IAEAInternational Atomic Energy Agency

Examples of Accident Investigations

LectureModule 12

IAEA

Types of accidents

• Acute, whole body, homogeneous

• Acute, whole body, heterogeneous

• Acute, part-body

• Protracted, continuous

• Protracted, fractionated

• Protracted, internal emitter

• Immediate discovery

• Delayed discovery

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Examples

Two examples of actual accidents will be given:

1. A lost gamma radiography source picked up by a non-radiation worker and put into a pocket

2. An inhalation of tritium in an industrial accident

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Case 1. Orphan source incident

• Iridium-192

• 7.4 TBq (200 Ci)

• Small shiny metal cylinder

• No warning markings on it

• Used to carry out gamma radiography in a factory construction site

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Orphan source

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Radiography apparatus

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Sequence of events (1)

• It happened on a Friday

• It was discovered on the following Monday

1. The radiographers carried out their normal radiography

2. The source fell out from the end of the guide tube

3. It was not noticed by the radiographer because of a faulty monitor

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Sequence of events (2)

4. Source had fallen out onto the floor

5. Three hours later a construction supervisor picked it up

6. He mistakenly thought it had fallen from a mobile crane

7. He picked it up and put it into his shirt breast pocket

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Sequence of events (3)

8. He travelled home in small bus with 6 colleagues

9. They got off bus at various places along their route

10. He got home after ~40 min, sat down and watched TV

11. 40 min later he felt ill, undressed, put shirt into cupboard and went to bed

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Sequence of events (4)

12. Saturday morning source was moved to drawer in bedside cabinet

13. Family went out for day

14. Saturday and Sunday nights the man slept closest to cabinet with 6y son between him and his wife

15. He returned to work on Monday morning

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Sequence of events (5)

16. Loss of source was discovered on Monday late morning when radiographers resumed work

17. Monitors (in working order) were used to search area - Nothing found

18. Replica source was shown to all workers

19. Man recognised it and source was recovered from his home

20. Regulatory authority was informed

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Next steps

• A health physics investigation started to establish what had happened

• The man, his family and colleagues placed under medical supervision

• Doses were estimated

Two approaches:

1. Physical reconstruction

2. Biodosimetry

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Physical dose reconstruction

Estimated timings and geometry

1. Source in shirt pocket

2. Source in bedroom cabinet

3. Calculated doses for other people –few tens of mGy

First estimate: averaged whole body dose 1.33 Gy

Later refined estimate: 1.06 Gy

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Isodose contours through torso

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Biodosimetry-scoring results

Person cells dic ring ace

Man 1000 86 2 60

Wife 500 2 0 10

Child 500 1 0 3

Coll x 500 0 0 1

Coll y 500 0 0 3

Coll z 500 0 0 5

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Biodosimetry

Averaged whole body dose estimates (Gy)

Person Dose LCL UCL

Man 1.01 0.57 1.78

Wife 0.12 <0 0.33

Child 0.06 <0 0.28

Coll x 0 <0 0.047

Coll y 0 <0 0.047

Coll z 0 <0 0.047

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Biodosimetry: non-uniform exposure

Cells with number of dicentrics

0 1 2 3 4 5 v:m u

Observed 932 56 9 1 1 1 1.57 12.74

Poisson 918 79 3 0 0 0

Based on expected Poisson distribution, which would be expected for uniform exposure, there were too many highly damaged and too few undamaged cells or cells with only 1 dicentric observed.

This suggests non-uniformity.

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Biodosimetry: non-uniform exposure

Contaminated Poisson method

Partial body dose: 2.63 ± 0.50 Gy

Irradiated fraction: 23%

Qdr method

Partial body dose: 2.69 ± 0.52 Gy

Irradiated fraction: 25%

Good agreement between the two methods

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Biodosimetry: dose rate

The exposure was not acute

• 2h 40 min in the pocket

• Weekend in bedside cabinet

Use the G-function adjustment to the dose response curve;

Y = αD + G(β)D2

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Biodosimetry: dose rate corrected

• Averaged whole body increased from 1.01 to 1.19 Gy

• Contaminated Poisson estimates increased from 2.62 Gy and 36% to 3.15 Gy and 40%

• Qdr estimates increased from 2.69 Gy and 25% to 4.99 Gy and 30%

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Main points from the biodosimetry

1. Only the man was seriously exposed

2. Clearly non-uniform exposure

3. General exposure to majority of his body was serious but not life threatening

Biodosimetry information reported to medical doctors indicated that 1. bone marrow injury would not be problem (they were searching for suitable marrow donor) and 2. there would be local injuries requiring treatment

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Local reactions- chest on day 17

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Local reactions- hands on day 24

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Chest -day 1139 after surgery

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Hands- day 1139

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Conclusion (1)

• This orphan source type accident involving non-radiation worker is not unique; it is typical of many similar scenarios

• This case is good example of how biodosimetry inter-relates with physical methods, calculations and account of events to come to overall view of case

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Conclusion (2)

• This case is good example of how biodosimetry can approach problems of:

- non-uniformity of exposure

- combined with protracted exposure

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Case 2. Incorporation of tritium

• Accident happened in factory manufacturing glass capillary tubes filled with tritium gas

• Patient was 33y female

• She inhaled aerosol of tritiated water droplets

• Cause of accident was over-pressure in tube filling apparatus

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Diagram of tube filling system

P

uranium trap capillaries

tap

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What happened (1)

It happened on Thursday morning

• Woman melted off the first capillary

• It did not seal but instead popped open

• She realised that this was unusual and asked senior supervisor for help

• He sealed off broken capillary and then tried to melt off capillary from second set. It ruptured too and hissing gas was heard

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What happened (2)

• He transferred remaining gas in system back to uranium trap

• He noticed that area radiation monitors were showing increase of tritium in room

• However significant release of tritium was not at first suspected

• Urine samples were collected

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What happened (3)

• Friday evening urine measurement results showed:

1.3 GBq / L (woman)

28 MBq / L (supervisor)

• Now realised that it was serious

• Woman told to delay starting holiday and drink lot of extra fluid

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What happened (4)

• Further delay because company management could not be contacted until Saturday evening

• Sunday morning conference

• At noon regulatory authority were informed

• Authority contacted a medical doctor

• Monday morning woman admitted to hospital for forced diuresis

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Investigation- What went wrong? (1)

• Pressure in the system was 1600 mb

• It should have been 600 mb

• Manometer dial was confusing; needle had gone twice round dial

• There was no separate mechanism to warn of over-pressure

• About 3.4 TBq had leaked from 2 sets of ruptured capillaries

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What went wrong? (2)

• Woman had removed part of the containment

• Her face was close to escaping tritium gas

• Gas passed through flame where it was oxidised to tritiated water droplets

• She inhaled about 35 GBq

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ICRP recommendation

Annual limits of intake

• 3 GBq for standard man

• This converts to about 2.2 GBq for this 53.5 kg woman

Committed dose equivalents:

0.8 Sv (woman)

0.025 Sv (supervisor)

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Tritium excretion in victim’s urine

Tritiumconc.,MBq l-1

Days0 10 20 30

2000

1000

500

200

100Hospital

0

T½ = 2.7 d, forced diuresis

T½ = 6.4 d, enhanced fluid intake

T½ = 10 d

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Dosimetry from urine measurements

Standard formula using parameters:

• conc. of H3 in urine integrated over time

• mean energy per disintegration

• ratio of total body water / soft tissue for females

Resultant dose to soft tissue = 0.47 Sv

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Committed Doses

Actual dose 0.47 Sv

Dose if untreated 0.80 Sv (T½ = 10d)

Dose if increased drinking 0.55 Sv (T½ = 6.4d)

Dose if earlier diuresis 0.41 Sv

Lesson learned: It was probably not worth psychological stress of forced diuresis for amount of dose saved

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Dose build-up from urine measurements

Days after accident

Dosetosofttissue,Gy

Hospital

20 30 40 5010

0.1

180

0.5

0.3

0

21318

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Biodosimetry

Blood sampled for the dicentric assay

• On days 4, 18, 39, 50, and 178

• 1000 metaphases scored per sample

• Dicentric yields referred to calibration curve

• Need to consider water content of lymphocytes

• Need to consider ratio of body water to soft tissue

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In vitro dose response for tritium

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Dose (Gy)

Dic

en

tric

s p

er

ce

ll

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Urine and biodosimetry dose estimates

Days after accident

Dosetosofttissue,Gy

Hospital

20 30 40 5010

0.1

180

0.5

0.3

0

21317

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Final dose estimates

Urine assay 0.47 Sv

Dicentrics 0.37 Gy

• Patient remained for several years under periodic medical surveillance and blood was again taken ~ 5, 6 and 11 years

• Analysed again for dicentrics and also for FISH translocations

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Tritium overdose

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Gy

Urine

Dic

DicDic

FISH

Dic

FISH

1985 1990 1991 1996

Follow-up blood sampling

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Overall conclusion

• Two quite different accidents have been described

• 1. External partial body exposure

• 2. Internal whole body exposure

• Confounding factors of dose protraction

• It was possible to compare information from biodosimetry with that from physics

• Good agreement was obtained

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