

3. Recommendations 103 of the International Commission on Radiological Protection 2007

4. https://www.mcgill.ca/ehs/laboratory/radiation/manual/3 last assessed 18th May 2017

5. Nigerian Basic Ionizing Radiation Regulation 2003

6. Introduction to Radiation Protection Cole et al 2012

7. Tochilin, E., N. Goldstein, and W. G. Miller. "Beryllium oxide as a thermo luminescent dosimeter." Health physics 16.1 (1969): 1-7.

8. Yamashita, T., et al. "Calcium sulfate activated by thulium or dysprosium for thermoluminescencedosimetry." Health physics 21.2 (1971): 295-300.

9. http://ozradonc.wikidot.com/thermoluminescent-dosimeters last assessed 18th May 2017

10. Mettler FA, Bhargavan M, Faulkner K, Gilley DB, Gray JE, Ibbott GS, et al.1950-2007

11. http://www.jzimaging.com/instadose_radiation_protection_dosimeter_x-Ray_badge.htm, www.nukeworker.com.

12. Rhodes, Edward J. (2011). "Optically stimulated luminescence dating of sediments over the past 200,000 years". Annual Review of Earth and Planetary Sciences. 39: 461–488. doi:10.1146/annurev-earth-040610-133425.

13. http://www.mgi.co.kr/pdf/Dosimeter.pdf

14. Draft “Nigerian Standard for Dosimetry – Personnel Dosimetry Performance - Criteria for Testing”. Friday, June

30, 201727

Friday, June

30, 201728

THANKS FOR YOUR ATTENTION